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Spectrophotometric determination of cobalt(II) based on the ion-pair of tetrathiocyanatocobaltate(II) with neotetrazolium chloride
Analytica Chimica Acta, 172 (1985) 303-306 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
SPECTROPHOTOMETRIC DETERMINATION OF COBALT(I1) BASED ON THE ION-PAIR OF TETRATHIOCYANATOCOBALTATE(I1) WITH NEOTETRAZOLIUM CHLORIDE
A. K. SINGH*, D. KUMAR and M. KATYALa Department
of Chemistry,
Indian Institute
of Technology,
New Delhi 110 016 (India)
(Received 26th July 1984)
Summary. The ion-pair formed between tetrathiocyanatocobaltate(I1) and [ 3,3’-(4,4’biphenylene)bis(2,5-diphenyl)]-ZH-tetrazolium chloride (neotetraxolium chloride) can be extracted into 4-methylpentan-a-one and used for the spectrophotometric determination of cobalt (2.5-9.8 pg ml-‘) at pH 3.5-5.0. The Sandell sensitivity of the method is 0.02 pg Co cm-‘. With suitable masking, the method is quite selective and is applicable to nickel wire alloys.
Thiocyanate is commonly used as a spectrophotometric reagent for cobalt [l-3]. Tetrathiocyanatocobaltate(I1) and its ion-pairs with various large cations have been used to determine the metal [2, 31, but Sandell sensitivities less than 0.04 pg cme2 (for 0.001 absorbance) are rare. In a study of tetrazolium salts, it was observed that the ion-pair formed between neotetrazolium chloride (NTC) and [CO(SCN),]~- absorbs strongly in 4-methylpentan2-one, providing a sensitive procedure for cobalt. The ion-pair formed with malachite green in carbon tetrachloride/cyclohexanone provides a greater absorptivity [ 41 but the coloured complex is not very stable. An aqueous solution of tetrathiocyanatocobaltate(I1) on mixing with NTC solution forms a greenish-blue precipitate which can be extracted into organic solvents. The absorbance of this ion-pair is greatest in cyclohexane (E = 6.4 X lo4 mol” cm-’ ) but the colour is unstable. In 4-methylpentan-2-one (MIBK), the absorbance remains stable for about 12 h, thus MIBK was chosen for extraction. The tolerance to other anions is improved in this system compared to other thiocyanate systems for cobalt. Thus this spectrophotometric method provides reasonable selectivity as well as sensitivity. Experimental Apparatus.
Unicam SP-500 and SP-700 spectrophotometers and an Elico pH meter model LI-10 were used. Reagents. A stock solution (0.01 M) of cobalt(I1) was prepared by dissolvaPresent address: St. Stephens College, Delhi 110007, 0003-2670/85/$03.30
India.
o 1985 Elsevier Science Publishers B.V.
304
ing cobalt(I1) sulphate heptahydrate (BDH Chemicals) in doubly distilled water; it was standardized by titration with EDTA before use. The solution (0.4% w/v) of NTC (Aldrich Chemical Co.) was prepared in 50% (v/v) ethanol. Acetate buffers (pH 3-6) were used for pH adjustment. Ammonium thiocyanate solution (10% w/v) was prepared in doubly distilled water. All solutions, including those for the interference study, were prepared by dissolving analytical-grade chemicals in doubly distilled water. Recommended procedure. To a suitable aliquot (2-3 ml) containing 20-70 pg of cobalt, add acetate buffer (pH 4.5), 4 ml of the thiocyanate solution and 4 ml of the NTC solution. Then add 2 ml of water and 5 ml of dimethylformamide to dissolve the precipitate. Extract the complex with 7 ml of MIBK and measure the absorbance of the organic layer after centrifugation at 620 nm against a water blank. Calculate the metal content from a calibration graph prepared from data obtained with standard solutions. Results and discussion
The ion-pair absorbs maximally at 620 nm. The absorbance remains constant and maximal in pH range 3.5-5.0. Variations in pH do not affect the wavelength of maximum absorbance of the complex. The reagent blank does not absorb at 620 nm. For 1 ml of 1 mM cobalt(I1) solution, maximum absorbance is attained in the presence of >3.7 ml of NTC solution (0.4%). At least 3 ml of 10% thiocyanate solution is needed for full colour development. Excess of either reagent does not affect the absorbance. Samples up to 10 ml in volume (containing 20-70 pg of cobalt) can be successfully treated by the recommended procedure if the concentration of dimethylformamide (DMF) in the aqueous phase is maintained around 30%. At least 4.5 ml of DMF must be added in the determination of 1 ml of 1 mM cobalt(I1) (with a final volume of aqueous phase around 20 ml); up to 8 ml does not have any effect but more than that makes the extraction erratic because of slow and irreproducible phase separation. The colour in the extract is stable for 12 h. Characteristics of the complex. Beer’s law is obeyed up to 13 pg ml-’ of cobalt in the extract at pH 4.5. The optimum concentration, evaluated by a Ringbom plot, is 2.5-9.8 pg ml-‘. The Sandell sensitivity is 0.02 ,ug Co cm”. The molar absorptivity is 3.0 X lo3 1 mol-’ cm-‘. The metal/NTC ratio, as found by Job’s method of continuous variations, is 1: 1. Thus NTC[ Co( SCN),] is probably responsible for the absorption. The standard deviation for a mean absorbance of 0.345 corresponding to 6.5 pg ml-’ of cobalt was found to be 0.006. Effect of diverse ions. Synthetic solutions containing known amounts of cobalt(I1) and varying amounts of diverse ions were prepared and the recommended procedure was followed. An error of 2% in the absorbance reading was considered tolerable. In the determination of 8.4 bg ml-’ cobalt, the anions tolerated (in pg ml-’ given in parentheses) were as follows: fluoride, chloride, iodide, nitrate (1000 each); bromide (625); phosphate (1250);
305
thiosulphate (600); nitrite (1200); oxalate (1280); citrate or tartrate (1800); and thiourea (150). The metal ions tolerated were: Ca(II), Sr(II), Ba(II), Ni(II), Cd(H), Mn(II), Al(III), Ti(IV) (250 pg ml-’ each); Zn(I1) or Fe(I1) (50 pg ml-‘); and Cu(I1) (10 pg ml-‘). EDTA interfered seriously. All other metal ions which form coloured complexes [l] with thiocyanate also interfere. However, iron(II1) (400 pg ml-‘) can be masked with fluoride. The tolerance for copper can be improved to 50 pg ml-’ by using thiourea as masking agent. Similarly, a mixture of citrate and tartrate improves the tolerance for nickel to 1000 pg ml-‘. Determination of cobalt in alloys. Weighed samples (0.1 g) of “K” Monel wire and Nilo “K” wire were dissolved in concentrated nitric acid. Excess of acid was boiled out and the residues were dissolved in 1000 and 50 ml of doubly distilled water, respectively, for the Nilo and Monel wires. The recommended procedure was applied to 3-4 ml of these solutions to quantify the cobalt content. To mask copper, nickel and iron in these alloys, the requisite amounts of thiourea, citrate/tartrate (3 mg each) and fluoride (15 mg) were added. The results are reported in Table 1. TABLE
1
Determination
of cobalt in alloys Cobalt (%)
Alloy
Reported “K” Monel wire Nilo “K” wire aMean of 6 determinations TABLE
0.51
R.s.d. (%) Founda 0.50
17.4
4.8 2.1
17.6
with relative standard deviation (r.s.d.).
2
Sensitivities of thiocyanate (II)
methods for the spectrophotometric
determination
of cobalt-
Other reactant
Wavelength (nm)
Sandell sensitivity (pg Co cm-‘)
Ref.
(Isoamyl alcohol) Tetraphenylarsonium (chloroform) Methyldiantipyrylmethane Tri-n-butylammonium 2,3,5-Triphenyltetraolium Malachite green Neotetrazolium chloride
620 620
0.055 0.034
1, 5 1, 5
625 615 625 630 620
0.075 0.045 0.055 0.0007 0.02
5 5 1, 6 4 This work
306
Comparison with other methods based on thiocyanates. The present method is better in sensitivity than most thiocyanate methods. Sandell sensitivities are compared in Table 2. The high molecular weight of NTC is responsible for better sensitivity. However for malachite green the higher values are reported in CCL, + cyclohexanone medium but the colour fades too quickly. The Co-SCN-NTC ion association complex formed in the presence of less amounts of SCN- also has e value 6.4 X lo4 1 mol-’ cm-’ in cyclohexane medium but suffers from the poor stability. REFERENCES 1 E. B. Sandell, Calorimetric Determinatioh of Traces of Metals, 3rd edn., Interscience, New York, 1959, p. 414. 2 F. D. Snell, Photometric and Fluorimetric Methods of Analysis (Metal), Part I, Wiley, New York, 1978. 3 E. B. Sandell and H. Onishi, Photometric Determination of Traces of Metals, 4th edn., Wiley, New York, 1979, p. 536. 4 L. I. Kotelyanskaya and P. P. Kish, Zh. Anal. Khim., 28 (1973) 1999. 5 I. V. Pyatnitskii, Analytical Chemistry of Cobalt, Israel Programme for Scientific Translations, Jerusalem, 1966, p. 104. 6 M. C. Mehra and D. LeBlanc, Microchem. J., 24 (1979) 435.
Short Communication
SPECTROPHOTOMETRIC DETERMINATION OF COBALT(I1) BASED ON THE ION-PAIR OF TETRATHIOCYANATOCOBALTATE(I1) WITH NEOTETRAZOLIUM CHLORIDE
A. K. SINGH*, D. KUMAR and M. KATYALa Department
of Chemistry,
Indian Institute
of Technology,
New Delhi 110 016 (India)
(Received 26th July 1984)
Summary. The ion-pair formed between tetrathiocyanatocobaltate(I1) and [ 3,3’-(4,4’biphenylene)bis(2,5-diphenyl)]-ZH-tetrazolium chloride (neotetraxolium chloride) can be extracted into 4-methylpentan-a-one and used for the spectrophotometric determination of cobalt (2.5-9.8 pg ml-‘) at pH 3.5-5.0. The Sandell sensitivity of the method is 0.02 pg Co cm-‘. With suitable masking, the method is quite selective and is applicable to nickel wire alloys.
Thiocyanate is commonly used as a spectrophotometric reagent for cobalt [l-3]. Tetrathiocyanatocobaltate(I1) and its ion-pairs with various large cations have been used to determine the metal [2, 31, but Sandell sensitivities less than 0.04 pg cme2 (for 0.001 absorbance) are rare. In a study of tetrazolium salts, it was observed that the ion-pair formed between neotetrazolium chloride (NTC) and [CO(SCN),]~- absorbs strongly in 4-methylpentan2-one, providing a sensitive procedure for cobalt. The ion-pair formed with malachite green in carbon tetrachloride/cyclohexanone provides a greater absorptivity [ 41 but the coloured complex is not very stable. An aqueous solution of tetrathiocyanatocobaltate(I1) on mixing with NTC solution forms a greenish-blue precipitate which can be extracted into organic solvents. The absorbance of this ion-pair is greatest in cyclohexane (E = 6.4 X lo4 mol” cm-’ ) but the colour is unstable. In 4-methylpentan-2-one (MIBK), the absorbance remains stable for about 12 h, thus MIBK was chosen for extraction. The tolerance to other anions is improved in this system compared to other thiocyanate systems for cobalt. Thus this spectrophotometric method provides reasonable selectivity as well as sensitivity. Experimental Apparatus.
Unicam SP-500 and SP-700 spectrophotometers and an Elico pH meter model LI-10 were used. Reagents. A stock solution (0.01 M) of cobalt(I1) was prepared by dissolvaPresent address: St. Stephens College, Delhi 110007, 0003-2670/85/$03.30
India.
o 1985 Elsevier Science Publishers B.V.
304
ing cobalt(I1) sulphate heptahydrate (BDH Chemicals) in doubly distilled water; it was standardized by titration with EDTA before use. The solution (0.4% w/v) of NTC (Aldrich Chemical Co.) was prepared in 50% (v/v) ethanol. Acetate buffers (pH 3-6) were used for pH adjustment. Ammonium thiocyanate solution (10% w/v) was prepared in doubly distilled water. All solutions, including those for the interference study, were prepared by dissolving analytical-grade chemicals in doubly distilled water. Recommended procedure. To a suitable aliquot (2-3 ml) containing 20-70 pg of cobalt, add acetate buffer (pH 4.5), 4 ml of the thiocyanate solution and 4 ml of the NTC solution. Then add 2 ml of water and 5 ml of dimethylformamide to dissolve the precipitate. Extract the complex with 7 ml of MIBK and measure the absorbance of the organic layer after centrifugation at 620 nm against a water blank. Calculate the metal content from a calibration graph prepared from data obtained with standard solutions. Results and discussion
The ion-pair absorbs maximally at 620 nm. The absorbance remains constant and maximal in pH range 3.5-5.0. Variations in pH do not affect the wavelength of maximum absorbance of the complex. The reagent blank does not absorb at 620 nm. For 1 ml of 1 mM cobalt(I1) solution, maximum absorbance is attained in the presence of >3.7 ml of NTC solution (0.4%). At least 3 ml of 10% thiocyanate solution is needed for full colour development. Excess of either reagent does not affect the absorbance. Samples up to 10 ml in volume (containing 20-70 pg of cobalt) can be successfully treated by the recommended procedure if the concentration of dimethylformamide (DMF) in the aqueous phase is maintained around 30%. At least 4.5 ml of DMF must be added in the determination of 1 ml of 1 mM cobalt(I1) (with a final volume of aqueous phase around 20 ml); up to 8 ml does not have any effect but more than that makes the extraction erratic because of slow and irreproducible phase separation. The colour in the extract is stable for 12 h. Characteristics of the complex. Beer’s law is obeyed up to 13 pg ml-’ of cobalt in the extract at pH 4.5. The optimum concentration, evaluated by a Ringbom plot, is 2.5-9.8 pg ml-‘. The Sandell sensitivity is 0.02 ,ug Co cm”. The molar absorptivity is 3.0 X lo3 1 mol-’ cm-‘. The metal/NTC ratio, as found by Job’s method of continuous variations, is 1: 1. Thus NTC[ Co( SCN),] is probably responsible for the absorption. The standard deviation for a mean absorbance of 0.345 corresponding to 6.5 pg ml-’ of cobalt was found to be 0.006. Effect of diverse ions. Synthetic solutions containing known amounts of cobalt(I1) and varying amounts of diverse ions were prepared and the recommended procedure was followed. An error of 2% in the absorbance reading was considered tolerable. In the determination of 8.4 bg ml-’ cobalt, the anions tolerated (in pg ml-’ given in parentheses) were as follows: fluoride, chloride, iodide, nitrate (1000 each); bromide (625); phosphate (1250);
305
thiosulphate (600); nitrite (1200); oxalate (1280); citrate or tartrate (1800); and thiourea (150). The metal ions tolerated were: Ca(II), Sr(II), Ba(II), Ni(II), Cd(H), Mn(II), Al(III), Ti(IV) (250 pg ml-’ each); Zn(I1) or Fe(I1) (50 pg ml-‘); and Cu(I1) (10 pg ml-‘). EDTA interfered seriously. All other metal ions which form coloured complexes [l] with thiocyanate also interfere. However, iron(II1) (400 pg ml-‘) can be masked with fluoride. The tolerance for copper can be improved to 50 pg ml-’ by using thiourea as masking agent. Similarly, a mixture of citrate and tartrate improves the tolerance for nickel to 1000 pg ml-‘. Determination of cobalt in alloys. Weighed samples (0.1 g) of “K” Monel wire and Nilo “K” wire were dissolved in concentrated nitric acid. Excess of acid was boiled out and the residues were dissolved in 1000 and 50 ml of doubly distilled water, respectively, for the Nilo and Monel wires. The recommended procedure was applied to 3-4 ml of these solutions to quantify the cobalt content. To mask copper, nickel and iron in these alloys, the requisite amounts of thiourea, citrate/tartrate (3 mg each) and fluoride (15 mg) were added. The results are reported in Table 1. TABLE
1
Determination
of cobalt in alloys Cobalt (%)
Alloy
Reported “K” Monel wire Nilo “K” wire aMean of 6 determinations TABLE
0.51
R.s.d. (%) Founda 0.50
17.4
4.8 2.1
17.6
with relative standard deviation (r.s.d.).
2
Sensitivities of thiocyanate (II)
methods for the spectrophotometric
determination
of cobalt-
Other reactant
Wavelength (nm)
Sandell sensitivity (pg Co cm-‘)
Ref.
(Isoamyl alcohol) Tetraphenylarsonium (chloroform) Methyldiantipyrylmethane Tri-n-butylammonium 2,3,5-Triphenyltetraolium Malachite green Neotetrazolium chloride
620 620
0.055 0.034
1, 5 1, 5
625 615 625 630 620
0.075 0.045 0.055 0.0007 0.02
5 5 1, 6 4 This work
306
Comparison with other methods based on thiocyanates. The present method is better in sensitivity than most thiocyanate methods. Sandell sensitivities are compared in Table 2. The high molecular weight of NTC is responsible for better sensitivity. However for malachite green the higher values are reported in CCL, + cyclohexanone medium but the colour fades too quickly. The Co-SCN-NTC ion association complex formed in the presence of less amounts of SCN- also has e value 6.4 X lo4 1 mol-’ cm-’ in cyclohexane medium but suffers from the poor stability. REFERENCES 1 E. B. Sandell, Calorimetric Determinatioh of Traces of Metals, 3rd edn., Interscience, New York, 1959, p. 414. 2 F. D. Snell, Photometric and Fluorimetric Methods of Analysis (Metal), Part I, Wiley, New York, 1978. 3 E. B. Sandell and H. Onishi, Photometric Determination of Traces of Metals, 4th edn., Wiley, New York, 1979, p. 536. 4 L. I. Kotelyanskaya and P. P. Kish, Zh. Anal. Khim., 28 (1973) 1999. 5 I. V. Pyatnitskii, Analytical Chemistry of Cobalt, Israel Programme for Scientific Translations, Jerusalem, 1966, p. 104. 6 M. C. Mehra and D. LeBlanc, Microchem. J., 24 (1979) 435.