Indian Journal of Chemistry Vol. 42A, December 2003, pp. 3000-3005

Liquid-liquid extraction, preconcentration and trace determination of selenium with rotaxane

Y K Agrawal*, S K Menon & Yauvan Pancholi

Chemistry Department, School of Sciences, Gujarat University, Ahmedabad 380009, India

E-mail: drykagrawal@yahoo.com

Received /0 January 2003

A rapid and sensitive method for the extraction and spectrophotometric determination of Se(lV) with [2]-( l ,lO-bi s trityloxy decane)( I, 4, 7, 10, 13, 16, 19, 22, 25, 28-decaoxa cyclotriacontane)rotaxane (TDDCR), has be,~n described. Selenium forms a colourless complex with TDDCR which is extracted in chloroform (molar absorptivity 3.18x103 I mol- I em- I) and obeys Beer's law in the range of 0.62-16.25 ppm for Se(lV). The extract is directly inserted in the plasma for ICP­AES measurement of selenium which enhances the sensiti vity several fo lds with the detection limits of 1.5 ppb. The overall formation ((lOgf32Kc) and distribution (Ke) constants calculated are 11.56±O.03 and 2.0±0.05x lO-8

, respectively. Selenium has been separated and determined in presence of tellurium.

Selenium can be both essential and toxic depending on the species and concentration present. The need for the analysis and speciation of selenium is driven by the release of Se in the environment from fossils fuel combustion and industrial discharges and its involvement in biochemical processes . Several reagents are reported for the spectrophotometric determination of selenium; however, these are less sensi tive and not free from interferences t

-4

. The atomic absorption spectrometry (AAS) hydride method is sensitive fo r selenium but a number of transition metals, mainly those Groups VIII and IB, can cause severe signal depressions in hydride generation AAS5. Selenium at lower levels cannot be determined directly by inductively coupled plasma (ICP) since it has to be reduced to lower oxidation states before determination by hydride-ICP. But reduction of Se(lV) is tricky because the usual mild reduction of Se(lV) with KI brings about the formation of Se(O), which is non-reactive with tetrahydroborate ion5

,6. A hybrid liquid-liquid extraction and plasma ionization technique can be very useful in trace determination of selenium. The rotaxanes have achieved considerable importance for the complexation of various ions, however, so far these have not been used as an extractants 7 . In the present investigation a newly synthesized [2] -rotaxane (TDDCR) (I) is reported for the selective extraction, separation, preconcentration and determination of selenium in presence of telleurim.

Materials and Methods

The pH measurements were made on a Systronics pH-meter (model 331) equipped with glass and calomel electrodes. A Hitachi 3210 UV-visible spectrophotometer with matched 10 mm quartz-cells was used fOf spectral measurements.

A Plasma Scan (model 710) Sequential Inductively Coupled Plasma Atomic Emission Spectrophotometer with Plasma scan multitasking computer and peristaltic pump was used. The following operating conditions were set for ICP-AES. Rf, 27.12 MHz; Incident power, 2000 W; GMK Nebulizer; Rf power, S W; Observation height, 14 mm; Arg,:m coolant, flow rate, 10 I min- I, Carrier flow rate, 11 min- I; Intergraph period, 10 s; Resolution, 0.04 nm; Peristaltic pump flow rate, 1 ml min-I; Wavelength, 196.03 nm.

All chemicals used were of analytical grade of B.D.H. or E. Merck, unless otherwise specified. Glass distilled and de-ionized water was used throughout. The ligand, [2]-rotaxane (TDDCR), was synthesized

(I)

AGRAWAL et al.: DETERMINATION OF Se(IV) WITH ROTAXANE 3001

as described elsewhere8. Its 0.1 % solution was

prepared in chloroform. A 0.1 % (w/v) solution of cetrimide was prepared by

dissolving 0.1 g of hexadecyl trimethyl ammonium bromide (cetrirnide, CTN+Br-) in 100 ml doubly distilled water.

A l.27xl0-4 M standard stock solution of selenium was prepared by dissolving 0.7029 g of selenium dioxide in doubly distilled water in a 500 ml standard flask. Its working solution was made by suitable dilution with doubly distilled water and the final concentration was determined spectrophoto­metrically '0. Buffer solutions were prepared as described elsewhere9

.

Sample preparation The synthetic samples of steels, alloys, ores, etc. ,

were digested with conc. HCI and perchloric acid. The hot solution was filtered and centrifuged to remove any silicious residue. The filtrate was evaporated and diluted to 100 ml with 0.1 M HCI.

Plant and soil samples Depending upon the concentration of trace metals

present in plants or soils, about 10 g of sample was digested with an excess of conc. HCI and 10-20 ml of perchloric acid. The filtrate was evaporated and diluted to 100 ml with 0.01 M HCI. An appropriate aliquot of this solution was used for the determination of the trace metals.

Extraction of selenium A suitable aliquot of the sample solution containing

(155-406.25 ppm) of selenium was transferred into a 60-ml separatory funnel, and the molarity of the aqueous phase was adjusted to 3 M with conc . HCI. To this , 1 ml of 0.1 % cetrimide was added. Then 5 ml of 0.1 % reagent solution, TDDCR in chloroform was added and the contents were shaken gently for 5 min. The organic phase was separated, dried over anhydrous sodium sulphate and transferred into a 25-ml volumetric flask. To ensure the complete recovery of selenium, the extraction was repeated with 5 ml of the reagent solution in chloroform. The sodium sulphate was also washed with 2x3 ml of chloroform and the washings were collected. The combined extracts and washings were diluted up to the mark with chloroform. The absorbance of organic phase was measured at the A max 284 nm against the reagent bl ank prepared in a simil ar way without the metal ion. The extract after appropriate dilu tion with chloroform

was directly inserted into plasma for ICP-AES measurements.

Results and Discussion The Se(lV)-[2]-rotaxane, (TDDCR), complex is

colourless and has a maximum absorbance at 284 nm. The chloroform reagent blank does not absorb at this wavelength. The molar absorptivity is 3.18x 1031 mol- 'cm- '. The system obeys Beer's law in the range of 0.62-16.25 ppm of selenium and the regression analysis represents, selenium conc. (ppm) = 2l.70 x abs +0.023 with a correlation coefficient of 0.998. Under the optimum conditions for ICP-AES determination, a linear calibration was obtained from 5-110 ppb of selenium.

Influence of Hel molarity and shaking time Maximum extraction of selenium with rotaxane

was obtained between 3.0 and 4.0 M of HCI, but as the molarity of HCl was decreased, the percentage of extraction also decreased. The increase in the molarity of HCl beyond 4.5 M showed a steady decrease in the percentage of extraction . Therefore, all the extraction studies were carried out at 3.0 M HCI. Shaking time of 5 min was sufficient for the quantitative extraction of selenium.

Effect of reagent concentration Se(IV) was extracted with varying concentrations

of TDDCR. It was found that maximum extraction was obtained with 0.1 % (w/v) of the reagent solution. For quantitative extraction, 3-5 ml of 0.1 % solution was sufficient. Low concentration of reagent solution led to a decrease in percentage extraction of Se(IV) . Large excess of reagent could be used without any difficulty.

Effect of cetrimide concentration For complete formation of the complex, 1.0 ml of

0.1 % (w/v) solution of cetrimide was sufficient. Lower concentration of cetrimide gave incomplete extraction . Excess of cetrimide could not be used as the solution becames turbid.

Effect of solvent Various solvents, viz., chloroform, dichloro­

methane, carbon tetrachloride, toluene and isoamyl alcohol , etc. , were tes ted for maximum extraction and spectrophotometric determinat ion of Se(lV) with [2]­rotaxane. Chloroform was found to be the ideal solvent for extract ion of the complex.

3002 INDIAN] CHEM, SEC A, DECEMBER 2003

Stoichiometry of the complex The nature of the Se(lV)-[2]-rotaxane (TO OCR)

complex was ascertained by plouing the graph of logarithm of distribution coefficient of the metal [logDl111 against the negative logarithm of the ligand concentration [-Iog(ligand)] at a fixed selenium concentration. A linear graph with slope 0.93 was obtained. Therefore, the probable composition of the extracted species is 1: 1, i.e., 1 mol of rotaxane is required for 1 mol of selenium metal.

The extraction of selenium with [2]-rotaxane (TOOCR) (as R) is represented as:

HzSe03+2(CTN+Br-)+(R) ~ [Se03(CTN+)2](R)+2HBr

The equilibrium between an aqueous solution contall1lng selenium, cetrimide (CTN+Br-) and rotaxane [R] can be represented as:

mSeO(aq)+n(CTN+)+pR(org) ~ [SeOm(CTN+)I1Rp](org)

. .. (i)

where the subscript (org) refers to the organic phase and (aq) the aqueous phase. Omitting the charges for si mplicity, the extraction constant

Kex=[SeOm(CTN+)I1 Rp] (Orgy'[SeO]I1l(aq)[CTN+]n[R]p(org)

... (ii)

Assuming only one species is formed 111 a given reagent concentration. It can be

.. . (iii)

where A= absorbance, E = molar absorptivity in (I mol- Icm- I) and t = cell path length. From Eqs (ii) and (iii)

10gA = m 10g[SeO] + n 10g[CTN+] + P 10gR +logKEt

The values were found to be m = 1.02, n = 2.02 and p = 0.93.

The stoichiometry of the complex was determined by the mole ratio method and by the plot of log DM agai nst - log[R] which shows that selenium-rotaxane complex is I: 1 while selenium with [CTN+] forms a 1:2 complex.

The two phase stability constant can be obtained as

where 10g13zKe and Ke are the respective overall stability and distribution constants of the chelate. The values of log132Ke and Ke calculated are 11.56±0.03 and 2.0±0.05x lO-8

, respectively.

Effect of diverse ions Varying amounts of diverse ions were added to a

fixed amount of selenium, extracted according to the above procedure and determined both spectrophotometrically and by ICP-AES. Most of the metal ions associated with selenium do not interfere. Selenium was extracted in the presence of associated alkali metals, alkaline earths, transiti on metals, etc., at 3 M HC\. The interference of large amounts of Te4+ can be tolerated by stripping it with thioglycolic acid. This confirms that the KSe values are greater than KM

n+ or KA"- for competing metal cations or anions which were determined independently at 3 M HCI II ,12, The results, given in Table I, show that the selectivity factor, KSe (Kse =132KeIKMn+ or=132KcIKAn-) for selenium TOOR complex has a high selectivity with most of the cations and anions.

Sample analysis [n order to test the accuracy and applicability of the

proposed method to the analysis of environmental samples, high purity grade metals and alloys were analyzed, Matrix interference was verified by comparison of the slopes of calibration graphs with those of standard methods. The results of the analysis of various samples of selenium are given in Tables 2 and 3 .

Determination of selenium in the presence of tellurium

Selenium and tellurium have similar analytical and chemical properties and usually occur together. Hence, separation of selenium from tellurium is necessary,

Selenium is extracted with rotaxane (TOOCR) at 3 M HCI and tellurium at 2 M HC\. At 3 M HCl, only 60% tellurium is extracted with TOOCR. However, in the present investigation, the interference of tellurium was selectively removed by stripping tellurium with thioglycollic acid prior to the extraction of selenium (Table 4).

Transport of selenium through liquid membrane Transport experiments were performed using

rotaxane (TOOCR) in chloroform as a liquid

AGRA WAL et al.: DETERMINATION OF Se(lY) WITH ROTAXANE 3003

Table I-Effect of diverse ions on the determination of selenium [Selenium: 5 ppm; HCI: 3 M; Solvent: Chloroform; Amax: 284 nm]

Ions Added as Amt added log K~+or Ksc Selenium found (QQm) (mg) l og K~ - Spectrophotometry ICP-AES

A13+ AI(N03h 60 11.56 1.00 4.98±0.05 5.003±0.007 As3+ AsF3 60 Ba2+ BaCI2 60 Be2+ BeS04 80 Bi3+ Bi(N03h 5H2O 60 Ca2+ Ca(N03h 80 Cd2+ CdS04 60 C02+ Co(C2H30 2)z. 4H2O 60 Cr3+ Cr20 3 40 Te4+ Te(CI04)4 40 Pb2+ Pb(C2H30 2)2. 4H2O 40 y S+ NH4Y03 40 Cu2+ CUS04 60 Zn2+ ZnCI2 60 Ni2+ NiCI2 40 Ti4+ TiCI4 80 Zr4+ Zr(N03)4. 5H20 60 Hf HfCI4 60 Fe2+ FeS04 40 Mg2+ Mg(N03h 40 F- NaF 60 CI- NaCi 40 Nbs+ Nb20 S 40 Tas+ Ta20 S 40 M06+ (NH4)6 M070 24 40 Mn2+ MnCl2 40

membrane system. Source phase aqueous 10 ml of 6.87x I0-4 M selenium solution in 3 M HCI and 20 ml chloroform with 1.0x lO-3 M DDCR as carrier liquid membrane phase were taken in a glass Pressman cell and stirred with teflon-coated magnetic stirrer derived by Hust Sychronous meter. The receiving phase was 10 ml of 0.01 M HC\. The selenium concentration in the aqueous compartment was monitored as function of time by ICP-AES. The transport data are the average of at least six runs where experimental error is less than 1.8%. No movement of selenium was observed unless a carrier TDDCR was used. Selenium transport and retention with the time is shown in Fig. I. The selenium concentration in membrane varies linearly till 20 min. The transport of selenium is of pseduo-unimetal reaction of first order. It has been observed that the maximum transportation is observed until 20 m~ with tl/2 equal to 9 min .

., A precbncentration study was carried out with 5 /.lg of selenium in 1000 ml aqueous phase by extracting 5 times, each 200 ml of aqueous phase with the same 10 mlofchloroform solution of TDDCR (selenium gets

1.0 0.50 0.80 1.20 0.86 1.56 1.54 1.80 5.56 1.00 1.26 1.86 1.76 1.56 2.00 2. 15 2.14 2.00 0.56 0.56 1.00 1.80 1.56 1.56 1.46

1010.56 4.99±0.05 4.996±0.007 1010.06

5.02±0.03 5.002±0.005 1010.76

5.01±0.02 4.999±0.004 1010.46 5.03±0.06 5.00 I ±0.005 101070

5.02±0.04 5.006±0.008 1010.00

4.99±0.03 4.998±0.005 1010.12

4.97±0.05 4.997±0.005 109.76

4.97±0.05 4.998±0.004 106.00

4.98±0.04 4.995±0.007 101056 5.02±0.04 4.995±0.006 1010.40 5.00±0.03 5.000±0.003 109.70 4.99±0.03 4.998±0.005 109.80

5.02±0.04 4.995±0.006 1010.00 4 .99±0.03 4.998±0.005 109.56

5.01±0.02 4.999±0.004 109.4 1 5.02±0.03 5.002±0.005 109.41 5.00±0.04 4.998±0.002 109.56 5.00±0.04 5.000±0.003 1011.00 5.02±0.03 5.005±0.005 1011.00 4.99±0.05 4.996±0.007 10 10 56

4.98±0.04 4.995±0.006 109.76

4.99±0.05 5.000±0.005 10 10.00

4.96±0.03 4.998±0.003 101000

5.00±0.05 5.002±0.005 101000

4.97±0.08 4 .995±0.007

Table 2-Determination of se lenium in high purity grade metal s and alloys

Sample Selenium (%)

Cert. -=-__ -:----.:F:...:o::..:u:::n.::.d --::-=-=---,--:=-_ Spectrophotometry ICP-AES

Stainless Steel(303 Se) 0.247 0.249±0.005 0.248±0.003

Copper Benz Mark 0.500 0.503±0.004 0.499±0.002

Steel(AlS/4340) 3. 160 3. 158±0.004 3. I 62±0.003

363 Cr-Y 0.130 0.132±0.004 0.130±0.002

NBS.36I t 40.0 39.85±0.50

NBS.364t 2.0 I. 89±0. 15

tSelenium in /:!;g/g.

concentrated in 10 ml chloroform). For evaluation of the efficiency of preconcentration, expressed as recovery, the concentration of selenium in organic phase and that remaining in aqueous phase was measured by ICP-AES against calibrated standard solution in chloroform. The recovery [R %] was calculated from the equation, R% =[Asland-AJ As1d]xlOO, where ASland is the absorbance value for the

3004 INDIAN J CHEM, SEC A, DECEMBER 2003

Table 3-Determination of selenium in environmental samples

Sample Selenium found (ppm) Spectrophotometry ICP-AES

Soil (Nandesari ,Baroda) 0.005-0.09±0.OO4 0.OO4-0.08±0.OO3 Soil (GIDC, Baroda) 0.050-I .OI±0.006 0.05-1 .03±0.004 Plan teN andesari, Baroda) 0.020-5.00±0.005 O.02-5.10±0.005 Television Tube Manufacture 0.080-0.510±0.005 0.07-0.53±0.004 Petrochemical Effluents 0. IOO-5.500±0.OO5 0.10-5.8±0.004 Whole Blood 0.540-1.720±0.OO6 0.50-1.45±0.005 Fish 3.000-10.000±0.008 2.98-9 .98±0.005

Oyster tissue NBBSIl566

Wheat flour (NBSI567) Bovine liver (NBS 1568) Animal muscle (IAEA H-4) Biological ti ssue Lobster Scallops Plaice Tissue Sea water (Bombay, Mahim) Swimming Pool

Table 4-Determination of selenium in presence of tell urium

Selenium Tellurium Selenium found* (ppm) taken (ppm) added (mg) Spectrophotometry ICP-AES

0.50 40 0.49±0.03 0.499±0.003

1.00 40 1.01±0.03 I .OOO±0.OO2

2.00 40 1.98±0.04 2.001±0.002

5.00 40 4.97±0.04 4.987±0.008

10.00 40 9.96±0.05 I 0.00 I ±0.002

20.00 40 20.02±0.03 19.993±0.008

* Average of eight determinations

sample containing known amount of standard selenium added to the sample solution prior to the extraction, A Sld is the absorbance value for the standard selenium in chloroform with the same analyte concentration as the standard addition and As is the absorbance value of the sample. The quantitative collection of titanium was possible from matrix such as alkali metals and other metals with recovery of 99.35% and preconcentration factor of 102.

The sample of sea water was filtered and pumped to the mixing chamber (Fig. 2). The molarity, 3 M

Selenium (ppb) Cert. 2.10

1.10

1. 10

0.28

125 1

100

E :I 'c

75 II> 4i .. '0 .. II>

50 '0 E ~

25

0 .-~

0 5

Found (ICP-AES) 2.07±0.05

0.98±0.02

1.I0±0.01

0.34±0.01

5.70±0.04 0.72±0.05 2.8±0.05

20.0±0.15 80.0±0.50 1.00±0.20

10 15 20

Time, min

)( )(

25 30

Fig. I-Percentage of selenium transported (x- x) and retained (t.- t.) by the membrane with time.

HCl, was adjusted and valve V I was opened to pass the liquid to the separator. Valve V2 was opened to add 10 ml TDDCR in chloroform. After 5 min. the organic layer was transferred to the extraction chamber through valve V 3 for preconcentration and

AGRAWAL etal. : DETERMINATION OF Se(IV) WITH ROTAXANE 3005

sea water or waste Filter

Mixing Coil

HCIl-----'

ICP-Af ;

Fig. 2-Preconcentration and recovery of selenium

recirculation. The sea water or waste was drawn out from the separator through valve V 4. The chloroform layer was recycled via valve V 5 into the reagent chamber. The selenium extract was also inserted into the plasma for ICP-AES estimates. The data on sea water are given in Table 3.

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8 Pancholi Y A, Supramolecules:Studies on rotaxanes, Ph.D Thesis (Gujarat University, Ahmedabad) 2002.

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