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Spectrophotometric determination of tellurium in trace quantities by use of an ion-association complex
Th~nro. Vol. 26. pp. 323. 325 0 Pergamon PressLtd 1979.Printed in Great Britam
M)39-914079 0401-0323s02.00~0
SHORT COtiMUNICATIONS SPECTROPHOTOMETRIC DETERMINATION OF TELLURIUM IN TRACE QUANTITIES BY USE OF AN ION-ASSOCIATION COMPLEX M. VIJAYAKUMAR, T. V. RAMAKRISHNA@ and G. ARAVAMUDAN Department of Chemistry, Indian Institute of Technology, Madras. 600 036, India (Receiced 16 August 1978. Accepted 11 October 1978) Summary-The formation of an ion-association complex by the interaction of iodotellurate(IV) with cetyltrimethylammonium bromide is used as the basis of an extractive procedure to determine tellurium in the range 2.5-12.5 Rg in a final aqueous phase volume of 20 ml. The method is simple, reliable and sensitive. Selectivity is achieved by separation of tellurium on aluminium hydroxide as collector.
Relatively few spectrophotometric procedures are available for the routine determination of tellurium in different kinds of materials. Both the iodotellurate(IV) procedure reported by Johnson and Kwan’ and the bismuthiol II method reported independently by Jankovsky and Ksir’ and by Cheng’ are quite sensitive, but they are restricted because under some conditions the reagents can undergo aerial oxidation to form products which absorb at the absorption maximum of the tellurium reagent complexes. The methods are also not particularly selective. Bode’s diethyldithiocarbamate procedure4 is highly selective under certain experimental conditions, but its sensitivity is very poor (e = 3.2 x lo3 l.mole-‘.cm-‘). Efforts to extract the iodotellurate(IV) species selectively into organic solvents to increase its analytical utility have been reported in the literature.‘-* A strongly acidic medium is used and hence there is extensive liberation of iodine by aerial oxidation of the iodide. These procedures are therefore more useful for separating tellurium from other elements’ than for its determination. The analytical application of ion-association intersction between iodotellurate(IV) and cationic surfactants has not been studied so far. Our work with such surfactants has indicated that cetyltrimethylammonium bromide (CTAB) is worth examining. This paper presents the details of the study.
EXPERIMENTAL
Reagents Telhrium(W) solution (2.5 ppm). Dissolve 0.050 g of metallic tellurium in 10 ml of concentrated nitric acid and boil carefully to remove nitrous fumes. Cool, transfer into a SOO-mlstandard flask and dilute to the mark to obtain a lOO-ppm solution. Dilute this solution 40-fold. CTAJ3 solution (0.75%). Potassium iodide solution (5%). Dissolve 12.5 g of potas-
sium iodide and 5 g of sodium hypophosphite and dilute to 250ml.
in water
Procedure
Transfer a suitable portion of sample solution containing not more than 12 Rg of tellurium into a 60-ml separating funnel. Add, with mixing, 2 ml of 5N sulphuric acid, 1 ml of CTAB solution and 2.5 ml of potassium iodide solution. Dilute to cu. 20 ml and shake for 2-3 min with 5 ml of chloroform. Drain the organic extract into a dry lO-ml volumetric flask containing a pinch of anhydrous sodium sulphate. Measure the absorbance of the extract at 360 nm in 5-mm cells against a reagent blank. Prepare a calibration graph covering the range 2.5-12.5 Rg of tellurium by the above procedure. RESULTS AND DISCUSSION
As the ion-association complex formed by iodotellurate(IV) and CTAB gradually precipitates, its extractability into organic solvents was examined. Chloroform seemed best. However, the results were found to be very erratic because of the liberation of iodine by aerial oxidation of iodide [the I; thus formed reacts with CTAB similarly to iodotellurate(IV)]. Experiments conducted in the presence of reducing agents capable of suppressing iodine liberation indicated that hypophosphite is very effective [under these conditions, Te(IVj is not reduced]. Hence, hypophosphite was incorporated in the iodide reagent solution itself. Absorption
spectra
Figure 1 shows the absorption spectrum of the complex at different concentrations of tellurium. The complex has absorption peaks at 300 and 360 nm. Although the absorption at 300 mn is the higher, the blank values were also higher in this region and therefore the 360~nm peak was employed for analytical purposes. Optimization
of experimental
conditions
Figure 2 shows the minimum concentration of iodide required for the formation of maximum amount of the ion-pair at various acidities. A final concentration’ of OSN sulphuric acid and 0.035M 323
SHORT COMMUNICATIONS
324
mlof
wavelength, nm Fig. 1. Absorption spectra (A): 2 ml of 5N H,SO,, 2.5 ml of 5% KI-2% NaH2P02 solution and 1 ml of 0.75% CTAB solution, diluted to 20 ml and extracted with 5 ml of CHCI,. (B, C, D): as in (A) with the addition of 1 ml (B), 2 ml (C) and 3 ml (D) of 2.5 ppm Te(IV) solution. potassium iodide was therefore chosen as optimal. A minimum of 2.5 ml of 0.15% CTAB solution was required to ensure maximum extraction of 1Opg of tellurium (Fig. 3). A higher concentration, viz. 1 ml of 0.75%. was employed in practice, as CTAB was found to help in suppression of aerial oxidation of iodide, for reasons not yet clarified. It was also observed that hypophosphite does not affect the extraction. Shaking for 2 min was found sufficient to ensure maximum extraction of the ionpair. The organic extracts were found to be stable for at least 90 min in daylight and for 25 hr in the dark. Slight variation in the final volume of aqueous phase did not affect the extraction significantly.
WI,
mhf
Fig. 2. Minimum KI concentration required for maximum extraction of the complex, as a function of acidity.
0.15%
CTAB
Fig. 3. Effect of CTAB concentration: 4 ml of 2.5 ppm Te(IV), 2 ml of 5N H2S04, G4 ml of 0.15% CTAB solution and 2.5 ml of 5% KI-2% NaH,PO,, solution diluted to 20 ml and extracted with 5 ml of CHCI,.
Beer’s law and precision Beer’s law was applicable to the system in the range O-12.5 pg of tellurium in a final aqueous phase volume of 20ml. The molar absorptivity of the ionpair was 4.9 x lo4 1. mole-’ .cm-r. For 28 determinations of 10 pg of tellurium, the relative standard deviation was 2%.
Interference studies The influence of various ions on the determination of tellurium was examined, with 1 mg of ion along with 10 pg of tellurium. The results are presented in Table 1. Separation of tellurium from the matrix by co-precipitation with aluminium hydroxide has been reported.’ To deal with the interferents, a slightly modified version of this procedure was employed. Tellurium was selectively collected by adding 5 ml of a cu. 1% slurry of freshly precipitated aluminium hydroxide to the sample solution at pH 4-6 and containing 5 ml of 0.05M EDTA. The precipitate was dissolved in 2 ml of 5N sulphuric acid and the recommended procedure followed (the additional 2 ml of acid can be omitted but does no harm). It was thus separated from all the interfering ions listed in Table 1 except Fe(III), Se(W), Sb(V) and V(V). The interference of Fe(III), Se(IV) and V(V) was eliminated by reduction with hydroxylammonium chloride in 2N sulphuric acid medium. The selenium formed was filtered off or extracted into chloroform before proceeding with the determination of tellurium. Sb(V) was reduced to Sb(II1) with iodide and the Sb(II1) masked with fluoride, the iodine liberated being extracted into chloroform before the addition of CTAB. Thus all the interferences encountered can be eliminated and no tellurium appears to be lost during the process.
SHORT
325
COMMUNICATIONS
Table 1. Interference studies Li+, Mg’+, Ca’+, Sr*+, Ba*+, Zn2+, Be’+, BO:-, A13+, La3+, Th4*, SnZ+, PO:-, NO;, Cr’+, SsO:-, MnZ+, F-, ClO;, Fe’+, Co’+, Ni*+, oxatate, tartrate, citrate and EDTA Asrr$iGzNO;b IO;, SeO:-, VO;, 5+ Fe3+ C”z+ 3 27, 3 , Ag’. Tl+, Pb’+, Bi3+ wo:MOO:Cd’+, Hg2+, Sb’+, PdZ+, Pt4+
CONCLUSION
The method developed is an improved version of the iodotellurate(IV) method due to Johnson and Kwan. It is superior to the existing spectrophotometric methods in sensitivity, speed and flexibility of the experimental conditions and comparable to them in selectivity. Even though the iodotelhtrate(IV) system is used, aerial oxidation of iodide is suppressed by incorporating a reducing agent. Good selectivity is achieved by selective collection of tellurium on aluminium hydroxide. During this study, it was established that the presence of CTAB is essential for the extraction of iodotellurate(IV) species, so the extracted complex must be an ion-association complex. Its composition could not be established by conventional methods because of the concomitant extraction of the cetyltrimethyl-
No interference
Interfere by oxidizing iodide to iodine Interfere by precipitating as iodides Interferes by precipitating as tungstic acid Reacts with CTAB to give a white precipitate Interfere by forming their iodo-complexCTAB ion-association species; Cd’+ reduces the absorbance, while the rest enhance it
ammonium iodide ion pair, but this does not affect the analytical utility of the system. Acknowledgement-One of us (MV) is grateful to the Council of Scientific and Industrial Research, New Delhi for financial support. REFERENCES
1. R. A. Johnson and F. P. Kwan, Anal. Chem., 1951, 23, 651. 2. J. Jankovsky and 0. Ksir, Talanra, 1960, 5, 238. 3. K. L. Cheng, ibid., 1961, 8, 301. 4. H. Bode. Z. Anal. Chem., 1954, 142, 414; 1955, 144, 90. 5. P. W. West and J. E. Carlton, Anal. Chim. Acra, 1952, 6, 406. 6. C. L. Chakrabharti, ibid., 1961, 39, 293. 7. E. Gagliardi and P. Tummler, Tulanta, 1970, 17, 93. 8. I. Havezov and M. Stoeppler, Z. Anal. Chem., 1972,
258, 189. 9. R. Bock and P. Tschopel. ibid., 1969, 246, 81.
M)39-914079 0401-0323s02.00~0
SHORT COtiMUNICATIONS SPECTROPHOTOMETRIC DETERMINATION OF TELLURIUM IN TRACE QUANTITIES BY USE OF AN ION-ASSOCIATION COMPLEX M. VIJAYAKUMAR, T. V. RAMAKRISHNA@ and G. ARAVAMUDAN Department of Chemistry, Indian Institute of Technology, Madras. 600 036, India (Receiced 16 August 1978. Accepted 11 October 1978) Summary-The formation of an ion-association complex by the interaction of iodotellurate(IV) with cetyltrimethylammonium bromide is used as the basis of an extractive procedure to determine tellurium in the range 2.5-12.5 Rg in a final aqueous phase volume of 20 ml. The method is simple, reliable and sensitive. Selectivity is achieved by separation of tellurium on aluminium hydroxide as collector.
Relatively few spectrophotometric procedures are available for the routine determination of tellurium in different kinds of materials. Both the iodotellurate(IV) procedure reported by Johnson and Kwan’ and the bismuthiol II method reported independently by Jankovsky and Ksir’ and by Cheng’ are quite sensitive, but they are restricted because under some conditions the reagents can undergo aerial oxidation to form products which absorb at the absorption maximum of the tellurium reagent complexes. The methods are also not particularly selective. Bode’s diethyldithiocarbamate procedure4 is highly selective under certain experimental conditions, but its sensitivity is very poor (e = 3.2 x lo3 l.mole-‘.cm-‘). Efforts to extract the iodotellurate(IV) species selectively into organic solvents to increase its analytical utility have been reported in the literature.‘-* A strongly acidic medium is used and hence there is extensive liberation of iodine by aerial oxidation of the iodide. These procedures are therefore more useful for separating tellurium from other elements’ than for its determination. The analytical application of ion-association intersction between iodotellurate(IV) and cationic surfactants has not been studied so far. Our work with such surfactants has indicated that cetyltrimethylammonium bromide (CTAB) is worth examining. This paper presents the details of the study.
EXPERIMENTAL
Reagents Telhrium(W) solution (2.5 ppm). Dissolve 0.050 g of metallic tellurium in 10 ml of concentrated nitric acid and boil carefully to remove nitrous fumes. Cool, transfer into a SOO-mlstandard flask and dilute to the mark to obtain a lOO-ppm solution. Dilute this solution 40-fold. CTAJ3 solution (0.75%). Potassium iodide solution (5%). Dissolve 12.5 g of potas-
sium iodide and 5 g of sodium hypophosphite and dilute to 250ml.
in water
Procedure
Transfer a suitable portion of sample solution containing not more than 12 Rg of tellurium into a 60-ml separating funnel. Add, with mixing, 2 ml of 5N sulphuric acid, 1 ml of CTAB solution and 2.5 ml of potassium iodide solution. Dilute to cu. 20 ml and shake for 2-3 min with 5 ml of chloroform. Drain the organic extract into a dry lO-ml volumetric flask containing a pinch of anhydrous sodium sulphate. Measure the absorbance of the extract at 360 nm in 5-mm cells against a reagent blank. Prepare a calibration graph covering the range 2.5-12.5 Rg of tellurium by the above procedure. RESULTS AND DISCUSSION
As the ion-association complex formed by iodotellurate(IV) and CTAB gradually precipitates, its extractability into organic solvents was examined. Chloroform seemed best. However, the results were found to be very erratic because of the liberation of iodine by aerial oxidation of iodide [the I; thus formed reacts with CTAB similarly to iodotellurate(IV)]. Experiments conducted in the presence of reducing agents capable of suppressing iodine liberation indicated that hypophosphite is very effective [under these conditions, Te(IVj is not reduced]. Hence, hypophosphite was incorporated in the iodide reagent solution itself. Absorption
spectra
Figure 1 shows the absorption spectrum of the complex at different concentrations of tellurium. The complex has absorption peaks at 300 and 360 nm. Although the absorption at 300 mn is the higher, the blank values were also higher in this region and therefore the 360~nm peak was employed for analytical purposes. Optimization
of experimental
conditions
Figure 2 shows the minimum concentration of iodide required for the formation of maximum amount of the ion-pair at various acidities. A final concentration’ of OSN sulphuric acid and 0.035M 323
SHORT COMMUNICATIONS
324
mlof
wavelength, nm Fig. 1. Absorption spectra (A): 2 ml of 5N H,SO,, 2.5 ml of 5% KI-2% NaH2P02 solution and 1 ml of 0.75% CTAB solution, diluted to 20 ml and extracted with 5 ml of CHCI,. (B, C, D): as in (A) with the addition of 1 ml (B), 2 ml (C) and 3 ml (D) of 2.5 ppm Te(IV) solution. potassium iodide was therefore chosen as optimal. A minimum of 2.5 ml of 0.15% CTAB solution was required to ensure maximum extraction of 1Opg of tellurium (Fig. 3). A higher concentration, viz. 1 ml of 0.75%. was employed in practice, as CTAB was found to help in suppression of aerial oxidation of iodide, for reasons not yet clarified. It was also observed that hypophosphite does not affect the extraction. Shaking for 2 min was found sufficient to ensure maximum extraction of the ionpair. The organic extracts were found to be stable for at least 90 min in daylight and for 25 hr in the dark. Slight variation in the final volume of aqueous phase did not affect the extraction significantly.
WI,
mhf
Fig. 2. Minimum KI concentration required for maximum extraction of the complex, as a function of acidity.
0.15%
CTAB
Fig. 3. Effect of CTAB concentration: 4 ml of 2.5 ppm Te(IV), 2 ml of 5N H2S04, G4 ml of 0.15% CTAB solution and 2.5 ml of 5% KI-2% NaH,PO,, solution diluted to 20 ml and extracted with 5 ml of CHCI,.
Beer’s law and precision Beer’s law was applicable to the system in the range O-12.5 pg of tellurium in a final aqueous phase volume of 20ml. The molar absorptivity of the ionpair was 4.9 x lo4 1. mole-’ .cm-r. For 28 determinations of 10 pg of tellurium, the relative standard deviation was 2%.
Interference studies The influence of various ions on the determination of tellurium was examined, with 1 mg of ion along with 10 pg of tellurium. The results are presented in Table 1. Separation of tellurium from the matrix by co-precipitation with aluminium hydroxide has been reported.’ To deal with the interferents, a slightly modified version of this procedure was employed. Tellurium was selectively collected by adding 5 ml of a cu. 1% slurry of freshly precipitated aluminium hydroxide to the sample solution at pH 4-6 and containing 5 ml of 0.05M EDTA. The precipitate was dissolved in 2 ml of 5N sulphuric acid and the recommended procedure followed (the additional 2 ml of acid can be omitted but does no harm). It was thus separated from all the interfering ions listed in Table 1 except Fe(III), Se(W), Sb(V) and V(V). The interference of Fe(III), Se(IV) and V(V) was eliminated by reduction with hydroxylammonium chloride in 2N sulphuric acid medium. The selenium formed was filtered off or extracted into chloroform before proceeding with the determination of tellurium. Sb(V) was reduced to Sb(II1) with iodide and the Sb(II1) masked with fluoride, the iodine liberated being extracted into chloroform before the addition of CTAB. Thus all the interferences encountered can be eliminated and no tellurium appears to be lost during the process.
SHORT
325
COMMUNICATIONS
Table 1. Interference studies Li+, Mg’+, Ca’+, Sr*+, Ba*+, Zn2+, Be’+, BO:-, A13+, La3+, Th4*, SnZ+, PO:-, NO;, Cr’+, SsO:-, MnZ+, F-, ClO;, Fe’+, Co’+, Ni*+, oxatate, tartrate, citrate and EDTA Asrr$iGzNO;b IO;, SeO:-, VO;, 5+ Fe3+ C”z+ 3 27, 3 , Ag’. Tl+, Pb’+, Bi3+ wo:MOO:Cd’+, Hg2+, Sb’+, PdZ+, Pt4+
CONCLUSION
The method developed is an improved version of the iodotellurate(IV) method due to Johnson and Kwan. It is superior to the existing spectrophotometric methods in sensitivity, speed and flexibility of the experimental conditions and comparable to them in selectivity. Even though the iodotelhtrate(IV) system is used, aerial oxidation of iodide is suppressed by incorporating a reducing agent. Good selectivity is achieved by selective collection of tellurium on aluminium hydroxide. During this study, it was established that the presence of CTAB is essential for the extraction of iodotellurate(IV) species, so the extracted complex must be an ion-association complex. Its composition could not be established by conventional methods because of the concomitant extraction of the cetyltrimethyl-
No interference
Interfere by oxidizing iodide to iodine Interfere by precipitating as iodides Interferes by precipitating as tungstic acid Reacts with CTAB to give a white precipitate Interfere by forming their iodo-complexCTAB ion-association species; Cd’+ reduces the absorbance, while the rest enhance it
ammonium iodide ion pair, but this does not affect the analytical utility of the system. Acknowledgement-One of us (MV) is grateful to the Council of Scientific and Industrial Research, New Delhi for financial support. REFERENCES
1. R. A. Johnson and F. P. Kwan, Anal. Chem., 1951, 23, 651. 2. J. Jankovsky and 0. Ksir, Talanra, 1960, 5, 238. 3. K. L. Cheng, ibid., 1961, 8, 301. 4. H. Bode. Z. Anal. Chem., 1954, 142, 414; 1955, 144, 90. 5. P. W. West and J. E. Carlton, Anal. Chim. Acra, 1952, 6, 406. 6. C. L. Chakrabharti, ibid., 1961, 39, 293. 7. E. Gagliardi and P. Tummler, Tulanta, 1970, 17, 93. 8. I. Havezov and M. Stoeppler, Z. Anal. Chem., 1972,
258, 189. 9. R. Bock and P. Tschopel. ibid., 1969, 246, 81.