THIN-LAYER MICELLAR CHROMATOGRAPHY OF COINAGE METAL CATIONS

DISSERTATION S U B M I T T E D IN PARTIAL FULFILMENT OF THE REQUIREMENTS

FOR THE AWARD OF T H E DEGREE OF

dllasiter of ^I)tIos;opl)p IN

Applied Chemistry

BY

YASIR HAMID SIRWAL

DEPARTMENT OF APPLIED CHEMISTRY H. COLLEGE OF ENGINEERING & TECHNOLOGY

ALIGARH M U S L I M UNIVERSITY ALIGARH ( INDIA)

2001

DS3279

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Idicat^d ID

* mmssfi

DEPARTMENT OF APPLIED CHEMISTRY

Dr. Ali Mohammad Ph.D., D.Sc. (F.N.A.Sc.)

EDITOR : Chemical & Environmental Research

Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh-202 002, India 564-230 AMU IN (Telex) . 91-571-700920, 700921 (Tel) . 456 (Ext.) . mohammadali4u@rediffmail.com (E-mail)

OEm^^ICJiTfE

Certifiecf that tfie wor^incorporatedin this thesis entitled

"1hin.-Layer 'MiceCfar Chromatography of Coinage MetaC

Cations" Seviig suSmitted 6y 'Mr Tasir Jdamid SirwaC is in

partiaCfudfiCfment of the requirements for the award of the

degree of'Master of (philosophy in Applied Chemistry of Aligarh

Muslim. Vniversity, Aligarh.

The •wor{^em5odiedin the thesis is original and Sonafide

record of the research carried out under my supervision.

(ALI MO^KAMMAO)

SV(FE^ISO(^

Residence : "AL-RAIYAN" Iqra Colony, New Sir Syed Nagar, Aligarh-202002 (INDIA), 9 : 0571-408196

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inspiring guiiiancE. Entl]ufiiafitir support. sgmpatl|Etic attituiiE. f|Ealti]g

anil confitructiUE criticism, ualuafak guidancE ani constant supEruision.

Mu sincErE tijanks ars buE to l^xof. ^M. lKi|an. ffiljairman anii

?arof. 2C.(I. liarsiinEg. lEx-ctfairman. lEptt. of AppliEii ctfEmistrQ for

prouiibing rESEarct} facilities.

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colkaguES. Sr. €ram 3raqi. Br. 3.A. iKl|an. MB. BinsEta Agrauial.

ills. 5s'al|tii ilabEsn anb ills. &ufia iiina for tl|Eir gooJi uiist}ES,

constant encouragEniEnt anb cljEErful assistancE during tljis uiork.

gipECial ti|ank arr also hm to mg friEnis Amir. iHE|balj,

iiari (§m. Nanboo. dl|uja. Srfan. Hikmat. Aquil anh ^atfi for tl|Eir

cooperation and timElg IfEip.

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bring tljis uiork usrg fruitful.

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inspiration in accomplislfing tifis tasks anb also bsliolbEn to oti|Er

mEmbErs of tifE familg EspEciallg mg ElbEr sistsr for l|Er affEctionatE

EncouragEment anb intsrEst in mg acabEmic pursuits.

i

- 1 -

CONTENTS

Acknowledgements i CHAPTER-!: GENERAL INTRODUCTION I - 40 1.1. Analytical Chemistry 1.2. Chromatography 1.3. Thin-layer Chromatography 1.4. History of TLC 1.5. TLC Methodology 1.6. Principles and Techniques 1.7. Nature of Phase Interactions 1.8. Sample Application 1.9. Development 1.10. Visualization 1.11. Quantitation 1.12. Advantages of TLC 1.13. Latest Developments in TLC 1.14. Combination of TLC with other Analytical Techniques 1.15. Coinage Metals: Copper, Silver and Gold 1.16. Terrestrial Abundance and Distribution 1.17. Biological Aspects of Copper, Silver and Gold 1.18. Aqueous Chemistry and Complexes of Cu (II) 1.19. Compounds of Silver (I) 1.20. Complexes of Au (III) <• Table of literature *l* References CHAPTER - 2: THIN-LAYER MICELLAR CHROMATOGRAPHY

OF COINAGE METAL CATIONS 41 - 67

2.1. 2.2. 2.3. • : •

• : •

• : •

Introduction Experimental Results and Discussion Tables Figures References

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1.1 ANALAYTICAL CHEMISTRY

Analytical chemistry is a branch of chemistry basically concerned with the

determination of the composition of matter. The modem aspects of analytical

chemistry are the identification of a substance, the elucidation of its structure and

quantitative analysis of its composition. The ever-increasing impact of analytical

methods reveals that no other branch of science finds extensive applications as

analytical chemistry for two reasons. It finds numerous applications in various

disciplines of chemistry such as inorganic, organic, physical and biological

chemistry. Secondly, it finds applications in other fields of science such as

environmental, agricultural, biomedical and clinical, solid state and space sciences.

Owing to the great importance of pollution, the environmental chemistry is being

more relevant. Instrumental and non- instrumental methods are being developed for

the analysis of air, water and soil pollutants depending solely on its chemical

analysis, which may be qualitative or quantitative. A quantitative analysis deals with

the methods dealing with the determination of actual amount of a given species

present in sample. Instrumental and non- instrumental methods are used in analytical

chemistry. Instrumental methods are usually faster and more sensitive, whereas non-

instrumental methods, which form the basis of standardization of the instruments, are

considered more accurate. However, it is difficult to draw a clear borderline between

instrumental and non- instrumental analytical methods.

Despite distinct advantages of instrumental methods in many directions, their

wide spread adsorption has not rendered the classical methods obsolete. The non-

instrumental methods being simple, inexpensive and versatile have to be

strengthened. In fact, a close critical comparison of all available analytical methods

demonstrates the truth of well known rule of life that "nobody (here: nothing) is

perfect. The team is always stronger than the individual and nothing can be replaced

completely without loss. Co- operation is best." It is, therefore, concluded that all the

analytical techniques should be developed continuously without discrimination.

Several analytical techniques are available for the analysis of substances but

the choice of a particular method depends upon the simplicity, selectivity, rapidity,

cost effectiveness, sensitivity and ease of application. This is especially important in

the determination of very small quantities of substances. One has to look into the

possibilities of using various separation techniques such as precipitation, distillation,

dialysis, ring- oven technique, ion- exchange, electrophoresis, solvent extraction and

chromatography while handling a new system.

1.2 CHROMATOGRAPHY

According to Keulemans "Chromatography is a physical method of

separation, in which the components to be separated are distributed between two

phases, one of which constituting a stationary bed of large surface area, the other

being a fluid that percolates through or along the stationary phase." The separation of

individual components results primarily due to differences in their affinity for the

stationary and mobile phases. The word "chromatography", derived from the Greek

words "chromatus" and "graphein", meaning "color" and "to write" was used by him.

After the initial work of Tswett, a wide variety of independent techniques that have

little to do with color have come to be called chromatography. From its infancy,

chromatography has grown in prominence and popularity to become a leading

technique of analysis. The salient features of common chromatographic techniques

are summarized in Table 1.1.

Chromatographic Systems

According to the physical arrangement, chromatographic systems can be

divided as planar or column depending on the geometry of the support. The planar

arrangement are represented by paper and thin- layer chromatography. According to

development procedures, the planar systems can be further classifieds as ascendent,

horizontal, descendent and occasionally centrifugal.

The four possible chromatographic systems derived from solid, liquid, and

gaseous phases are:

a. Liquid - Liquid

b. Liquid - Solid

c. Gas - Liquid

d. Gas-Solid

Of these, liquid - liquid and liquid - solid systems constitute "liquid

chromatography (LC)." Column chromatography, paper chromatography and thin-

layer chromatography (TLC) are the variants of LC. Of these, TLC is more efficient

and rapid. However, the high performance liquid chromatography (HPLC), similar in

efficiency to GLC has widened the applicability of LC. As the work being reported

here involves the use of TLC as analytical technique, it is appropriate to summarize

some salient features of this most popular and commonly used separation technique.

1.3 THIN- LAYER CHROMATOGRAPHY

TLC is a subdivision of LC, where the mobile phase (a liquid) migrates

through the stationary phase (a thin layer of porous sorbent on a planar inert surface)

by capillary action. It is a rapid, simple, versatile, reasonably sensitive and

inexpensive analytical tool which is applicable for (a) qualitative (b) quantitative and

(c) preparative analyses.

1.4 HISTORY OF TLC

The history of TLC has been reviewed by Stahl (1), Kirchner (2, 3) and Pelick

et. al. (4). In fact, the beginning of TLC may be attributed to Beyerinck who reported

the separation of sulfuric and hydrochloric acids in the form of rings on thin layers of

gelatin (5). Following the same technique, Wijsman separated enzymes from malt

diastase (6). In 1938, Izmailov and Shraiber separated certain medicinal compounds

on binder free horizontal thin layer of alumina spread over a glass plate (7). As the

development was carried out by placing solvent drops on the glass plate containing

sample and adsorbent, their method was called "drop chromatography." However,

this method could not catch the eyes of scientists until two American chemists,

Minhard and Hall used a mixture of aluminum oxide (adsorbent) and celite (binder)

as a layer on a microscopic slide to separate inorganic ions (8). They called this

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technique as "surface chromatography" and this was the first application of layer

chromatography in the separation of inorganics. Since 1958, when Stahl introduced

the term, "thin- layer chromatography" and standardized procedures, materials and

nomenclature (9, 10), the effectiveness of this technique for separation was realized.

A major breakthrough in the field of TLC came in the early 1960's with the

availability of precoated plates (11). It has recently been realized that modem high

performance thin layer chromatography (HPTLC) initiated in 1975, rivals high

pressure liquid chromatography (HPLC) and gas chromatography (GC) in its ability

to resolve complex mixtures and to provide analyte quantification.

1.5 TLC METHODOLOGY

The complete process of TLC is summarized in Fig. LL Solute identification

in TLC is based on Rp, known as retardation factor which is calculated as:

Distance travelled by the compound from the origin

Distance travelled by the mobile phase from the origin. The Rp values in TLC are dependent upon many variables (nature of the

sorbent, layer thickness, layer-activation, temperature, chamber saturation, and nature

of mobile phase, pH of the medium, development technique, room temperature,

sample size and relative humidity); which must be regulated carefully during the

preparation and evaluation of the chromatography to obtain reproducible results. Rp

values vary between 0.0 (solute remaining at the point of application) and 0.999

(solute migrates up to the solvent front) and have no unit. The differential migration

results because of varying degrees of affinity of components in a mixture, for the

stationary and mobile phases.

1.6 PRINCIPLE AND TECHNIQUES

In TLC, separation of components in a mixture is achieved by optimizing the

experimental conditions. The desired separation can be achieved by proper selection

of adsorbent (stationary phase) and solvent (mobile phase).

Prepara t ion of Sample

Applied on Chromatop la te By Spotting or S t reaking

Development

Drying of Chromatogram

Detection Visual , UV-scanning, reagent sp ray

C o m p o n e n t Removal (optional)

Documentation

Figure 1.1 The process of thin-layer ch roma tog raphy

1.7 NATURE OF PHASE INTERACTIONS

Some of the important physical and chemical characteristic that determine the

degree of interactions of mobile phase - solute, sorbent- solute, and mobile phase -

sorbent are given in the following paragraphs.

a. Intermolecular Forces: These forces hold neutral molecules

together in the liquid or solid state. These forces are physical,

characterized by low equilibrium and results in good

chromatographic separation.

b. Inductive Forces: These forces exist when a chemical bond has a

permanent electric field associated with it (e.g. C-Cl, C-NO2

groups). Under influence of this field, the electrons of an adjacent

atom, group or molecule are polarized so as to give an induced

dipole moment. This is a major contributing factor in the total

adsorptive energy on alumina.

c. Hydrogen Bonding: It makes a strong contribution in adsorption

energies between solute or solvents having a proton donor group and

a nucleophilic polar surface such as that of alumina or silica gel.

d. Charge Transfers: Charge transfers between components of the

mobile phase and the sorbent can also take place to form a complex

of the type S" A" (where S = solvent or solute, and A = surface site of

sorbent). This is prominent in ion- exchange chromatography.

e. Covalent Bonds: These bonds can be formed between solute and / or

the mobile phase and the sorbent. These are strong forces and results

in poor chromatographic separation.

1. STATIONARY PHASE (ADSORBENT)

A large number of sorbents are available which can be used in TLC. However,

more commonly used sorbents are silica gel, alumina, cellulose and kieselguhr

(diatomaceous earth).

Silica gel is the most frequently used layer material. It is slightly acidic in

nature. At the surface of silica gel the free valencies of the oxygen are connected with

hydrogen (Si- OH, silanol groups) or with other silicon atom (Si- O - Si, siloxane

groups) as shown in Fig.1.2. The silanol groups represent adsorption active surface

centers that are able to interact with solute molecules. The ability of the silanol

groups to react chemically with appropriate reagents is used for controlled surface

modifications. Hence, silica gel is considered as the most favored layer material in

chromatography.

Alumina or aluminum oxide is also generally used as a sorbent. It is basic in

nature and more reactive than silica gel. Kieselguhr is chemically neutral sorbent

consisting of about 90% SiOs along with AI2O3, FcjOs, MgO, NajO, K2O, CaOj and

Ti02 in different proportions representing, the remaining 10% of the kieselguhr

material. It has very low surface activity. Cellulose can be used as a sorbent in TLC

when it is convenient to perfomi a given paper chromatographic separation by TLC

in order to decrease the time required for the separation and increase the sensitivity of

detection.

In addition, the importance of using surface modified sorbents in TLC has

increased. Both hydrophilic and hydrophobic modified sorbents have been used in

recent years. Both commercially available layers as well as laboratory made sorbent

layers have been used by chromatographers. However, more emphasis has been on

precoated HPTLC plates.

II. MOBILE PHASE (SOLVENT SYSTEM)

In liquid Chromatography including TLC, the mobile phase exerts decisive

influence on the separation. Various optimization schemes (windows diagram,

overlapping resolution maps, simplex method and PRISMA model) are proposed for

normal-phase and reversed-phase TLC. A large number of mobile phases have been

reported, of them some are enlisted below:

a) Organic Solvents: The single component mobile phase including acetone,

acetonitrile, benzene. Carbon tetrachloride, chloroform, dioxane, ethanol,

OH

Figure 1.2 Structure of Silica gel

10

ethylacetate, methanol, o-xylene, petroleum ether, toluene, n-octanol,

n-nonane, cyclohexane and binary/ ternary mixtures of alcohols, amines,

ketones, phenols, haloalkanes have been used.

b) Inorganic Solvents: Being non-toxic and non-volatile, solvent systems of

this group have been widely used in TLC of inorganics and

organometallics. This group includes the solution of mineral acids, alkalies

and inorganic salts prepared in double distilled water or water-methanol

mixture.

c) Mixed Solvents: Mixtures of two or more different solvents, most of

which have either a base (NaOH, NH4OH and amine) or an acid (mineral

or carboxylic) as a component are used to develop the TLC plates.

d) Surfactant- Mediated Solvents: Solutions of surfactants (SDS, CTAB, or

Triton X-100) have also been used to lesser extent as mobile phase in TLC.

SURFACTANT - MEDIATED SYSTEMS

These systems contain surfactant as one of the components of the mobile

phase. Surfactants in the aqueous mobile phase can be used in the following ways:

a) As monomer surfactants where the concentration of surfactant in aqueous

mobile phase is restricted to well below the critical micelle concentration

(CMC) of the surfactant. These mobile phases are most suited to separate

ionic species by ion-pair chromatography (IPC). In this technique, a small

concentration of ion-pairing reagent, which has an opposite charge to the

ionic solutes (i.e. cationic surfactant for anionic solutes and anionic

surfactant for cationic solutes), is added to the aqueous mobile phase and

its concentration is kept low to avoid the formation of micelles.

b) As surfactant micelles where the surfactant concentration is kept well

above its CMC value. In such cases, the mobile phase is composed of

surfactant molecules in the form of monomers and aggregates (or

micelles). These mobile phases are very useful for simultaneous

11

separation of ionic and non-ionic compounds by micellar liquid

chromatography (MLC).

c) As microemulsion where surfactant in the presence of water, an oil

(hydrocarbon) and co-surfactant (i.e. medium chain length amine or

alcohol) is used as transparent solution.

Surfactants are long chain amphiphilic organic or organometallic molecules

containing a highly polar (hydrophilic or lipophobic) or "ionic head group" attached

to a non-polar (hydrophobic or lipophilic) hydrocarbon tail of varying chain length.

The "head group" is either cationic (e.g. ammonium or pyridinium ion), anionic (e.g.

hydroxy compounds) or zwitterionic (e.g. amine oxide, carboxylate or sulphonate

betain) and the hydrocarbon tail which may contain at least 8 carbon atoms.

Depending upon the nature of hydrophilic group, surfactant can be classified as

anionic [R-X 'M ^); cationic (R-N '•(CH3)3 X •]; zwitterionic [R-(CH3)2 N ^CHjX "]

and nonionic [R(0CH2 CH2)],r,0H, where R is a long aliphatic hydrocarbon chain,

M^ is a metal ion, X " is a halogen, COO " or SO4 " and m is an integer. A list of some

common surfactants is provided in Table 1.2.

MICELLES

Surfactant (or amphiphilic) molecules comprising of hydrophobic and

hydrophilic moieties tend to exhibit a considerable degree of self organization when

dissolved in aqueous solutions. Above a certain concentration level, termed as critical

micelle concentration (CMC), the surfactant molecules in solutions (water or organic

solvents) aggregate to form micelles. The process of micelle formation is called

"micellization". Micelles do not exist at all concentrations and temperatures. There is

a very small concentration range below which aggregation to micelles is absent and

above which association leads to micelle formation. This narrow concentration range

during which micelle formation occurs is called the CMC. At low concentration i.e.

below CMC and at temperature above the critical micelle concentration (e.g. Kraft

temperature), the surfactant is dispersed in the aqueous media at the molecular level

as a monomer. The average number of monomers per micelle is called the

12

Table 1.2 Typical Surfactants and their CMCs, Aggregation

Numbers, Kraft Point Values*

Surfactant CMC (M) Aggregation Kraft

Number Point

(°C) b

Aqueous (norma!) Anionic Sodium dodecvl sulfate (SDS), g l x l O " ^ 62 9 CH,(CH:)nOS03"Na" Potassium perfluorohcptanoate, 3.0 x 10"' c 25.6 CvFi.COOK' Sodium polvoxvethvlene( 12)dodec\l ether. 2.0 x 10"' 81 <0 CH3(CH:)M(OC'H:CH,:),:OS03"Na""(SDS 12E0)

Cationic Cei\lp\"ridinium chloride.'^ 1.2 x 10" 95 c C:oH;;NX.H5Cr Cet\ I trimethvl ammonium bromide (CTAB). 9 0 x 10" 78 23 CH--(CH:),.N"(CH.OjBr

.N 0 n i 0 n i c Pol\ox\cthvlene(6)dodecanol. 9 .0x10 '^ 400 c CH;,(CH:),,'(0CH;CH;)60H Pol\oxvethvlene(23)dodccanol(Brij-35). l . O x l O ' 40 c CH;(CH:)n(0CH:CH:):30H

Zvr itterionic N-Dodec\l-N,N-dimethvlammonium-3- 3.0 x 10" 55 <0 propane-1-sulfonic acid (SB-12). CH;(CH;)nN'(CH3):(CH:)3S03-N.N-Dimethyl-N- 25x10"" 24 <0 (carboxymethvi)oct\lammonium salt, CgH.,7N"(CH3):CH;COO"(octylbetaine)

Nonaqueous (reversed) Bis(2-ethvlhexvl) sodium sulfosuccinate 6.0 x 10"" c c (AOT).' " Na05SCH(CH2COOCsH,7)COOC8H,7

"values for aqueous solution at 25 C ^Temperature at which the solubility of an ionic surfactant is equal to

the CMC. "Not available or not defined

'^In 0.0175 MNaCl. ^In hexane.

13

aggregation number (N). At 25° C and 1 atm., the CMC is typically less than 20 mM,

with each micelle-consisting of 40-140 monomers. A conventional model of micelles

is that proposed by Hartly (Fig. 1.3) which is very useful for visualization of a

micelle. The various structures formed in aqueous solution on increasing the

concentration of surfactant is illustrated in Fig. 1.4. There are mainly two types of

micelles:

a) Normal Micelles: The molecular organization of surfactant molecules in

aqueous solutions results in the formation of normal micelles. Above CMC.

the surfactant molecules are self aggregated in such a manner that the

hydrophobic moieties (i.e. hydrocarbon tails) are oriented inward forming a

non-polar core and hydrophilic (polar) head groups are outward keeping

themselves in contact with the bulk aqueous phase. Normal aqueous micelles

are generally formed from a singly-chain surfactants and chain-branching

inhibits micellization.

Micelles are considered to be dynamic in nature, with continuous

exchange of surfactant molecules, in and out of the aggregates occurring in

the milliseconds to microseconds range. Thus, individual surfactant molecules

(called monomers) are thought to be distributed throughout the aqueous phase

surrounding the micelles.

b) Reverse Micelles: In contrast to the normal micelles which are formed in

polar (i.e. aqueous media) solvents, reverse micelles are formed in non-polar

solvents like hexane or chloroform and a trace of water where the polar head

groups of the surfactant are directed towards the interior of the aggregate and

the hydrocarbon chains are in contact with the non-polar solvent. Compared

to normal micelles, reverse micelles are more complex and less understood.

Reverse micelles offer the same potential advantages for analysis as do

normal micelles i.e. the ability to solubilize polar species that would be

excluded from normal micelles. An interesting aspect of reverse micelles is

their capability to solubilize water in the interior of micelle structure.

14

{

«

MicGl lar core

"_ Hycrocorbon "-T' inter ior

Aqueous ex ter io r

Gouy - Chapman layer

Stern layer

Figure 1.3 Hartley model of a spherical micelle

15

—O Q - ^

ex.

o

< -J t- < o - —o o o

o— —o o o o— —o o o O - —GO O o— —o o o

TO

o ,TO

CO

<u

o

u. o

z ic: o < Q.

_ i < 2 : 0

< UJ X

LU Q 2 — —1 >-0

cr 1x1

<

0

> c ^ 0

0

c5 £ iZ

£ <u

*~~ L-

0

CO u.

o o to

< §

s

G3

O

o

16

From macroscopic perspective, miceliar solutions are homogeneous and

cannot be filtered. However, the unique characteristics of miceliar aggregates stem

from their microscopically non-homogeneous nature i.e., they provide a

microenvironment which is distinctly different from the bulk solvent. The most

important property of micelles is their ability to solubilize substances that are

otherwise insoluble (or sparingly soluble) in water.

1.8 SAMPLE APPLICATION

Sample application is one of the most important steps in the technology

of TLC. Improperly applied samples result in poor chromatograms. Samples are

applied as spots or stripes on the sorbent layer, about 2-3 cm above from the lower

edge of the TLC plate so that only sorbent layer makes contact with the mobile phase

and the sample does not dissolve in the mobile phase. The sample should be

completely dried before placing the plate in the development chamber. Micropipette,

micro syringe, melting point capillaries etc. have been used to apply the sample on

the plates. A number of automatic spotters of varying design are also available in the

market.

L9 DEVELOPMENT

Development in TLC is the process in which the mobile phase moves across

the sorbent layer to effect separation of the sample substances. Ascending

development or linear development is the commonly used mode of development in

TLC in which the mobile phase moves up (ascends) the plate. Any close container

that will hold the plate upright is usable while performing the development and

saturation of chamber apart from other factors. It has been observed that the angle of

development i.e. the angle at which the plate is supported, effects the rate of

developments as well as the shapes of the spots (12). An angle of 75° C is optimum

for development. If a desired separation is not achieved by simple development,

some development options such as (i) multiple development (ii) stepwise

development (iii) continuous development (iv) two- dimensional development

17

(v) circular development and (vi) reverse-phase partition development are available

to achieve the desired separation.

1.10 VISUALIZATION

The methods of visualization (detection used in TLC are of three major types,

(i) physical (ii) chemical and (iii) enzymatic or biological. Among the physical

methods, visualization in UV-light is most common. This method is highly sensitive,

non-destructive, and amenable to the visualization of spots before undertaking

quantitative studies. Chemical methods of detection involve the spraying of

chromatoplates with a suitable reagent, which forms colored compounds with the

separated species. Both selective and non-selective reagents may be chosen for the

location of the separated zone.

1.11 QUANTITATION

The three main approaches associated with quantitation in TLC are (i) visual

estimation (ii) zone-elution (iii) in-situ densitometry. Amongst these, in-situ

densitometr}' is the preferred technique for quantitative TLC. Substances separated

by TLC or HPTLC are quantified by in-situ measurement of adsorbed visible or UV-

light, or emitted florescence upon excitation with UV-light. Adsorption of UV-light

is measured either on regular layers or on layer with incorporated phosphor.

1.12 ADVANTAGES O F TLC

TLC is the most versatile and flexible chromatographic method. It is rapid

because precoated layers are available for use as received, without preparation. It has

highest sample throughout, because up to 30 individual samples and standards can be

applied to a single plate and separated at the same time. The automated sample

applications and developers allow high accuracy and precision in quantification.

There is a wide choice of layers, developers and detection methods. The wide choice

of detection reagents leads to unsurpassed specificity. Less pure samples can be

applied as the layers are normally not reused. Being an "off line" method, different

steps of the procedure are carried out independently.

18

1.13 LATEST DEVELOPMENTS IN TLC

As a result of recent innovations, several new techniques such as high

performance thin- layer chi-omatography (HPTLC), over pressurized thin- layer

chromatography (OPTLC), centrifugal layer chromatography (CLC), reversed-phase

thin layer chromatography (RPTLC), radial chromatography, hot plate

chromatography, bioautoradiography, immunostanning and enzyme inhibition

techniques came into light. HPTLC layers being thinner and made of sorbent with

more uniform particle size are developed for a shorter distance. All these factors lead

to faster separations, reduced zone diffusion, lower detection limits, less solvent

consumption and better separation efficiency.

1.14 COMBINATION OF TLC WITH OTHER ANALYTICAL TECHNIQUES

The careful combination of TLC with other analytical techniques is more

useful to collect information regarding the analysis of a complex sample.

Spectrophotometiy, high-performance liquid chromatography and gas

cluomatography, in conjugation with TLC are the three most widely used techniques.

However, mass/ GC, infrared and thermal analytical techniques in combination with

TLC has also been used. One of the newest techniques used in combination with

TLC is photoaccuoustic spectrometry, which is capable to locate compounds in -situ

on the plate. Issaq and BaiT (13) combined TLC with flameless atomic absorption

spectrometry (FAAS) to identify an inorganic compound in an impure organometallic

complex and to determine the recovery and purity of organometallic samples.

The examples cited above reveal, how the separation method of TLC

complement the analytical methods necessary for the absolute identification of a

substance. TLC provides an excellent purification method for separating a substance

of interest from other contaminants in the sample. Analytical techniques can then be

applied to identify the separated substances.

19

1.15 COINAGE METALS: COPPER, SILVER AND GOLD

Cu, Ag, and Au are collectively known as the 'Coinage Metal' because of

their former usage, these elements were almost certainly the first three metals known

to man. These metals have been used as primitive money long before the introduction

of Au coins in Egypt around 3400 BC.

Cold hammering was used in the late Stone Age to produce plates of gold for

ornamental purposes, and this metal has always been synonymous with beautv,

wealth and power. The coffin of Tutankhamun (a minor pharoh who was only 18

when he died) contained no less than 112kg of gold, and the legendary Aztec and

Inca hoards in Mexico and Peru were a major reasons for the Spanish conquests of

Central and South America in the early sixteenth century. Today the greatest hoard of

gold is the 30,000 tonnes of the bullion (i.e. bars) lying in the Vaults of the US

Federal Reserve Bank in New York and belonging to eighty different nations. By

about 3500 BC copper was obtained from its ores in the Middle East by charcoal

reduction and by 3000 BC the advantage of adding tin in order to produce the harder

bronze was appreciated in India, Mesopotamia, and Greece. This established the

"Bronze Age", and copper has continued to be one of man's most important metals.

The monetary use of silver may well be as old as that of gold but the

abundance of the native metal was probably far less, so that comparable supplies

were not available until a method of winning the metal from its ores was discovered.

It appears that by perhaps 3000 B.C a form of cupellation was in operation in Asia

Minor and its use gradually spread, so that silver coinage was of crucial economic

importance to all subsequent classical Mediterranean civilizations. The name copper

and the symbol Cu are derived from Cyprium (later Cuprum), since it was from

Cyprus that the Romans first obtained their copper metal. The words silver and gold

are Anglo- Saxon in origin but the chemical symbols for these elements (Ag and Au)

are derived from the Latin argentum and aurum, gold.

20

1.16 TERRESTRIAL ABUNDANCE AND DISTRIBUTION

The relative abundances of these three metals in the earth's crust (Cu 68 ppm.

Ag 0.08 ppm, Au 0.004 ppm) are comparable to those of the preceding triad ~ Ni, Pd,

and Pt. Copper is found mainly as the sulfide, oxide, or carbonate, its major ores

being copper pyrites (chalcopyrite), CuFeSa, which is estimated to account for about

50% of all Cu deposits; copper glance (chalcocite), CuaS; cuperite, CU2O, and,

malachite, CU2CO3 (OH) 2- A rarer mineral, CuAlg (PO4) 4(0H) g. 4H2O is the valued

blue gemstone turquoise. Large deposits are found in various parts of North and

South America, and in Africa, Russia and its former alliances. The native copper

found near Lake Superior is extremely pure but the vast majority of current supplies

of copper are obtained from low - grade ores containing only about 1% Cu.

Silver is widely distributed in sulfide ores of which silver glance (argentite).

Ag2S is the most important. Native silver is sometimes associated with these ores as a

result of their chemical reduction, while the action of sah water is probabK

responsible for their conversion into "horn silver", AgCl, which is found in Chile and

New South Wales. The Spanish Americas provided most of world's silver for three

centuries after about 1520, to be succeeded in the nineteenth century by Russia.

Appreciable quantities are now obtained as a byproduct in the production of other

metals such as copper, and the main "Western" producers are Mexico, Peru, Canada,

the USA, and Australia.

Gold too, is widely, distributed both native and in tellurides, and is almost

invariably associated with quartz or pyrite, both in Veins and in alluvial or placer

deposits laid down after the weathering of gold - bearing rocks, ft is also present in

seawater to the extent of around 1x10"- ppm, depending on location. Prior to about

1830, a large proportion of the world's stock of gold was derived from ancient and

South American civilizations (recycling is not a new idea), and the annual output of

new gold was no more than 12 tonnes pa. This supply gradually increased with the

discovery of gold in Siberia followed by "gold rushes in 1849 (California: as a result

2 1

of which the American West was settled), 1851 (New South Wales: within 7 yrs the

population of Australia doubled to 1 million, 1884 (Transvaal), 1896 (Klondike,

North-West Canada), and, finally, 1900 (Nome area of Alaska) as a result which by

1890 world production has risen to 150 tormes pa. It is now 8 times that amount,

-1200 tonnes pa.

Cu is a soft metal and is often used in alloys - brass (for plumbing fixtures)

and bronze (for statues). Also, it is often a minor component in nickel and silver

alloys Tablel.3.

Table 1.3 Important Alloys of Copper:

Alloy

Brass

Bronze

Nickel coins

Sterling Silver

Approximate Composition

77% Cu, 23% Zn

80% Cu, 10% Sn, 10% Zn

75% Ni, 25% Cu

92.5% Ag, 7.5% Cu

Properties

Harder than Cu

Harder than Brass

Corrosion resistant

More durable than pure silver

Au is commonly alloyed with other metals in order to make it harder and

cheaper. The proportion of gold is exposed in carats, a carat being a twenty - fourth

part by weight of the metal so that pure gold is 24 carats. The term is derived from

the name of the small and very uniform seeds of the carob tree, which in antiquity

were used to weight precious metals and stones. The two main uses of gold are in

setting international debts and in the manufacture of jewellery, but other important

uses are in dentistry, the electronics industry (corrosion free contacts), and the aero­

space industry (brazing alloys and heat reflection), while in office building it has

been found that a mere 20 pm film on the inside face of windows cuts down heat

losses in winter and reflects unwanted radiation in summer.

1.17 BIOLOGICAL ASPECTS OF COPPER, SILVER AND GOLD

Copper is the third most biologically important transition metal after iron and

zinc. About 5 mg are required in the daily human diet. A deficiency of this element

renders the body unable to use ironstones in the liver. There are numerous copper

22

proteins throughout the living world, the most intriguing being the hemocyanins.

These molecules are common oxygen carriers in the invertebrate world: crabs,

lobsters, octopi, scorpions, and snails, all have bright blue blood. At the same time,

an excess of copper is highly poisonous, particularly to fish. This is why copper coins

should never be thrown into fish pools for 'good luck". Humans usually excrete

many excess, but a biochemical (genetic) defect can result in copper accumulation in

the liver, kidneys and brain. This illness, Wilson's disease, can be treated by

administering chelating agents, which complex the metal ion and allow it to be

excreted harmlessly. Both silver and gold have specific medical applications. Silver

ion is a bactericide, and dilute solutions of silver nitrate are placed in the eyes of

newborn babies to prevent infection. Gold compounds, such as the drug auranofin,

are used in the treatment of rheumatoid arthritis.

Some properties of the elements Cu, Ag, and Au that can be directly related to

the d"* s' electronic configuration are summarized in Tablel.4.

1.18 AQUEOUS CHEMISTRY AND COMPLEXES OF COPPER (II)

Most Cu (II) salts dissolve readily in water and give the aqua ion. Addition of

ligands to such aqueous solutions leads to the formation of complexes by successive

displacement of water molecules. With NH3, for example, the species

[Cu (NH3) 2(H20) 5] ", - ' -, [Cu (NH3) 4(H20) 4] ^ are formed in the nonnal way.

Addition of the fifth NH3 can occur in aqueous solution (14), but the sixth occurs

only in liquid ammonia. The reason for this unusual behavior is connected with the

John - Teller effect. Because of it, the Cu (II) ion does not bind the fifth and sixth

ligands strongly (even the H2O). When this intrinsic weak binding of the fifth and

sixth ligands is added to the normally expected decrease in the stepwise formation

constants, the formation constants K5 and Kg are very small indeed. Similarly, it is

found with ethylenediamine that [Cuen (H2O) 4] " and [Cuen2 (H2O) 2] ^ form

readily, but [Cu (en) 3] * are formed only at extremely high concentration. Many

other amine complexes of Cu (II) are known, and all are much more intensely blue

than the aqua ion. This is because the amines produce a strong ligand field, which

23

causes the absorption band to move from the far red to the middle of the red region of

the spectrum, e.g. in the aqua ion the absorption maximum is at -800 nm, whereas in

[Cu (NH3) 4(H20) 2] ^ it is at -600 nm as shown in Fig.1.5.

Table 1.4 Some Properties of the Elements Copper, Silver and Gold:

Property Atomic Number Number of naturally occurring Isotopes Atomic Weight Electronic configuration Electronegativity

Cu 29 2

63.546 (+0.003) [Ar]3d'°4s"^

1.9 Metal radius (12 - coordinate)(pm) 128 Effective ionic radius (6-coordinate)(pm)

Ionization energy (KJmol"')

MP CQ BP (°C) AHfus (KJ mol'') AHvap(KJ mof') A H (nionatomic 2as)(K.J m o l )

Density *''(20°C)(gcm-^) Electrical resistivity at 20°C(|j.ohm cm)

V III II I ^st

2nd i r d

-

54 73 77

745.3 1957.3 3577.6 1083 2570 13.0

307(+6) 337(+6) 8.95

1.673

As 47 2

107.8682 (+3) [Kr]4d'°5s'

1.9 144

-

75 94 115

730.8 2072.6 359.4 961 2155 11.1 258(+6) 284(+4) 10.49

1.59

Au 79 1

196.9665 [Xe]4f''5d'°6s'

2.4 144

57 85 -137

889.9 1973.3 (2895) 1064 2808 12.8 343(+ll) 379(+8) 19.32

2.35 Depends on mechanical history of sample.

In haiide solutions the equilibrium concentrations of the various possible

species depend on the conditions; although CuCls^" has only a low formation

constant, it is precipitated from solutions by large cations of similar charge. Treating

aqueous solution with ligands may isolate many other Cu (II) complexes. When the

ligands are such as to form neutral, water - insoluble complexes, the complexes are

precipitated and can be purified by recrystallization from organic solvents. The

bis- (acetylacetanato) copper (II) complex is another example of this type.

Although as noted previously, addition of CN" normally leads to reduction to

CuCN, in the presence of nitrogen donors like 1,10 - phenanthroline reduction is

inhibited and five - coordinate complexes like [Cuphen2 (CN)]" and [Cuphen2 (CN)2]

are obtained. Multidentate ligands that coordinate through oxygen or nitrogen, such

24

as amino acids, from Cu (II) complexes, often of considerable complexity. The well

known blue solutions formed by the addition of tartarate to Cu solutions (known as

Fehling's solution when basic and when mesotartarate is used) may contain

monomeric, dimeric, or polymeric species at different, pH values. One of the dimers,

Na2[Cu{(+ ) C4O6H2}].5H20, has square coordinate Cu (II), two tartarate bridges, and

a Cu - Cu distance of 2.99A.

10,000 15,000

Frequency (cm" 20,000

Figure 1.5 Absorption spectra of {A) (Cu(H,0),]^- and of the ammines in 2 M ammonium nitrate of 25°C: {B) [Cu(NH3)(H,0),]-; (C) [Cu(NH:,MH,0),]- ; (D) [CuCNHO^H.O),]-- (£)

1.19 COMPOUNDS OF SILVER (I)

The salts AgNOs, AgClOs and AgC104 are water soluble but Ag2S04 and

AgOOCCHs are sparingly soluble. The salts of oxo anions are primarily ionic, but

although the water - insoluble halides AgCl and AgBr have the NaCl structure, there

appears to be appreciable covalent character in the Ag X interactions, whereas in

compounds such as AgCN and AgSCN, which have chain structures as given below,

the bonds are considered to be predominantly covalent.

- A g - C - N - A g - C - N Ag C

25

Silver oxide is more soluble in strongly alkaline solution than in water,

and AgOH and Ag(OH)" 2 are formed. The treatment of water-soluble halides with a

suspension of silver oxide is a useful way of preparing hydroxides, since the silver

halides are insoluble. Analogous to copper and gold, alkali metal silver oxides

contain Ag20''"4 units. The action of hydrogen sulfide on argentous solution gives

black Ag2 S, which is the least soluble in water of all silver compounds. The fluoride

is unique in forming hydrates such as AgF.4H20, which are obtained by crystallizing

solutions of Ag2 O in aqueous HF. The other well-known halides are precipitated by

the addition of X to Ag^ solutions; the color and insolubility in water increase

C l < B r < l .

Many complexes of Ag with nitrogen ligands form readily in aqueous

solution, NH2 being the most important of the nitrogen ligands. Aqueous pyridine

and substituted pyridines form Ag (py) * and Ag (py) 2 ions (15) but when non­

aqueous conditions are employed (e.g. CHCI3/ py mixtures) solids containing other

complexes can be obtained. For example, [Ag (py) 4] C104 contains essentially

tetrahedral cations (16). Cationic species such as Ag2X and AgsX are formed when

the silver halides are dissolved in aqueous solution of AgNOs or AgC104

Silver (I) has a relatively low affinity for oxygen donors, although compounds

and complexes containing carboxylate ions, DMSO, DMF, and crown ethers (17) are

known. However, it forms numerous complexes with the donor atoms S, Se, P, and

As. With sulfur the thiosulfate complexes, [Ag(S203)]' and [Ag(S203)2]^' are quite

stable and silver chloride and bromide will dissolve in aqueous thiosulfate, thus

providing a means of "fixing" photographic images. Another important sulfur ligands

for Ag (I) are thiolate anions (18), which give oligomers, (AgSR) „, dithiocarbamate

ions, SCN', thioureas, and thioethers. Silver (I) binds to peptides and proteins with a

preference for the thioether sulfiir atoms and imidazole nitrogen atoms (19).

26

1.20 COMPLEXES OF GOLD (III)

Au (III) like Pt (II) displays predominantly square coordination. Although

there is little evidence for any persisting five- coordinate complexes, there is no

doubt that, as with Pt (II) species, substitution reactions precede via five - coordinate

intermediates. There is no evidence for a simple aqua ion, [Au (H2O) 4] '^ but mixed

chloro - aqua and chloro - hydroxo complexes are formed by hydrolysis of [AUCI4]'.

Complexes of the type [AUX4] ' where X is a halogen or pseudo halogen are well

characterized. By action of BrFs on a mixture of gold and an alkali metal chloride the

M'AUF4 compounds may be obtained. Gold is readily recovered from such solutions

(e.g. by precipitation with SO2). The [AuBr4]" ion is similar to [AuCU]" but [AUI4] ",

obtainable in solid compounds, is subject to the decomposition in solution:

[AUI4]" = [AUI2]" + I2, K = 0.005

Other [AUX4] • complexes are those with X' = SCN (S- bonded), N3' (20), and CN".

The reaction of AUCI3 with ECI4 (E = S, Se and Te) gives compounds that

contain distorted [AuCU] " ions with one CI bridging to the ECls" ion (21). The

cationic complex [Au (NH3) 4] "* is well characterized and numerous others exist, e.g.

[Au py2 CI2] ^ and [Au dien CI] *. Interaction of triphenylposphine complexes of

gold (III) and McsAuPPhs with methyl lithium in ether gives [AuMe4]', which can be

isolated as lithium amine salts and the most stable organogold compounds.

27

Table 1.5 Thin- Layer Chromatographic Studies Carried Out During Last

Twenty Years on Inorganics Including Copper, Silver and Gold:

ION/METAL S.P M.P COMMENT REF.

Forty- nine ions S\ Mi

including W, Ha,

TI, Mo, Re, Sn, Pb,

Cr, Ni, Cd, Zr, Co,

Te, Sc, Sb, Cu, Ag,

Be, Fe, Mg, Th, Y,

and Ge

Forty- eight metals S2 M2

ions

Au, Ir, Ft, Pd, Ir, S3 M3

Ru, and Rh

Development time 35-45 min; layer thickness

0.25 mm; run 17cm; qualitative separation.

Qualitative Separations

Qualitative Separations

22

24

Forty- metals ion S4

Hg, Cu, Cd, and Ag S,

Cu, Hg, Zr, Zn, Cr, S5

Y, Gd, Ho, Nd, La,

Pr, Ce, Pd, Au, Pt,

Co, Ni, Fe, and Bi

Twenty-five Se

Cations

Forty- eight metal S2

ions

Cu, Cd, Zn, Ni, Co, Sy Fe, Pb, Cr, Al, Zr, V, Th, U 0 2 ^ Ag, Se, and VO^

M4

Ms

M.

My

M«

Mg

Qualitative Separations 25

Concentration, separation and detection of 26

microgram amounts of Cu, Ag, Hg, and Cd 26"

salts in fresh water.

Spectrophotometric determination of Pd^ ' after 27

elution from TLC plate, using p-

nitrosodimethylaniline as chromogenic reagent.

Development time 15-25 min, TLC on 28

microscope slides.

Qualitative sepeirations 29

Ascending technique; run 10cm; development time 12-15 mins; 0.25 mm layer, semiquantitive determination of nine - cations on silica gel impregnated with sodium molybdate.

30

28

Pb, Cd, Hg, Te, W,

Mn, Ag, Fe, Be, Ni,

Mg, Pt, Ga, Cu, Ti,

Se, Co, and As

Se, Te, and Au

Bi, Au, Mn, Co,

Zn, Ti, Cu, Ag, Ni,

Pt, Cd, Pb, Hg, and

TI

Noble Metals

Ag, Al, As, Au, Ba,

Be, Bi. Ca, Ce,

Ss

Sg

Sio

Sn

Sl2

Mio

M„

Mi2

-

M B

Mio Ascending technique; run 17 cm; development 31

time 60 - 80 min; 0.25 mm layer; qualitative

separations.

Plates developed in Stahl chamber 32

Ascending technique; run 11 cm; development 33

time 2 hours; quantitative separation of Bi ^

from some ternary and quaternary mixtures of

metals.

Separation of noble metals from commonly 34

present metals.

Qualitative separations on treatment with PEI - 35

cellulose.

Ce ^, Co, Cr, Cu,

Dy, Fe, Ga, Ge,

Hg"", Hg, Ln, La,

Mg, Mn, and Mo^^

Ag, Al, Cu, Th, Si3 Mi4 Semiquantitative determination of Pb, Ag and 36

Mo, Zn, Cd, Pb, Se, Th by spot-area measurement method and

Zr, Ni, W, U02^^, spectrophotometer determination of UO2 ^ after

VO " , Fe, TI, and separation from other metal ions.

Co

Ru, Au, Pd, Pt, Rh, Si4 M15 Qualitative analysis 37

Os, Ir, and Ag no

Fe, Cd, Zn, Cu, Bi, S2 M16 Ascending technique; run 10 cm; development ^°

T F , Ag, Hg, Pb, Se, time 1 5 - 2 0 mins; 0.25 layers; examination of

Mn, U02^*, Ni, Al, effect of sample concentration, element

Zr, Ti, and Th concentration, pH of salt solutions, pH of

mobile phase and presence of anions in the

sample solution on the separation of Cd^* from

Zn^^ and of Cu "' from Ni^^, Co^^ and Cd^^ with

1.0 M HCOONa - 1.0 M KI (9:1).

29

Al, Fe *, Fe "*", Co, S15

Ni, Cu, Zn, Ag, Cd,

Hg, Tl, Pb, and Bi

Ni, Co, Zn, Cd, Ni, S2

Cu, UO2, V "", Fe^^

Fe^^ Ai, Th, Ti,

Mo'", Sc, W, Hg,

V^^ Tl^ Pb, Bi,

and Ag

UO2, Ti, Al, W, Ni, S,6

Fe, Co, Mn, Cu, Bi,

Zn, Cd, Ta, Se, Pb,

Ce, Ag, Th, Zr, Hg,

andTl

Twenty-eight metal Sn

ions including Ru,

Pd, W, Pt, Au, Mo

and UO2

Noble metals (Ru, Sis

Rh, Pd, Pt, Ir, and

Au)

Ag, Cd, Tl, Pb, Bi, S,9

Mg, Hg, Fe, Al, Co, S2

Ni, Zn, and Cu

Noble Metals (Au, Sig

Pd, Pt, Ru Ir, and

Rh)

Au, Se, and Tl S9

Ml7 Ascending technique; run 10 cm; quantitative 39

separation of Ni and Fe, Zn, Cd and Pb.

M, Ascending technique; run 10 cm; 0.25 layer;

loading volume 5 \i\; examination of the effect

of sample pH and the presence of alumina in

the stationary phase on separation of V '*' from 76+ 6+

M21 All noble metals were completely separated.

M22 Much shorter development time for NH4CI-

impregnated silica gel layers; clearer detection

and more compact spot formation on KI-

impregnated silica gel layers; better results

when KBr is used as eluent rather than as an

impregnant, but reverse is true for KI.

M23 Qualitative separations

40

W°" and Mo

Mi9 Ascending technique; run 10-12 cm, qualitative 41

separations.

M20 Quantitative separation of Ru " from other 42

metal ions.

44

45

M24 Simple and rapid separation of Au' ' , Se * and ^°

Tl 4+

30

Au, Pt, Pd, Cr, Mn, S3

Fe, Co, Ni, Cu, Ba,

Al, Bi, Pb, Zn, and

Ag

Cu, Co, Cd, Hg Ni, S20

and Ag

Ni, Cu, Zn, Pd, Cd, S2

Cr, Fe, Ru, Rh, La,

Au, Tl, Zr, Pt, Nb,

Ta, Mn, Ag, Hg,

Co, Mo and W

Cu. Co, Cd, Ni, Ag, S21

and Hg S20

Twenty-one cations Se

Ag, Ni, Cu, Co, Cd, S22

and Hg

M25 Application of method for the analysis of 47

platinum powder and two kinds of Au alloy.

Cu, Mg, Al, Ca, V,

Zn, Ge, Y, Zr, Mo,

Ag, Cd, IN, La, Ce,

Eu, Tb, Tl, Pb, and

Bi

3d metal ions

Twenty-six cations

Au, Ru, Rh, Pd,

Os, andPt

S23

S24

S25

S7

M31

M32

M33

M34

M26 Qualitative separations and determination of

chromatographic parameters as a function of

the concentration of MeOH, NH3, AcOH and

inorganic salts in the mobile phase.

M27 Run 11cm; correlation between Rp values on

impregnated layers developed with DMSO -

THF (1+10) and the atomic numbers of the

metal ions.

M28 Qualitative TLC separation of metal cations.

M29 Separation and identification of 21 cations on

cellulose layer.

M30 Separation of mixtures of inorganic ions on

chitin layers; possible use of chitin and chitosan

layers in wastewater and seawater purifying

systems.

Detection limits and Rp values of fluorescent

cations separated on porous glass sheet were

reported.

Reversed-phase TLC of 3d metal ions.

Qualitative analysis of twenty-six cations on

cellulose layer using six detection reagents.

Rs value of each pair of ions is 1.0 except that

ofRu (III) and Pd (II), limit of de tec t ion is

4.0 [ig.

48

49

50

51

52

53

54

55

56

31

Forty-nine

inorganic ions

526

Til, Fe, Al, Bi, S27

UO2, Cd, Hg, Ni,

Co, Cu, Pb, Ag, and

Ti

Fe, Zn, Cu, Ca, Co, S28

Hg, UO2, VO, Ag,

Mn, and Bi

Thirty-three metal S7

ions

Heavy metal S29

cations

Cu. Pb, Hg. Ag. Tl, S30

Cd, Fe. Bi. Ni. Co

and Zn

M35 Reversed-phase TLC of 49 inorganic ions and 57

their separation from multi-component

mixtures.

M36 Ascending technique; run 10 cm; layer 58

thickness 0.25 mm, limits of detection falls in

the range of 0.22-3.4 jig.

M37 TLC separation and colorimetric determination 59

of SCN" as applied to water and wastewater.

M38 TLC behavior of 33 metal ions was examined 60

and a theoretical model was proposed.

M39 Detection, identification and separation of 61

heavy metals.

M40 TLC separation of Ag^ from other metal cations 62

and colorimetric determination of Ag^ in

synthetically prepared ores.

The metal ions were in their usual valency states.

S.P.: Stationar>' Phase

M.P. : Mobile Phase

REF. : Reference

32

Tablel.6 LIST OF STATIONARY PHASES USED:

Si Cellulose phosphate (Whahnan P41, U.K) and cellulose phosphate +

microcrystalline cellulose (3:1)

52 Silica gel G

53 Ecteola - cellulose

54 Dowex 1 or 50 mixed with cellulose (Avicle SF)

85 Ti (IV) antimonate (H^ form)

Se Cellulose

S7 Silica gel

Sg Cellulose phosphate in H^ form (Whatman P41, U.K)

S9 Alumina

Sio Hydrous Zirconium oxide

Si 1 Semi cr}'stalline Sn phosphate ion-exchanger + Silica gel G

S12 PEI cellulose

Si3 Silica gel impregnated with 0.5M NH4CI and a saturated aqueous solution of

barium nitrate

Si4 Tin pyrophosphate and silica gel containing sodium CM cellulose as binder

Si5 Silica gel impregnated with O.IM aqueous sodium nitrite, sodium molybdate

and potassium dihydrogen ortho phosphate

516 Silica gel loaded with different concentrations of TBA

517 Binder free Zr (IV) antimonate in the H" form, Silica gel G and mixture of

Zr (IV) antimonate and silica gel G (1:1)

33

Si8 Microcrystalline cellulose modijfied with silica gel G

519 Silica gel impregnated with 0.1 - l.OM aqueous solutions of NaCl, NH4CI,

KBr, or KI

520 Chitosan

821 Chitin, chitosan and their derivatives

522 Chitin

523 Porous glass sheets

524 Silica gel coated with high molecular weight amines (Primine JM-T,

Amberlite LA-1, Alumina 36, Aliquat 336)

525 Microcrystalline cellulose

526 Silica gel impregnated with amberlite LA-2

527 Silica gel and alumina impregnated with 0.1-1.0% LiCl

528 Cellulose microcrystalline/cellulose + kieselguhr (4:1, 3: 2, 1:1); Kieselguhr

529 Silica gel, silica gel mixed with Sn'*" arsenosilicate and impregnated with

tributyl amine

530 Microcrystalline cellulose, alumina and their mixtures

34

Table 1.7 LIST OF MOBILE PHASE USED:

Ml Aqueous acetic acid (0.1 - 3.0 M) and mixtures of acetic acid and ammonium

acetate solutions

M2 Dimethyl sulphoxide- HCl (1 - 6 M) (1+9, 3+7, 5+5, 7+3 and 9+1, v/v)

M3 Different concentrations of HCl and aqueous chloride solutions of Li, Mg, Na,

Ca, and Sr

M4 HO Ac - HCl

M5 Distilled water, aq. sodium chloride (0.1 - 1%)

Mg HNO3 (0.001-1.0 M),DMF-0.1M HNO3 (1+0, 4+1, 1+4, 4+6, 6+4)

M7 Binary solvent systems

Mg Oxalic acid - oxalate systems

M9 Demineralized water, mixtures of sodium formate and formic acid solutions in

different ratios

M,o 0.5 M H2S04 - acetone or methanol (1+0, 4+1, 3+2, and 2+3)

M]i Aqueous solutions of inorganic acids (HCIO4, HCl, HBr, HNOs H2SO4 and

H3PO4), their mixtures and some of their salts

M12 Aqueous HCl and HNO3 at different pH levels and their mixtures with organic

solvents in different ratios

M,3 0.01 - l . O M HCl

Mi4 Aqueous solutions of HCOOH, HCOONa, CH3COOH, CH3C00Na,

CH3COOH - CHsCOONa (1+1) and HCOOH - HCOONa (1+1)

Ml5 Acids (nitric, tartaric, citric, perchloric, formic), bases [NH4OH, (CH3)3N],

neutral compounds (NH4CI, NH4NO3, ACONH4) or mixture of these with

organic solvents (EtOH, MeOH, n - PrOH, Me2C0)

35

M,6 Aqueous HCOONa (10"\ 10'. 0.1, and 5.0M), 0.1 M HCOONa - O.IM

HCOOH (1+9, 3+7, 1 + 1, 7+3 and 9+1), 1.0 M HCOONa-1.0 MNaCl (1 + 1,

1+2, 2+1, 1+9 and 9+1) and mixtures of 0.1 or 1.0 M HCOONa and 0.1 or

1.0 M NaCl, KBr, KI or KBrOs in different ratios

Mi7 Mixtures of aqueous 1.0 M formic acid and alkaline salt solutions

Mi8 Ternary mixtures of TBA, fonnic acid and ammonium acetate in different

ratios

Mi9 Aqueous formic acid (10""' - 2.0 M), aqueous sodium formate (10' - 5 M) and

mixtures of 1.0 M formic acid and 1.0 M sodium formate (1 + 1, 4+6, 6+4, 2+8,

and 8+2)

M20 HNO3 (10"-- 1.0 M), DMSO - 0.1 M HNO3 (1+0, 4+1, 3+2, 1+4), dioxane-

O.IM HNO3 (1+0, 4+1, 3+2. 2+3. 1+4)

M21 n - Butanol - HCl - acetone (100-1 + 100)

M22 Demineralized water, O.IM formic acid; l.OM KI, KBr or NaCl; O.IM formic

acid-0.1 MKIorKBr(l+9), 1.0 M formic acid - l.OM HBr (1+9 and 9+1),

l.OM fon-nic and -1.0 M NaCl, NH4CI, KBr or KI (1+9, 3+7, 1+1, 7+3 and

9+1)

M23 n - BuOH - IBMK - P^" - HCl (140 + 100 + 15 +18) and (140 +100+15 + 1)

M24 Aqueous solutions of HCl, HBr, H2SO4, HNO3, H3PO4 and various organic

acids

36

M25 Mixtures of 2.5 M HCI, 2.5 M NaCl and 0.6% hydrogen peroxide in different

ratios

M26 l.OM inorganic salt solutions in aqueous methanol

M27 DMSO - l.OM HNO3 (1+1); DMSO - THF (1+10); n - butanol - acetone -

HNO3 (6+6+1); diisopropyl ether systems

M28 Several aqueous mobile phases

M29 Acetylacetone - acetone - cone. HCI (5:5:1)

M30 Aqueous methanolic solutions (1+1, 3+1, v/v) of ammonia or AcOH with

ammonium nitrate, or sometimes ammonium acetate

M31 n - Butanol - benzene - IM PfN03 - IM HCI (75+69+4+2, v/v) or acetone -

3M HCI (99+1, v/v)

M32 Aqueous solutions of succinic acid

M33 Numerous solvent systems

M34 Sulfuric acid and sulfuric acid - ammonium sulfate media

M35 HCOOH (1.0 M); HCOONa (l.OM) and their mixture

M36 NH4OH (l.OM)-acetone (1:9, 3:7, 1:1,7:3,9:1)

M37 Methylisobutylketone - formic acid mixtures

Msg MeOH containing aqueous mobile phases

M39 Aqueous solutions of ammonia, acetic acid, sodium acetate and sodium nitrate

37

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39

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40

t * J haBt^P

2.1 INTRODUCTION

Mutual Separation of copper (29CU), silver (47Ag) and gold (79AU), the

elements of IB group of the periodic table is analytically important because of their

similarities in chemical properties. These metals with general configuration (n-1) d'°

ns have a tendency to form complex salts in which the metal may be complex cation

or complex anion. Copper is associated with silver in copper glance (Cu, Ag) 2 S ore

and hence its separation is needed to get pure silver from the ore. On the other hand,

the presence of silver ions has been found to decrease the rate of adsorption of Au" ^

on activated carbon from thiourea solution (I).

Thus, the presence of one metal in small quantities offers deleterious effects

on performance of other metals. Because of industrial, commercial and medicinal

importance of these metals, several analytical techniques such as ion-exchange

chromatography (2,3,4), potentiometry (5,6), capillary zone electrophoresis (7),

solvent extraction (8,9), single sweep oscillopolarography (10), ion pair - reversed-

phase HPLC (II), size - exclusion chromatography (12), titrimetry (13,14), reversed-

phase column chromatography (15,16), reflectance spectroscopy (17), flame or

graphite furnace atomic absorption spectrometry (18,19), laser excited atomic

fluorescence spectroscopy (20), neutron activation analysis, ion paired (21, 22) and

foam plastic (23) chromatography etc. have been used in addition to hyphenated -

techniques e.g. TLC- spectrophotometry (24), solvent extraction - AAS/FAAS (25,

26), ICP-AES and ICP - MS (27,28) and ion-exchange chromatography-photometry

(29), for the separation and determination of Au''' ,Cu " ,and Ag" from a variety of

matrices. Ion chromatography of Au-cyanide complexes and problems arised during

analysis of gold by titrimetric and spectrophotometric methods has been reviewed

recently (30, 31). ;

Recently, new materials including chelate forming plastics containing amino-

thiourea (32), VS-II anion exchange fibre (33), ion-exchange resin loaded with

41

bismuthiol (34), silica gel bound thia-crowns (35), chemically modified silica gel

with p-dimethylaminobenzylidenerhodanine (36), chitosan treated with

dithiocarbamates (37) and nanofilter membrane (38) have been developed and

utilized by several workers for preconcentration and separation of gold, silver, and

copper.

Amongst analytical techniques used, thin layer chromatography (TLC) being

inexpensive and versatile is still enjoying popularity among analytical scientists

specially working in India, China, Japan and European Countries. As a result, TLC

systems comprising of ECTEOLA- cellulose and HCl + NaCl +H2O for separation of

gold, platinum and palladium from associated base metals (39); chitin and aqueous

buffer solutions for separation of Cu~ and Ag^ (40), alumina and aqueous solutions

of both organic and inorganic acids as well as some sodium salts for rapid separation

of Au'" trom Te''^ and Se ^ (41,42) and silica gel containing sodium or ammonium

acetate and toluene for separation of Cu"" complex from transition metal complexes

(43) have been reported.

Analytical techniques listed above have been successfully applied to separate

Au"" either from Cu or Ag but the work on mutual separation of these metal ions

from their three-component mixtures is lacking. As far as, we are aware, not a single

reference is available on the separation of co-existing Au " , Cu " and Ag^ by TLC

using surfactant mediated mobile phase (or micellar system). During our previous

study (44) on micellar thin layer chromatography of heavy metal cations, we realized

that micellar mobile phases have unique separation capabilities to provide unusual

selectivity, enhanced detection sensitivity and faster analysis. Efficiency of micellar

systems in the separation of cations (45, 46,) and anions (47, 48) has been reported

and reviewed by Okada (49). Surprisingly, very little work appears to have been done

on the use of micellar mobile phases in TLC of inorganics (50, 51), none of these

studies examined the separation of metal cations. It was therefore decided to identify

novel micellar mobile phase systems to achieve highly selective separations of metal

42

cations. As a result, simultaneous separation of Au , Cu , and Ag^ from their

mixtures has been achieved on silica layer using buffered anionic micellar eluent

with added amino acids. The proposed TLC method is selective and rapid with

development time averaging 5 min.

2.2 EXPERIMENTAL

Apparatus

A TLC applicator (Toshniwal, India) was used for preparation of 20 x 3.5 cm

glass plates. The chromatography was performed in 24 x 6 cm glass jars. A glass

sprayer was used to spray reagents on the plate to detect the spot.

Chemical and Reagents

Silica gel 'G' (Merck, India); Sodium dodecyl sulphate (BDH, India);

L - arginine, L-histidine and DL- phenylalanine, phenols, amines and anions (CDH,

India); and L-tryptophan (Loba - chemie, India), were used. All reagents were of

Analar Reagent grade.

Metal - Cations Studied

Ve\ Cu'\ Ni'\ Co^^ UO,'^ VO^^ Cd^^ Zn2\ Ag^ P b ^ Tf, Bi^^ Hg^^

A P , T i ^ a n d A u ' ^

Materials Used in Separation Tests

1. Gold-plated printed-circuit board (GPCB) (Au, Ni, Cu) from Toyama

Electric, Bangalore.

2. Silver mirror scrap (SMS) (Ag, Cu) and Silver mirror spent solution

(SMSS) (Ag, Cu), both from Ship Mirror Industries, Bangalore.

3. High-copper dental amalgam (HCDA) (Ag, Hg, Cu, Zn, Sn) from the

Dental College, A.M.U., Aligarh.

4. Sterling silver scrap (SS) (Ag, Cu), (synthetic).

43

Test Solutions

1.0% aqueous solutions of following salts were used as test solution:

a. Nitrates of Cd^^ Zn^^ Pb^^ Tf, Bi^^ Al^^ and Ag^

b. Chlorides of Ni^^ Co^^ Fe^^ Hg^^ Ti^^ and Au^^

c. Sulfates ofCu^^VO^^ and UOz "".

All the solutions were prepared in demineralized water with a specific

conductivity (K = 2xl0'^ohm"* at 25°C). The solutions of nitrates of lead, silver and

bismuth, and the chloride of mercury also contained small quantities of

corresponding acid to limit the extent of hydrolysis.

The solutions (1%) of various anions were prepared in demineralized water.

The solutions (1%) of various amines and phenols were prepared in methanol.

Buffer Solutions

S. No.

1

2

4

5

Composition

0.04 M Boric acid-0.04 M phosphoric

acid

0.02 M Boric acid-0.04 M phosphoric

acid-0.24 M NaOH

0.04 M Boric acid-0.04 M phosphoric

acid- 0.24 M NaOH

0.04 M Boric acid-0.04 M phosphoric

acid- 0.24 M NaOH

0.04 M Boric acid-0.04 M phosphoric

acid -0.24 M NaOH

Volume Ratio

50:50

50:50:8

50:50:10

50:50:14

50:50:60

pH

2.3

3.4

5.7

7.0

11.9

Detection Reagents: For the detection of various cations, the following reagents were

used:

a. 8x10'^% (w/v) Dithizone in carbon tetrachloride for Cd , Zn , Ag , 2+ T.,+ r»:3+ 2+ ?h'\ Tf, Br, and Hg"".

44

x2+ b. Aqueous Potassium Ferrocyanide for Fe^ , Cu " , U02^^, VO'^, and

4+ Ti

c. Dimethylglyoxime for Ni^ , and Co^ . i 3 +

3+

d. Aluminon for Al , and

e. Yellow ammonium sulphide for Au "".

Stationary Phase: Silica gel 'G'

Mobile Phases: The following Solvent systems were used as mobile phases

Symbol

M,

M2

M3

M4

M5

Me

M7

Ms

^49

Composition

0.001, 0.005, 0.01 and 0.05M aqueous SDS

0.001, 0.005, 0.01 and 0.05M-buffered SDS (pH 2.3)

0.001, 0.005, 0.01 and 0.05M-buffered SDS (pH 3.4, 5.7, 7.0 and

11.9).

O.OIM SDS (pH 2.3) + O.OIM L-arginine (1:9, 3:7, 5:5, 7:3 and 9:1)

O.OIM SDS (pH 2.3) + O.OIM DL phenylalanine (1:9, 3:7, 5:5, 7:3

and 9:1)

O.OIM SDS (pH 2.3) + O.OIM L-tryptophan (1:9, 3:7, 5:5, 7:3 and

9:1)

O.OIM SDS (pH 2.3) + O.OIM L-histidine (1:9, 3:7, 5:5, 7:3 and 9:1).

0.001, 0.005, or 0.05M SDS (pH 2.3) + O.OIM L-arginine (9: 1).

O.OIM SDS (pH 2.3) + 0.00IM L-arginine, DL-phenylalanine,

L-tryptophan or L-histidine (1:9, 9:1)

Preparation of TLC Plates

Silica gel plates were prepared by mixing the adsorbent with double distilled

water in 1:3 ratios by weight. The resultant slurry was mechanically shaken for

10 min, after which it was applied to well-cleaned glass plates with the help of TLC

applicator to give a layer of approximately 0.25 mm thickness. The plates were air

45

dried at room temperature and then heated at 100 + 5 C for 1 h to activate them.

After activation, the plates were stored in a dessicator.

Preparation of Test Material

Peeling'. Peeling of silver mirror scrap (specimen surface area 19 cm ) was

performed with concentrated formic acid (90%, w/w) in a glass beaker. The acid was

heated at 110°C and the material was added into it. On completion of peeling (within

1 min.), the solution was separated and the peeled material was used as the 'source

material' for silver.

Leaching: Leaching of silver mirror scrap (specimen surface area 19 cm ) was

performed with 50% nitric acid in a glass beaker. On completion of leaching (within

1 min), the solution was separated from the leached residue and used for silver

separation. Similar leaching procedure was applied on other silver-containing

material (0.153 g sterling silver scrap and 0.25g high-copper dental amalgam.

Leaching of gold - plated printed- circuit board (specimen surface area 72.42

cm^ containing 16 large pins and 14 small pins) was performed with aqua -regia in a

glass beaker. On completion of leaching, the solution was separated from the residue

and used for gold separation.

Chromatography

About 10 JJ.L of test solution was applied using a micropipette about 2.0 cm

above the lower edge of the chromatoplates. The spots were dried, and the plates

were developed in glass jars containing the mobile phase using a one-dimensional

ascending technique. Before developing the plates, the glass jars that contained the

mobile phase were covered with a lid for about 20 min so that the glass jars would

get pre-saturated with the mobile phase vapors. The mobile phase (solvent) was

allowed to migrate up to 10 cm from the starting line in all cases. After development,

the plates were dried again and the cations were visualized as colored spots by

spraying with appropriate detection reagents. The cations were identified on the basis

46

of their Rp values, calculated from RL (Rp of leading front) and RT (RF of trailing

front) for each spot.

Separation

The test solution (0.0 ImL) of copper, silver and gold mixture was spotted on

TLC plate coated with silica gel 'G' and the chromatography was performed using

O.OIM SDS(pH2.3) + 0.01M L-tryptophan (1:9) or O.OIM SDS (pH 2.3) + O.OIM

L-histidine (1:9) as mobile phase. The resolved spots for these metal cations were

observed on TLC plates after spraying chromogenic reagents. The Rp values of Au^ ,

Cu^^ and Ag" in their mixture were found to vary marginally from their individual Rp

values.

Interference

For investigating the interference of inorganic anions, amines, and phenols on

the Rp values of Au^ , Cu " , and Ag" , an aliquot (O.OlmL) of impurity solution was

spotted along with the mixture (O.OlmL) of Au"" , Cu^ , and Ag" and chromatography

was performed as described above. The spots were detected and the Rp values of

separated metal ions were determined.

Limit of Detection

The identification limits of various cations were determined by spotting

different amounts of cationic solutions on the chromatoplates. The plates were

developed and the spots were detected as described above. The method was repeated

with successive lowering of the amount of cation until spots could no longer be

detected. The minimum amount of cation that could be detected was taken as the

limit of detection.

47

Applications

(i) Chromatography of Unspiked Materials

Chromatography of unspiked dental amalgam and printed circuit board was

done using O.OIM SDS (pH 2.3) + O.OIM L-histidine (1:9) and the spots of Ag^,

Hg^ , Zn " , Cu '* and Au' ' were detected and the Rp value of each cation was

determined.

(ii) Chromatography of Spiked Materials

The spiked samples of PCB, dental amalgam, silver mirror scrap, silver mirror

spent solutions, and sterling silver was prepared as follows:

(a) One ml of PCB/ dental amalgam solution was mixed with ImL of silver

test solution (1%). The chromatography was done using 0.0ImL solution of this

mixture for spotting.

(b) One ml of gold solution was added to 1.0 mL of each SMC, SMSS or SS

solution and 0.0ImL of the mixture was used for chromatographic separation of

Au^^- Cu" - Ag^. The spots were identified by their respective Rp values.

2.3 RESULTS AND DISCUSSION

The results of present study have been summarized in Figures 2.1- 2.3 and

Tables 2.1- 2.6. The unique features of this study are:

(1) Selection of micellar systems composed of anionic surfactant, sodium dodecyl

sulphate (SDS) as mobile phase, which have a fraction of negative charge and

tend to attract positively charged species including metal cations.

(2) Utilization of polar (arginine and histidine) and non-polar (phenylalanine and

tryptophan) amino acids as additives.

(3) Realization of mutual separation of Au^ , Cu^^ and Ag^, from their mixtures

and the investigation of effect of phenols, cations and anions on the separation

of coexisting Au "* , Cu " , and Ag^ ions.

48

(4) Application of proposed method to the analysis of several real and synthetic

samples for the presence of gold, silver and/ or copper.

Effect of Concentration andpH of SDS Solution

Results obtained with different concentrations of unbuffered aqueous SDS

(Ml) and buffered SDS (M2 and M3) solutions reveal the following trends.

(i) Metal ions such as K\^\ Ti^^ VO^^ Fe^^ Cu^^ Zn^^ Pb^^ Bi ^ and UOz ^

show either no mobility (Rp = 0.0) or very little mobility (Rp » 0.05) at all

concentration levels as well as over entire pH range of SDS solutions. A

slightly higher mobility (Rp = 0.3) in the case of Zn^^ was observed when

0.05M SDS solution of pH 2.3 was used as mobile phase.

(ii) Ni""", Co "" and T\* produce badly tailed spots (RL-RT > 0.3) almost with all

the mobile phases used with the exception of O.OIM SDS (pH 2.3).

Compared to buffered SDS solutions (pH 3.4, 5.7, 7.0 and 11.9), higher

mobility for these cations was realized with SDS solution (pH 2.3).

(iii) Au^^ as well-formed spot, always migrates with solvent front (Rp > 0.9)

regardless the concentration or pH of SDS solution and hence it can be

selectively separated from binary mixtures of other metal ions.

(iv) The mobility of certain metals (Cu^*, Zn " , Ag" , Bi' " and Al ^) was increased

marginally on substitution of 0.001 or O.OOSM-buffered SDS (pH 2.3) with

0.01 or0.05MSDS(pH2.3).

(v) The development time for 10 cm ascent was typically short i.e. 10-12 min for

all mobile phase systems used.

The Rp data of metal ions obtained with buffered SDS (pH 2.3) at different

concentrations (M2) levels are compared and presented in Figure 2.1. It is clear from

this figure that the mobility of metal ions is slightly influenced by the concentration

of SDS in the mobile phase.

49

Effect of Added Amino Acids

As micellar SDS solutions (buffered as well as non-buffered) were found to

resolve only limited number of two component mixtures of metal cations, it was

decided to improve the separation efficiency of SDS mobile phase system. Several

workers have reported (52-56) the improvement in chromatographic efficiency of

micellar systems as a result of alteration in micellar properties in the presence of

organic and inorganic additives. The additives tested so for include alcohols, diols,

dipolar aprotic solvents (DMSO, dioxane), alkylnitriles, alkanes, urea, NaCl, and

acetone etc. It is surprising that not a single reference is available on the use of

amino acids as additives in micellar TLC of inorganics. Amino acids being

amphiphilic substances are supposed to provide unique selectivities in the separation

of metal cations. Amino acids were chosen as additives for the following reasons.

(i) Amino acids contain both amino (-NH2) and carboxylic acid (R-COOH)

functional groups providing nitrogen and oxygen as electron donor centers to

coordinate with metals (57).

(ii) Amino acids form complexes with Cu ^ (58) and the presence of ionic

surfactant in the medium influences the binding of Cu^^ with certain amino

acids (59).

Cu^^ and Ag" have greater tendency to form coordination compounds with

nitrogen than that with oxygen (60).

(iii) In solution, amino acids exist in the following protonic equilibrium:

R-COOH <^R-COO-+H^

R-N^H3C:>R-NH2 + H^

Thus, the pH depended protonated (R-COOH and R-N^Hs) and proton

acceptor (R-COO^ and R-NH2) groups may influence the migration trend by

modifying the properties of micellar mobile phase.

50

(iv) The aromatic R-groups of phenylalanine and tryptophan are hydrophobic, a

property that has important consequences for the ordering of water molecules

in the mobile phase.

In the present study, the mobile phase systems consisting of variable

concentrations of amino acids (L-arginine, DL-phenylalanine, L-histidine or L-

tryptophan) and buffered SDS (pH 2.3) were formulated by adding required volumes

of O.OIM amino acids into SDS solution (0.001-0.05M) in volume ratio of 1:1, 3:7,

7:3, 9:1 and 1:9 keeping total volume constant in each case. The chromatography

performed with these mobile phase systems (M4 - M9) reveals the following trends:

(a) The solvent systems [M4-M7 (3:7, 5:5, 7:3)] containing amino acids (arginine,

phenylalanine, tryptophan or histidine) at concentration levels of 30 - 70%,

produce tailed spots (RL-RT> 0.3) for all the metal ions except Al" , Pb" and

Fe"' which stayed at the point of application and Au^^ that moved with the

solvent front. Thus, the presence of amino acids in SDS containing mobile

phases causes tailing in the spots of metal cations probably due to competitive

interactions among cations, charged amino acid and anionic SDS. Since the

experiments were performed at pH 2.3, the amino acids in this study are

supposed to bear a net partial positive charge.

(b) In case of mobile phases composed of either 10% SDS and 90%) amino acids

[M4-M7, (1:9)] or 10% amino acids and 90% SDS [M4-M7, (1:9)], number of

cations producing tailed spots was decreased as compared to mobile phases

discussed above in (a). The resuhs are compared in Table 2.1. Compared to

mobile phases containing 90%) SDS (O.OIM, pH 2.3) plus 10%) amino acids

(O.OIM), better separation possibilities were with those containing 10% SDS

(2.3 pH) plus 90%o amino acids.

(c) Against our hope, arginine (aliphatic amino acid with side chain containing

basic group) which has been utilized satisfactorily for resolution of amino acid

enantiomers via ligand exchange TLC (61) was found unsuitable for

separating metal cations. Similarly, phenylalanine (a non-cyclic aromatic

51

amino acid) was found ineffective to produce satisfactory results. However,

heterocyclic aromatic amino acids such as tryptophan and histidine were

found capable to provide better-resolved spots of metal cations. On the basis

of results presented in Table 2.1, the chromatographic performance (or

separation efficiency) of amino acids was in the following decreasing order:

L-Histidine > L-Tryptophan > DL-Phenylalanine > L-Arginine

Thus, the micellar solvent systems containing heterocyclic amino acids

like L-tryptophan or L- histidine proved superior to solvents containing aromatic or

aliphatic amino acids like phenylalanine or arginine for separation of metal cations.

(d) The solvent system consisting of O.OlM-buffered SDS (pH 2.3) and O.OIM

histidine (M7) or tryptophan (Mg) in 1:9 ratios was found most suitable for

separating co-existing Au "*", Cu^ , and Ag" ions (Table- 2.2).

(e) When chromatography was performed using mobile phases (Mg) obtained by

mixing O.OOIM, 0.005 or 0.05M SDS (pH 2.3) with O.OIM amino acid

(L-arginine) in 9:1 ratio, less tailed spots for Ni , Co , slightly higher

mobility for CvT* and T^ and increased compactness for Zn " and Cd * was

noticed with the increase in SDS concentration of mobile phase. However, the

mobility of Fe^^ Pb^^, Bi^^ Hg^ , AV\ Ti^\ UOi^^ or VO^* (Rp = 0.0) and

Au" " (Rp^ 0.9) remained unchanged over the entire SDS concentration range.

(f) When O.OIM amino acid in the mobile phases M4-M7 was substituted with

O.OOIM, little increase in mobility of Cu^^ Fe "", Ni " , Co " , Zn^^ and Tf was

noticed with mobile phases containing L-arginine or DL-phenylalanine.

Whereas opposite trend (i.e. decrease in the mobility) for these metal ions was

realized with L -tryptophan containing mobile phase. The mobility of all other

metal ions was found to remain unchanged. In the case of L-histidine, mobility

of all metal ions remains almost the same irrespective of the concentration of

histidine (0.01 or O.OOIM).

In order to provide a clearer picture about the variation of Rp values of

the metal ions as a function of concentration of amino acids in the mobile phase, a

52

representative plot of Rp Vs volume fraction of 0.01 M L-histidine was constructed

(Figure 2.2). The curves shown in (Figure 2.2) pass through maxima and minima

exhibiting the variation of Rp values (or mobility) of metal cations without any

regular pattern. This situation is probably arised due to the formation of occasional

tailed spots in certain cases as a result of multiple interactions.

In Figure 2.3, the A Rp [Rp in Me (1:9) and Rp in M7 (1:9)] was plotted against

metal cations to show the net effect of change in microenvironment of micellar

mobile phase on the mobility of metal ions. It is clear from this figure that metal ions

either migrate faster (positive A Rp value) in tryptophan (5 or 6 membered

heterocyclic amino acid) containing micellar system or show the same mobility (A Rp

values + 0.05) compared to their mobility in histidine (5 membered heterocyclic

amino acid) containing micellar system. Thus, an enhanced mobility of cations is

possible with micellar systems containing amino acid with large non-polar side chain

(e.g. tryptophan) compared to the micellar systems having amino acid with polar side

chain (e.g. histidine).

Separations

Separation of metal ions obtained on silica layer with micellar systems in the

presence and absence of amino acids has been listed in Table- 2.2. To widen the

applicability, the separation of Au " , Cu " , and Ag" was examined in the presence of

organic (phenols, urea, thiourea) and inorganic (cationic and anionic) impurities.

The results presented in Table 2.3 and 2.4 indicate that all the impurities are

ineffective in bringing about the change in the mobility of Au " and Ag^. Conversely,

the mobility of Cu "" is influenced by the impurities. The variation in Rp value of

Cu was from 0.30 (o-nitro phenol impurity) to 0.60 (thiourea impurity). However,

the simultaneous separation of Au " , Cu "", and Ag" from their mixtures was always

possible. The poorest separation was in the presence of K2S2O8 and ammonium

oxalate because of the formation of tailed spots of Cu " .

53

From the data provided in Table 2.3, it seems that the position of substituent

groups in the benzene ring control the mobility of Cu " . For example, the order of

increase in Rp value of Cu " (given in parenthesis) in the presence of o-, m -, and p-

isomers of nitro phenols and cresols was:

o-nitro phenol (0.3) < m - nitro phenol (0.50) < p-nitro phenol (0.53) and

p-cresol (0.45) < m - cresol (0.53) < o-cresol (0.55).

This reverse order in the mobility of Cu " can be attributed to the opposite

effects of NO2 (an electron withdrawing group) and CH3 (an electron releasing

group) attached to benzene ring. The A I^values (difference in Rp values of resolved

spots from binary mixtures of metal cations) obtained for Au'^^-Ag^, Au^^-Cu^" and

Cu~^- Ag' pairs on silica gel layer were 0.95, 0.30 and 0.65 respectively with 0.01 M

SDS and 0.01 M L-tr}'ptophan (1:9) eluent and 0.95, 0.42 and 0.53 respectively with

0.0IM SDS + 0.0IM L-histidine (1:9) eluent. From these data, it can be safely

concluded that SDS-tr\'ptophan system is better for resolving Cu^*- Ag^ (A Rp =

0.65) whereas SDS-histidine system is more useful for resolving Au' - Cu^^ mixture

(A Rf = 0.42). The detection limit ()ag) given in parenthesis for Cu^^ (6.6), Ag* (1-6),

Fe^^ (0.76). Zn^^ (0.022), Cd^ (0.038), and Hg"^ (0.029) is highly sensitive to detect

heavy metals at trace levels. Zn * and Hg " down to 0.02 and 0.03 (p-g) levels

respectively can be easily detected on TLC plates.

The data presented in Table 2.5 clearly demonstrate that the individual solvent

s\'stems are not of much practical utility for separation purposes. However, buffered

SDS (pH 2.3) in combination of amino acids (histidine/tryptophan), as discussed

above has enormous analytical potentialities to facilitate analytically important

separations.

Application: The proposed method was successfully applied for identification and

separation of Au''", Cu'", Ag^, Ni "" and Hg "" in a variety of matrices. The results are

summarized in Table- 2.6.

54

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56

Table 2.3 Effect of Certain Organic and Inorganic Additives on the

Separation of Au^\ Cu^^ and Ag^ Developed with O.OIM SDS (2.3 pH)

+ O.OIM L-histidine (1: 9):

1% aqueous solution of

different impurity

Urea

Thiourea

NaNOs

NaN02

Na M0O4

NH4SCN

NaH2P04

KIO3

KIO4

KI

K4[Fe(CN)6]

K3[Fe(CN)6]

KSCN

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Ammonium Oxalate

Au^"

0.94

0.95

0.92

0.93

0.94

0.94

0.93

0.92

0.95

0.95

0.93

0.95

0.94

0.92

0.95

Separations (RF)

Cu^^

0.56

0.6

0.44

0.52

0.44

0.55

0.58

0.5

0.44

0.55

0.52

0.52

0.55

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0.52T

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0.05

0.05

0.0

0.0

0.0

0.05

0.0

0.05

0.05

0.05

0.0

0.05

0.05

0.0

0.0

T= Tailed spot (RL - RT>0.3)

57

Table 2.4 Separation of Au^\ Cu^^ and Ag^ Ions from Their Mixture in

the Presence of Phenolic Compounds on Silica Layer Developed with

O.OIM SDS (2.3 pH) + O.OIM L-histidine (1: 9):

Phenolic compounds as

impurity

Phenol

Phloroglucinol

Pyrogallol

m-Nitro phenol

o-Nitro phenol

p-Nitro phenol

Vaniline

Pyrocatechol

m-Hydroxyacetophenone

Gallic acid

Orcinol

Picric acid

Hydroquinone

Resorcinol

o-Cresol

m-Cresol

p-Cresol

Au^^

0.93

0.95

0.94

0.96

0.96

0.95

0.95

0.95

0.95

0.95

0.93

0.96

0.95

0.93

0.96

0.95

0.92

Separations (RF)

Cu^^

0.55

0.56

0.51

0.50

0.30

0.53

0.57

0.49

0.52

0.56

0.35

0.59

0.55

0.53

0.55

0.53

0.45

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0.05

0.06

0.0

0.0

0.0

0.05

0.05

0.0

0.05

0.05

0.0

0.05

0.05

0.00

0.05

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0.05

58

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63

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67