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The spectrophotometric determination of cobalt after extraction of tetramethylene-bis(triphenyl-phosphonium) tetrathiocyanatocobaltate(II) with microcrystalline benzophenone
Analytica Chimica Acta, 182 (1986) 219-223 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
THE SPECTROPHOTOMETRIC DETERMINATION OF COBALT AFTER EXTRACTION OF TETRAMETHYLENE-BIS(TRIPHENYLPHOSPHONIUM) TETRATHIOCYANATOCOBALTATE(I1) WITH MICROCRYSTALLINE BENZOPHENONE
D. THORBURN
BURNS* and N. TUNGKANANURUK
Department of Analytical Chemistry, (Northern Ireland)
The Queen’s University, Belfast BT9 5AG
(Received 18th October 1985)
Summary. Cobalt (O-130 pg) is determined spectrophotometrically at 625 nm after its adsorptive extraction as tetramethylene-bis(triphenylphosphonium) tetrathiocyanatocobaltate(I1) on microcrystalline benzophenone at pH 6.5 after dissolution of the solid phase in acetylacetone. The system is applied to the determination of cobalt (0.2-10%) in high-speed tool steels without prior separation of iron.
A variety of onium cations has been proposed for the extraction of complex anions of transition metals [ 11. So far no applications have been repported for the tetramethylene-bis(triphenylphosphonium) cation which, being bifunctional, can be regarded as the ion-pairing equivalent for anions to a chelating ligand for cations [2]. Among the anions formed from transition metals, tetrathiocyanatocobaltate( II) has been widely studied in conventional liquid/liquid extraction [3] but not in solid/liquid systems [4], which have been applied mainly to extract cobalt(I1) chelates [5-lo]. Only two ion-pair systems, the tetraphenylborate ion-pairs with [ 4’-( 4-methoxyphenyl)2,2’:6’,6”-terpyridine]cobaltate(II) [ 111 and [3-(4-phenyl-2-pyridyl)-5,6diphenyl-1,2,4-triazine] cobaltate(I1) [ 121. All systems were extracted mto molten or microcrystalline naphthalene. The present communication reports on the novel absorptive extraction of tetramethylenebis(triphenylphosphonium) tetrathiocyanatocobaltate(I1) with microcrystalline benzophenone. The solid benzophenone may be dissolved in acetylacetone and the determinations completed spectrophotometrically at 625 nm. The system is applied to the determination of cobalt in tool steels. Experimental Apparatus.
Pye-Unicam SP8-400 and SP6-550 u.v.-visible spectrophotometers were used for recording absorption spectra and for routine measurements, respectively, with matched quartz l-cm cells.
0003-2670/86/$03.50
0 1986 Elsevier Science Publishers B.V.
220
Reagents and solutions. Tetramethylene-bis( triphenylphosphonium) bromide [ 1,4-bis(triphenylphosphonium)butane dibromide; Lancaster Synthesis; 98+%] was used as supplied. Elemental analysis gave 64.6% C, 5.2% H (theor. for C4,,H,,P2BrZ, 64.9% C, 5.2% H). A 0.003 M stock solution was made by dissolving 1.1107 g of the reagent in 500 ml of distilled water. This solution was stored in a dark brown glass bottle. A stock 1000 pg ml-’ cobalt(I1) solution was prepared by dissolving 2.630 g of anhydrous cobalt(I1) sulphate (analytical grade dried to constant weight at 400°C) in 1 1 of distilled water. More dilute standard solutions were prepared as required. For the pH 6.5 buffer, 10.89 g of potassium dihydrogenphosphate and 5.65 g of disodium hydrogenphosphate were dissolved in 1 1 of distilled water. The benzophenone solution was 20% (w/v) in acetone. All other reagents were of analytical grade. Twice-distilled water was used throughout. General procedure. Place an aliquot containing up to 130 pg of cobalt(I1) in an Erlenmeyer flask with ground-glass stopper. Add 3.0 ml of 5 M ammonium thiocyanate, 2 ml of pH 6.5 buffer and 2.0 ml of the 0.003 M reagent solution, and dilute to 10-15 ml with water. Swirl to mix. Add 2.0 ml of 20% benzophenone solution, stopper the flask and shake vigorously for 30 s. Filter the blue-coloured separated solid through a filter paper (Whatman No. 1) or a sintered glass filter (No. 2). Wash with water, drain or suck dry, dissolve the solid in acetylacetone and make up to volume with that solvent in a lo-ml volumetric flask. Dry the solution by addition of l-2 g of anhydrous sodium sulphate. Measure the absorbance at 625 nm against a reagent blank prepared in the same way. Procedure for steel samples. For steel samples containing 2-10% or 0.2-l% cobalt, dissolve accurately weighed 0.05- or 0.5-g samples, respectively, in a mixture of 3 or 30 ml of concentrated hydrochloric acid and 1 or 10 ml of concentrated nitric acid, respectively, in 250-ml conical flasks. Warm to aid dissolution. When dissolved, boil to near dryness, cool, add 1 or 10 ml of concentrated hydrochloric acid, and evaporate to near dryness. Cool, add 50 ml of distilled water and warm to dissolve the solid. Cool, and filter through a Whatman No. 1 paper into a lOO-mlvolumetric flask. Wash the residue (silica, tungstic acid) with a smallvolume of hot 2% (v/v) hydrochloric acid followed by distilled water and make up to volume with distilled water. Dilute (1 + 4) with distilled water. Transfer 5- or lo-ml aliquots containing 10-60 pg of cobalt to a ground-glass-stoppered Erlenmeyer flask. Add 5 ml of saturated ammonium-D(+)-tartrate solution (for a steel containing copper, add 2 ml of 10% (w/v) sodium thiosulphate solution after the tartrate) and proceed as in the general procedure. Prepare a calibration graph for the range O-130 pg of cobalt after adding 10 ml of iron(II1) nitrate solution, containing about the same amount of iron as in the steel sample solutions to the standard cobalt solution. Examination of main experimental variables Naphthalene, diphenyl, benzophenone and 1,Cdichlorobenzene were each examined for their adsorptive/extraction properties using the microcrystalline
221
solid formation procedure from acetone solution. Benzophenone showed the absorbance) of the solids examined when dissolved, highest recovery (i.e., and was adopted for use. A variety of solvents with a range of functional group types, including alcohols, ketones, esters, ethers and chlorinated and aromatic hydrocarbons, was examined to dissolve the complex along with the benzophenone. The ion-pair was soluble in acetylacetone, acetonitrile, propylene carbonate, diethyl ketone, methyl ethyl ketone, dichloromethane, dimethylformamide and dimethylsulphoxide but was not soluble in the other solvents examined. The solutions in dimethylformamide and dimethylsulphoxide were unstable. Acetylacetone was chosen for further study because the ion-pair gave the greatest molar absorptivity (at 625 nm) in this solvent. The effect of pH was examined for 80 pg of cobalt(I1) by addition of 1 M hydrochloric acid or 1 M ammonia solution prior to extraction. The absorbances were measured as before and were almost independent of pH in the range 2.0-8.0, decreasing rapidly outside these limits. In subsequent work the aqueous phase was buffered at pH 6.5. The effects of varying the amounts of ammonium thiocyanate and tetramethylene-bis(triphenylphosphonium)bromide were examined for 80 pg of cobalt(I1). For both reagents, the absorbance of the dissolved extracts increased up to a constant value with increase in volume of reagent. Convenient amounts in the plateau regions are specified in the general procedure. The extent of extraction was found to be unaffected by ionic strength (provided that the pH remained at 6.5), by phase volume ratios up to 1O:l water/ benzophenone in acetone, or by time of shaking. Dissolved extracts were stable for up to 3 days in direct daylight but for at least 10 days in diffuse daylight. The composition of the complex was established spectrophotometrically by Job’s method [ 131 and by the mole ratio method [ 14,151 to be [(C,H,),P(CH2)4P(C6HJ)j] [CO(NCS)~] . Elemental analysis of the precipitate formed from aqueous solution was consistent with that composition (required 60.6% C, 4.4% H, 6.4% N; found 60.5% C, 4.5% H, 6.3% N). Results and discussion A linear calibration graph was obtained over the range O-130 pg of cobalt in 10 ml of final solution in acetylacetone (molar absorptivity 1.5 X lo3 1 mol-’ cm-‘). For the determination of 40 pg of cobalt the relative standard deviation was 0.46% (10 results). The possible interferences of various anions and cations, chosen on the basis of previous studies of onium extraction of tetrathiocyanatocobaltate(I1) for the analysis of steels [ 161 were checked spectrophotometrically for 40 pg of cobalt. Slightly decreased extraction occurred at high adverse ratios of iron(II1) to cobalt (-2.8% for 40 mg of iron, -1.9% for 4 mg of iron) when 3.0 ml of saturated ammonium-D(+)-tartrate solution was used to mask iron(III). This effect may be compensated by addition of an equivalent amount
222
of iron to the standards and blank, for construction of a low concentration range calibration graph. The results of the interference and masking study are summarised in Table 1. The only ions which interfered significantly and are of interest in the analysis of steels were iron(II1) (>500:1), niobium and tin (>300:1) and copper (>200:1). The interferences of iron(III), niobium and tin can be masked by addition of ammonium tartrate and that of copper by sodium thiosulphate. Under the conditions of the procedure, the following cations in the weight ratio 5OO:l were without significant effect: Na+, K’, NH;, Mg’+, Ca’+, Cd2’, Ni2’, Mn2’, Al’+ and Zr(IV). Other cations which TABLE 1 Effect of various ions on the determination of 40 clg of Iona
Pd=+ Hg*+ Bi” C?+ Zn’+ D(IV) Sn(IV) Nb(V) Fez+
Ratio to Co(I1) (w/w) 100 500b 100 250c 5ooc 500 5ooc 300c 3ooc 300b
Error in absorbance (%Y
cobalt
Ion*
Ratio to Co(I1) (w/w)
Error in absorbance (%)’
0
Fe3+
0 +13 +lO -82 -24 +16 -7 -41
cl.l=+
-21
vo;
wo:CNEDTA
300 500C.d 200e 100 2500 500 250 2500 500 25 25
-79 -84 -17 0 -16 -7 0 -18 0 -100 -100
aCations added as chloride, sulphate or nitrate, anions added as sodium salts. b When 1 drop of hydrazine hydrate was added, there was no error in absorbance. CNo error with 1.5 ml of 30% (w/v) NH,F added. dNo error with 3 ml of saturated ammonium tartrated added. eNo error with 2 ml of 10% (w/v) Na,S,O, added. ‘Ions causing less than a 5% change in absorbance. TABLE 2 Analysis of high-speed tool steels BCS Steel
22012 24112 481 482 483 484 485
Cobalt content (%, w/w) Certified value
Certified range
Found*
0.32 5.70 0.21 0.24 1.94 10.20 5.06
0.31-0.33 5.66-5.73 0.19-0.22 0.21-0.27 1.90-1.99 10.10-10.39 5.00-5.11
0.330 + 0.015 5.73 * 0.04 0.220 f 0.002 0.235 f 0.013 1.955 a 0.013 10.25 + 0.08 5.03 f 0.05
aMean * 95% confidence
limits for 4 replicates.
223
interfered were Cr3+, U(IV), Hg2+ and Zn2+ at 500: 1 ratios and BP+ and Fe’+ at 3OO:l. The effects of iron(I1) and mercury(I1) were overcome by the addition of hydrazine hydrate and those of Bi(II1) and Cr(II1) by ammonium fluoride. Among ions which did not interfere were fluoride, chloride, bromide, iodide, hydrogenphosphate, acetate, perchlorate, sulphate, thiosulphate, molybdate, tartrate and ascorbate. Cyanide and EDTA should be absent. The results for the determination of cobalt in seven British Chemical Standard steel samples (Table 2) are in excellent agreement with the certificate values. The method is rapid and more precise than liquid/liquid extractions for the same complex anion. REFERENCES 1 A. J. Bowd, D. T. Burns and A. G. Fogg, Talanta, 16 (1969) 719. 2 D. T. Burns, Anal. Proc., 19 (1982) 355. 3 D. T. Burns and S. Kheawpintong, Anal. Chim. Acta, 162 (1984) 439 and references therein. 4 D. T. Burns, J. M. Jones and N. Tungkananuruk, Trends Anal. Chem., 4(Z) (1985) VI. 5 M. Gautam and B. K. Puri, Mikrochim. Acta, I (1979) 515. 6 M. Gautam, R. K. Bansal and B. K. Puri, Bull. Chem. Sot. Jpn., 54 (1981) 3178. 7 M. Satake, B. K. Puri, J. C. Yuh and L. F. Chang, Mem. Fat. Eng. Fukui Univ., 30 (1982) 63. 8 B. K. Puri, C. L. Sethi and A. Kumar, J. Chin. Chem. Sot. (Taipei), 28 (1982) 173. 9 A. Wasey, R. K. Bansai, M. Satake and B. K. Puri, Bunseki Kagaku, 32 (1983) E211. 10 C. L. Sethi, A. Kumar, B. K. Puri and M. Satake, Analyst (London), 108 (1983) 528. 11 T. Nagahiro, M. Satake, J. L. Lin and B. K. Puri, Analyst (London), 109 (1984) 163. 12 J. L. Lin, L. F. Chang, M. Katyal and M. Satake, Z. Anal. Chem., 319 (1984) 308. 13 P. Job, Ann. Chim. (Paris), 9 (1928) 113. 14 J. H. Yoe and A. L. Jones, Ind. Eng. Chem. Anal. Ed., 16 (1944) 111. 15 K. Momoki, J. Sekino, H. Sato and N. Yamaguchi, Anal. Chem., 41 (1969) 1286. 16 D. T. Burns, P. Hanprasopwattana and B. P. Murphy, Anal. Chim. Acta, 134 (1982) 397 and references therein.
Short Communication
THE SPECTROPHOTOMETRIC DETERMINATION OF COBALT AFTER EXTRACTION OF TETRAMETHYLENE-BIS(TRIPHENYLPHOSPHONIUM) TETRATHIOCYANATOCOBALTATE(I1) WITH MICROCRYSTALLINE BENZOPHENONE
D. THORBURN
BURNS* and N. TUNGKANANURUK
Department of Analytical Chemistry, (Northern Ireland)
The Queen’s University, Belfast BT9 5AG
(Received 18th October 1985)
Summary. Cobalt (O-130 pg) is determined spectrophotometrically at 625 nm after its adsorptive extraction as tetramethylene-bis(triphenylphosphonium) tetrathiocyanatocobaltate(I1) on microcrystalline benzophenone at pH 6.5 after dissolution of the solid phase in acetylacetone. The system is applied to the determination of cobalt (0.2-10%) in high-speed tool steels without prior separation of iron.
A variety of onium cations has been proposed for the extraction of complex anions of transition metals [ 11. So far no applications have been repported for the tetramethylene-bis(triphenylphosphonium) cation which, being bifunctional, can be regarded as the ion-pairing equivalent for anions to a chelating ligand for cations [2]. Among the anions formed from transition metals, tetrathiocyanatocobaltate( II) has been widely studied in conventional liquid/liquid extraction [3] but not in solid/liquid systems [4], which have been applied mainly to extract cobalt(I1) chelates [5-lo]. Only two ion-pair systems, the tetraphenylborate ion-pairs with [ 4’-( 4-methoxyphenyl)2,2’:6’,6”-terpyridine]cobaltate(II) [ 111 and [3-(4-phenyl-2-pyridyl)-5,6diphenyl-1,2,4-triazine] cobaltate(I1) [ 121. All systems were extracted mto molten or microcrystalline naphthalene. The present communication reports on the novel absorptive extraction of tetramethylenebis(triphenylphosphonium) tetrathiocyanatocobaltate(I1) with microcrystalline benzophenone. The solid benzophenone may be dissolved in acetylacetone and the determinations completed spectrophotometrically at 625 nm. The system is applied to the determination of cobalt in tool steels. Experimental Apparatus.
Pye-Unicam SP8-400 and SP6-550 u.v.-visible spectrophotometers were used for recording absorption spectra and for routine measurements, respectively, with matched quartz l-cm cells.
0003-2670/86/$03.50
0 1986 Elsevier Science Publishers B.V.
220
Reagents and solutions. Tetramethylene-bis( triphenylphosphonium) bromide [ 1,4-bis(triphenylphosphonium)butane dibromide; Lancaster Synthesis; 98+%] was used as supplied. Elemental analysis gave 64.6% C, 5.2% H (theor. for C4,,H,,P2BrZ, 64.9% C, 5.2% H). A 0.003 M stock solution was made by dissolving 1.1107 g of the reagent in 500 ml of distilled water. This solution was stored in a dark brown glass bottle. A stock 1000 pg ml-’ cobalt(I1) solution was prepared by dissolving 2.630 g of anhydrous cobalt(I1) sulphate (analytical grade dried to constant weight at 400°C) in 1 1 of distilled water. More dilute standard solutions were prepared as required. For the pH 6.5 buffer, 10.89 g of potassium dihydrogenphosphate and 5.65 g of disodium hydrogenphosphate were dissolved in 1 1 of distilled water. The benzophenone solution was 20% (w/v) in acetone. All other reagents were of analytical grade. Twice-distilled water was used throughout. General procedure. Place an aliquot containing up to 130 pg of cobalt(I1) in an Erlenmeyer flask with ground-glass stopper. Add 3.0 ml of 5 M ammonium thiocyanate, 2 ml of pH 6.5 buffer and 2.0 ml of the 0.003 M reagent solution, and dilute to 10-15 ml with water. Swirl to mix. Add 2.0 ml of 20% benzophenone solution, stopper the flask and shake vigorously for 30 s. Filter the blue-coloured separated solid through a filter paper (Whatman No. 1) or a sintered glass filter (No. 2). Wash with water, drain or suck dry, dissolve the solid in acetylacetone and make up to volume with that solvent in a lo-ml volumetric flask. Dry the solution by addition of l-2 g of anhydrous sodium sulphate. Measure the absorbance at 625 nm against a reagent blank prepared in the same way. Procedure for steel samples. For steel samples containing 2-10% or 0.2-l% cobalt, dissolve accurately weighed 0.05- or 0.5-g samples, respectively, in a mixture of 3 or 30 ml of concentrated hydrochloric acid and 1 or 10 ml of concentrated nitric acid, respectively, in 250-ml conical flasks. Warm to aid dissolution. When dissolved, boil to near dryness, cool, add 1 or 10 ml of concentrated hydrochloric acid, and evaporate to near dryness. Cool, add 50 ml of distilled water and warm to dissolve the solid. Cool, and filter through a Whatman No. 1 paper into a lOO-mlvolumetric flask. Wash the residue (silica, tungstic acid) with a smallvolume of hot 2% (v/v) hydrochloric acid followed by distilled water and make up to volume with distilled water. Dilute (1 + 4) with distilled water. Transfer 5- or lo-ml aliquots containing 10-60 pg of cobalt to a ground-glass-stoppered Erlenmeyer flask. Add 5 ml of saturated ammonium-D(+)-tartrate solution (for a steel containing copper, add 2 ml of 10% (w/v) sodium thiosulphate solution after the tartrate) and proceed as in the general procedure. Prepare a calibration graph for the range O-130 pg of cobalt after adding 10 ml of iron(II1) nitrate solution, containing about the same amount of iron as in the steel sample solutions to the standard cobalt solution. Examination of main experimental variables Naphthalene, diphenyl, benzophenone and 1,Cdichlorobenzene were each examined for their adsorptive/extraction properties using the microcrystalline
221
solid formation procedure from acetone solution. Benzophenone showed the absorbance) of the solids examined when dissolved, highest recovery (i.e., and was adopted for use. A variety of solvents with a range of functional group types, including alcohols, ketones, esters, ethers and chlorinated and aromatic hydrocarbons, was examined to dissolve the complex along with the benzophenone. The ion-pair was soluble in acetylacetone, acetonitrile, propylene carbonate, diethyl ketone, methyl ethyl ketone, dichloromethane, dimethylformamide and dimethylsulphoxide but was not soluble in the other solvents examined. The solutions in dimethylformamide and dimethylsulphoxide were unstable. Acetylacetone was chosen for further study because the ion-pair gave the greatest molar absorptivity (at 625 nm) in this solvent. The effect of pH was examined for 80 pg of cobalt(I1) by addition of 1 M hydrochloric acid or 1 M ammonia solution prior to extraction. The absorbances were measured as before and were almost independent of pH in the range 2.0-8.0, decreasing rapidly outside these limits. In subsequent work the aqueous phase was buffered at pH 6.5. The effects of varying the amounts of ammonium thiocyanate and tetramethylene-bis(triphenylphosphonium)bromide were examined for 80 pg of cobalt(I1). For both reagents, the absorbance of the dissolved extracts increased up to a constant value with increase in volume of reagent. Convenient amounts in the plateau regions are specified in the general procedure. The extent of extraction was found to be unaffected by ionic strength (provided that the pH remained at 6.5), by phase volume ratios up to 1O:l water/ benzophenone in acetone, or by time of shaking. Dissolved extracts were stable for up to 3 days in direct daylight but for at least 10 days in diffuse daylight. The composition of the complex was established spectrophotometrically by Job’s method [ 131 and by the mole ratio method [ 14,151 to be [(C,H,),P(CH2)4P(C6HJ)j] [CO(NCS)~] . Elemental analysis of the precipitate formed from aqueous solution was consistent with that composition (required 60.6% C, 4.4% H, 6.4% N; found 60.5% C, 4.5% H, 6.3% N). Results and discussion A linear calibration graph was obtained over the range O-130 pg of cobalt in 10 ml of final solution in acetylacetone (molar absorptivity 1.5 X lo3 1 mol-’ cm-‘). For the determination of 40 pg of cobalt the relative standard deviation was 0.46% (10 results). The possible interferences of various anions and cations, chosen on the basis of previous studies of onium extraction of tetrathiocyanatocobaltate(I1) for the analysis of steels [ 161 were checked spectrophotometrically for 40 pg of cobalt. Slightly decreased extraction occurred at high adverse ratios of iron(II1) to cobalt (-2.8% for 40 mg of iron, -1.9% for 4 mg of iron) when 3.0 ml of saturated ammonium-D(+)-tartrate solution was used to mask iron(III). This effect may be compensated by addition of an equivalent amount
222
of iron to the standards and blank, for construction of a low concentration range calibration graph. The results of the interference and masking study are summarised in Table 1. The only ions which interfered significantly and are of interest in the analysis of steels were iron(II1) (>500:1), niobium and tin (>300:1) and copper (>200:1). The interferences of iron(III), niobium and tin can be masked by addition of ammonium tartrate and that of copper by sodium thiosulphate. Under the conditions of the procedure, the following cations in the weight ratio 5OO:l were without significant effect: Na+, K’, NH;, Mg’+, Ca’+, Cd2’, Ni2’, Mn2’, Al’+ and Zr(IV). Other cations which TABLE 1 Effect of various ions on the determination of 40 clg of Iona
Pd=+ Hg*+ Bi” C?+ Zn’+ D(IV) Sn(IV) Nb(V) Fez+
Ratio to Co(I1) (w/w) 100 500b 100 250c 5ooc 500 5ooc 300c 3ooc 300b
Error in absorbance (%Y
cobalt
Ion*
Ratio to Co(I1) (w/w)
Error in absorbance (%)’
0
Fe3+
0 +13 +lO -82 -24 +16 -7 -41
cl.l=+
-21
vo;
wo:CNEDTA
300 500C.d 200e 100 2500 500 250 2500 500 25 25
-79 -84 -17 0 -16 -7 0 -18 0 -100 -100
aCations added as chloride, sulphate or nitrate, anions added as sodium salts. b When 1 drop of hydrazine hydrate was added, there was no error in absorbance. CNo error with 1.5 ml of 30% (w/v) NH,F added. dNo error with 3 ml of saturated ammonium tartrated added. eNo error with 2 ml of 10% (w/v) Na,S,O, added. ‘Ions causing less than a 5% change in absorbance. TABLE 2 Analysis of high-speed tool steels BCS Steel
22012 24112 481 482 483 484 485
Cobalt content (%, w/w) Certified value
Certified range
Found*
0.32 5.70 0.21 0.24 1.94 10.20 5.06
0.31-0.33 5.66-5.73 0.19-0.22 0.21-0.27 1.90-1.99 10.10-10.39 5.00-5.11
0.330 + 0.015 5.73 * 0.04 0.220 f 0.002 0.235 f 0.013 1.955 a 0.013 10.25 + 0.08 5.03 f 0.05
aMean * 95% confidence
limits for 4 replicates.
223
interfered were Cr3+, U(IV), Hg2+ and Zn2+ at 500: 1 ratios and BP+ and Fe’+ at 3OO:l. The effects of iron(I1) and mercury(I1) were overcome by the addition of hydrazine hydrate and those of Bi(II1) and Cr(II1) by ammonium fluoride. Among ions which did not interfere were fluoride, chloride, bromide, iodide, hydrogenphosphate, acetate, perchlorate, sulphate, thiosulphate, molybdate, tartrate and ascorbate. Cyanide and EDTA should be absent. The results for the determination of cobalt in seven British Chemical Standard steel samples (Table 2) are in excellent agreement with the certificate values. The method is rapid and more precise than liquid/liquid extractions for the same complex anion. REFERENCES 1 A. J. Bowd, D. T. Burns and A. G. Fogg, Talanta, 16 (1969) 719. 2 D. T. Burns, Anal. Proc., 19 (1982) 355. 3 D. T. Burns and S. Kheawpintong, Anal. Chim. Acta, 162 (1984) 439 and references therein. 4 D. T. Burns, J. M. Jones and N. Tungkananuruk, Trends Anal. Chem., 4(Z) (1985) VI. 5 M. Gautam and B. K. Puri, Mikrochim. Acta, I (1979) 515. 6 M. Gautam, R. K. Bansal and B. K. Puri, Bull. Chem. Sot. Jpn., 54 (1981) 3178. 7 M. Satake, B. K. Puri, J. C. Yuh and L. F. Chang, Mem. Fat. Eng. Fukui Univ., 30 (1982) 63. 8 B. K. Puri, C. L. Sethi and A. Kumar, J. Chin. Chem. Sot. (Taipei), 28 (1982) 173. 9 A. Wasey, R. K. Bansai, M. Satake and B. K. Puri, Bunseki Kagaku, 32 (1983) E211. 10 C. L. Sethi, A. Kumar, B. K. Puri and M. Satake, Analyst (London), 108 (1983) 528. 11 T. Nagahiro, M. Satake, J. L. Lin and B. K. Puri, Analyst (London), 109 (1984) 163. 12 J. L. Lin, L. F. Chang, M. Katyal and M. Satake, Z. Anal. Chem., 319 (1984) 308. 13 P. Job, Ann. Chim. (Paris), 9 (1928) 113. 14 J. H. Yoe and A. L. Jones, Ind. Eng. Chem. Anal. Ed., 16 (1944) 111. 15 K. Momoki, J. Sekino, H. Sato and N. Yamaguchi, Anal. Chem., 41 (1969) 1286. 16 D. T. Burns, P. Hanprasopwattana and B. P. Murphy, Anal. Chim. Acta, 134 (1982) 397 and references therein.