Download - SOLVENT EXTRACTION
NON-DESTRUCTI
VE TESTING
BY:- SWAPNIL NIGAM
INTRODUCTION Non-destructive Testing is an examination
or inspection performed on a component in any manner which will not impair its future use.
The terms Non-destructive Examination/Evaluation(NDE) and Non-destructive Inspection(NDI) are also used for this technology.
USES OF NDT METHODS Flaw detection and Evaluation. Leak Detection. Location Determination. Dimensional measurements. Structure and microstructure
characterization. Estimation of mech. & phy. Properties. Material sorting & chemical composition.
COMMON NDT METHODS Dye Penetrant Inspection Magnetic Particle Inspection. Ultrasonic Inspection Radiographic Inspection Eddy Current Inspection Acoustic Emission Inspection
DYE PENETRANT TEST INTRODUCTION :-
Oldest method of NDT. An extension of the visual inspection
method. Also called Liquid Penetrant test or
Penetrant Inspection. Used to detect casting, forging and
welding surface defects.
PRINCIPLE :- Based upon Capillary Action. A low surface tension fluid penetrates
into clean and drying surface breaking discontinuities.
After adequate penetration time has been allowed, excess penetrant is removed and developer is applied.
The developer A low surface tension fluid penetrates
into clean and drying surface breaking discontinuities.
After adequate penetration time has been allowed, excess penetrant is removed and developer is applied.
The law stated is thermodynamically rigorous ,i.e., it takes no account of the activities of the various species involved & thus, is expected to be applied in very dilute solutions.
Also, the law in its simple form is not applicable, when the distributing species undergo dissociation or association in either phase.
DISTRIBUTION RATIO:-In practical application, we are concerned with
the fraction of the total solute present in one or other phase, thus, its convenient to use “Distribution Ratio(D)”, i.e.,
D(CA)a
(CB)b
Where,CA denotes concentration of ‘A’ in all its form as determined analytically.
SEPARATION COEFFICIENT:-If the solution contains two solutes ‘A’ and ‘B’,
then it often happens that in conditions favouring complete extraction of ‘A’ , some ‘B’ is also extracted.
The effectiveness of separation then increases with the magnitude of the “Separation factor(or coefficient)” which is related to the individual distribution ratios as follows;
β DA
DB
β SEPARATION FACTOR
Where,
THEORY The greater the number of small extractions,
the greater the quantity of solute removed. This means,
For example, suppose we have
Amount of solute in aqueous phase(xo)=300g
Volume of aqueous phase = 100ml
Volume of organic phase to be added = 200ml
Distribution ratio of the particles = 0.5
NOW,
The total amount of solute (x n) left non-extracted in the aqueous phase (‘V’ ml) on adding the extractive organic phase (‘v’ ml) can be calculated using the formulae,
xn xoD
DV
V V
n
WHERE, xo = amount of solute present before adding extractive solvent D = Distribution ratio of the solute particles n = Number of times the solvent is added
CASE 1:
If n=1, i.e., the extractive solvent is added in one complete go, then, from the formulae,
we find that, x1 = 60g, that means 240g of the solute gets extracted.
CASE 2:
If n=2, i.e., if we add the extractive solvent in two parts each of 100 ml, then,
v=100ml, and using the formulae, we find x2 to be 33g, which implies that, 267g of the solute has been extracted.
Hence, from the above example it is clear that, it is more efficient to carry out small extraction with small equal portions of the extractive solvent, rather than using single large volume.
The inorganic solutes, with which we are concerned, tend to be more soluble in water rather than organic solvents. Also, there occurs a large loss of electrostatic solvation energy if, inorganic solutes are directly expected to be extracted by organic solvents.
Thus, for the extraction of inorganic solutes, we use appropriate reagents which can mask the water solubility of the inorganic ionic species by neutralizing their charge.
The masking of the water solubility of the inorganic ionic species present in water can be done in two ways:
By formation of a neutral metal chelate complex, or
By ion association.
Chelation complexes These complexes are often termed as
INNER COMPLEXES, when uncharged. In these complexes the central metal ion
coordinates with the poly-functional organic base to form a stable ring. For example, copper(II) acetylacetonate or iron(III) cupferrate.
CH3
CH3 C
C
O
O
CH2
CH3
CH3 C
C O
CH ½ Cu2+
CH3
CH3 C
C
O
OCH
Cu/2
CH3
CH3 C
C
O
OCH
Cu/2
Copper(II) acetylacetonate
H
OH
Factors which affect the stability of the chelate complexes:Basic strength of the chelating group
(more is the basic strength, more is the stability).
Nature of the donor atoms of chelating agent (soft-base type donor atoms form most stable complexes with the small group of class B metal ions, i.e., soft acid. Eg. Dithizone used for extraction of Pb2+.)
Size of the ring (five or six membered conjugated chelate rings are more stable, since they have minimum strain).
Resonance & steric effect (more are the resonance structures of the chelate ring, more will be its stability. Eg. Copper(II) acetylacetonate is more than copper chelate of salicylaldoxime. Also, the steric hindrance must be minimum).
CRITICAL INFLUENCE OF pH ON SOLVENT EXTRACTION OF METAL CHELATES:
The quantitative treatment of the solvent extraction of the neutral metal chelate can be done on the basis of the following assumptions;
Solvation plays no significant part in the extraction process.
The solutes are uncharged particle & their concentrations are so low that the solutions do not deviate much from ideality.
The reagent and the metal complex exist as undissociated molecules in both phases.
The formation of neutral metal(M) chelate complex, from a chelating reagent HR, takes place according to the following equation;
Also, the dissociation of the chelating reagent HR in the aqueous phase is given by the equation;
Mn+ n R- MRn
HR R-H+
Now, the above equilibria can be expressed in terms of the following thermodynamic constants;
Dissociation Constant (K), &Partition Coefficient (p).
in the following manner as described
in the equations
Dissociation constant of the
KMn+ R-
R
w w
w
n
c
complex
H+
MH n
r
reagent
Partition Coefficient of the
pMRn w
MRn oc
complex
HR
HRr
reagent
The Distribution Ratio(D) ,i.e., ratio of the amount of metal extracted into the organic phase as complex to that remaining in all forms in the aqueous phase is;
D MRn o
MRn w wMn+
The equation for Distribution Ratio can be further reduced to the form,
D oHR
wHR
n
K
Where,
K Kc
rK p
c
r*p*
( )n
And the % of solute extracted is given by;
D K*wHR
If the reagent concentration remains virtually constant, then
log( )E
log( )100-E - log( )K* npH
Thus, we observe that the distribution of the metal in the given system is a function of pH alone.
The equation of the % of solute extracted represents sigmoid curves, when E is plotted against pH. The slope of the curve depends upon ‘n’.
If pH1/2 is defined as the pH value at 50% extraction, i.e., E%=50, then,
pH1/2 n-1 log( )K*
The difference in pH1/2 values of two
metal ions in a system is a measure of the ease of separation of the two ions. If the values are far apart, excellent separation can be achieved by controlling pH.
The pH1/2 values may be altered using a competitive complexing agent.
Ion-association complex These complexes are formed when the
species to be extracted associates with oppositely charged ions to form neutral extractable species.
Such complexes can clusters with increasing concentration, particularly in an organic solvent of low dielectric constant.
Some types of ion-association complexes that have been recognized are:
Those formed from the reagents yielding large organic ions, eg. Tetraphenylarsonium[(C6H5)4As+], which tend to form large ionic clusters with oppositely charged ions, like ReO4
- . They do not have a hydration shell & thus, disrupt the water structure, due to which the tend to be pushed into the organic phase.
Those involving a cationic or anionic
chelate complex of the metal ion. Thus, the chelating reagent consists of two uncharged donor atoms. Eg. 1:10 phenanthroline forms cationic complexes.
Those in which solvent molecules are directly involved in the formation of ion- association complex. Eg. ethers, esters etc.
EXTRACTION REAGENTS NAME FORMULAE REMARKSACETYLACETO-PHENONE
CH3COCHCOCH3
DIMETHYL-GLYOXIME
CH3COCHCOCH3
Partition Chromatography II
Reverse Phase Chromatography– Nonpolar Stationary Phase– Polar Mobile Phase
Normal Phase Chromatography– Polar Stationary Phase– Nonpolar Mobile Phase
Column Selection Mobile-Phase Selection
Partition Chromatography III
Research Applications– Parathion in Insecticides: O – CH3CH2O P O NO2
CH3CH2O
– Cocaine in Fruit Flies: A Study of Neurotransmission by Prof. Jay Hirsh, UVa
Adsorption Chromatography
Classic Solvent Selection Non-polar Isomeric Mixtures Advantages/ Disadvantages Applications
What is Ion Chromatography?
Modern methods of separating and determining ions based on ion-exchange resins
Mid 1970s Anion or cation mixtures readily resolved on HPLC
column Applied to a variety of organic & biochemical systems
including drugs, their metabolites, serums, food preservatives, vitamin mixtures, sugars, pharmaceutical preparations
The Mobile Phases are...
Aqueous solutions – containing methanol, water-miscible organic solvents– also contain ionic species, in the form of a buffer – solvent strength & selectivity are determined by kind
and concentration of added ingredients– ions in this phase compete with analyte ions for the
active site in the packing
Properties of the Mobile Phase
Must– dissolve the sample– have a strong solvent strength leads to reasonable
retention times– interact with solutes in such a way as to lead to
selectivity
Ion-Exchange Packings
Types of packings– pellicular bead packing
• large (30-40 µm) nonporous, spherical, glass, polymer bead
• coated with synthetic ion-exchange resin• sample capacity of these particles is less
– coating porous microparticles of silica with a thin film of the exchanger• faster diffusion leads to enhanced efficiency
Ion-Exchange Equilibria
Exchange equilibria between ions in solution and ions on the surface of an insoluble, high molecular-weight solid
Cation exchange resins– sulfonic acid group, carboxylic acid group
Anion exchange resins– quaternary amine group, primary amine group
CM CelluloseCation Exchanger
DEAE CelluloseAnion Exchanger
Eluent Suppressor Technique
Made possible the conductometric detection of eluted ions.
Introduction of a eluent suppressor column immediately following the ion-exchange column.
Suppressor column– packed with a second ion-exchange resin
Cation analysis Anion analysis
Size Exclusion Chromatography(SEC) Gel permeation(GPC), gel filtration(GFC)
chromatography Technique applicable to separation of high-molecular
weight species Rapid determination of the molecular weight or
molecular-weight distribution of larger polymers or natural products
Solute and solvent molecules can diffuse into pores -- trapped and removed from the flow of the mobile phase
Specific pore sizes.average residence time in the pores depends on the effective size of the analyte molecules– larger molecules– smaller molecules– intermediate size molecules
SEC(continued)
SEC Column Packing
Small (~10 µm) silica or polymer particles containing a network of uniform pores
Two types (diameters of 5 ~ 10 µm)– Polymer beads– silica-based particles
Advantages of Size Exclusion Chromatography Short & well-defined separation times Narrow bands--> good sensitivity Freedom from sample loss, solutes do not interact
with the stationary phase Absence of column deactivation brought about by
interaction of solute with the packing
Disadvantages
Only limited number of bands can be accommodated because the time scale of the chromatogram is short
Inapplicability to samples of similar size, such as isomers. – At least 10% difference in molecular weight is required
for reasonable resolution
Instrumentation
Instruments required:– Mobile phase reservoir– Pump– Injector– Column– Detector – Data system
Schematic of liquid chromatograph
Mobile phase reservoir
Glass/stainless steel reservoir Removal of dissolved gases by degassers
– vacuum pumping system– heating/stirring of solvents– sparging– vacuum filtration
Elution methods
Isocratic elution– single solvent of constant composition
Gradient elution– 2 or more solvents of differing polarity used
Pumping System I
Provide a continuous constant flow of the solvent through the injector
Requirements– pressure outputs up to 6000 psi– pulse-free output– flow rates ranging from .1-10 mL/min– flow control and flow reproducibility
of .5% or better– corrosion-resistant components
Pumping System II
Two types:– constant-pressure– constant-flow
Reciprocating pumps– motor-driven piston– disadvantage: pulsed flow creates noise– advantages: small internal volume (35-400 L), high
output pressures (up to 10,000 psi), ready adaptability to gradient elution, constant flow rates
Pumping System III
Displacement pumps– syringe-like chambers activated by screw-driven
mechanism powered by a stepper motor– advantages: output is pulse free– disadvantage: limited solvent capacity (~20 mL) and
inconvenience when solvents need to be changed Flow control and programming system
– computer-controlled devices– measure flow rate– increase/decrease speed of pump motor
Sample Injection Systems
For injecting the solvent through the column Minimize possible flow disturbances Limiting factor in precision of liquid chromatographic
measurement Volumes must be small .1-500 L Sampling loops
– interchangeable loops (5-500 L at pressures up to 7000 psi)
LC column
LC injector
Liquid Chromatographic Column
Smooth-bore stainless steel or heavy-walled glass tubing
Hundreds of packed columns differing in size and packing are available from manufacturers ($200-$500)
Add columns together to increase length
Liquid Chromatographic Columns II Column thermostats
– maintaining column temperatures constant to a few tenths degree centigrade
– column heaters control column temperatures (from ambient to 150oC)
– columns fitted with water jackets fed from a constant temperature bath
Detector
Mostly optical Equipped with a flow cell Focus light beam at the center for
maximum energy transmission Cell ensures that the separated
bands do not widen
Some Properties of Detector
Adequate sensitivity Stability and reproducibility Wide linear dynamic range Short response time Minimum volume for reducing zone broadening
More Properties of Detector
High reliability and ease of use Similarity in response toward all analytes Selective response toward one or more classes of
analytes Non-destructive
Types of Detector
Refractive index UV/Visible Fluorescence Conductivity Evaporative light scattering Electrochemical
Refractive Index I
Measure displacement of beam with respect to photosensitive surface of dectector
Refractive Index II
Advantages– universal respond to nearly all solutes– reliable– unaffected by flow rate– low sensitive to dirt and air bubbles in the flow cell
Refractive Index III
Disadvantages– expensive– highly temperature sensitive– moderate sensitivity– cannot be used with gradient elution
UV/Visible I
Mercury lamp = 254nm = 250, 313, 334 and 365nm with filters Photocell measures absorbance Modern UV detector has filter wheels for rapidly
switching filters; used for repetitive and quantitative analysis
UV/Visible II
UV/Visible III
Advantages– high sensitivity– small sample volume required– linearity over wide concentration ranges– can be used with gradient elution
UV/Visible IV
Disadvantage– does not work with compounds that do not absorb light
at this wavelength region
Fluorescence I
For compounds having natural fluorescing capability
Fluorescence observed by photoelectric detector
Mercury or Xenon source with grating monochromator to isolate fluorescent radiation
Fluorescence II
Advantages– extremely high sensitivity– high selectivity
Disadvantage– may not yield linear response over wide range of
concentrations
Conductivity
Measure conductivity of column effluent
Sample indicated by change in conductivity
Best in ion-exchange chromatography
Cell instability
Evaporative Light Scattering I
Nebulizer converts eluent into mist Evaporation of mobile phase leads to formation of
fine analyte particles Particles passed through laser beam; scattered
radiation detected at right angles by silicon photodiode
Similar response for all nonvolatile solutes Good sensitivity
Evaporative Light Scattering II
Electrochemical I
Based on reduction or oxidation of the eluting compound at a suitable electrode and measurement of resulting current
Electrochemical II
Advantages– high sensitivity– ease of use
Disadvantages– mobile phase must be made conductive– mobile phase must be purified from oxygen, metal
contamination, halides
Data System
For better accuracy and precision Routine analysis
– pre-programmed computing integrator Data station/computer needed for higher control levels
– add automation options– complex data becomes more feasible– software safeguard prevents misuse of data system
Electrophoresis…charged species migrate in electric fieldSeparation based on charge or mobility
Capillary electrophoresishigher voltages can be used as the heat can be dissipated
Capillary electrophoresis