Spectrophotometric determination of micro amounts of thiosulfate by cyanolysis and solvent extraction

Abstract

In the previous paper, a spectrophotometric method for the determination of ultramicro amounts of polythionate when only one of tetra-, penta- or hexathionate is present has been described; the method is based on the formation of thiocyanate equivalent to the polythionates and on the extraction of an ion-pair between Methylene Blue and thiocyanate with an organic solvent. In the present paper, a spectrophotometric method for the determination of thiosulfate has been devised as a fundamental experiment for the determination of polythionates when two species of them (tetra-, penta- and hexathionate) in amounts such as 10^-6 M coexist. The method is based on the formation of thiocyanate from thiosulfate by the following reaction in the presence of copper(II):
S₂O₃^2-+CN^-=SO₃^2-+SCN^-
and on the extraction of an ion-pair between Methylene Blue and thiocyanate with 1, 2-dichloroethane. The analytical procedure is as follows.
Add 10.0 ml of sample solution, 0.5 ml of 0.13 M sodium carbonate solution, 2.0 ml of 0.05 M potassium cyanide solution, and 1.0 ml of 0.06 M copper(II) sulfate solution to an approximately 50 mlseparatory funnel; the pH of the solution is thereby brought to 7.9. Then, shake the mixture vigorously by hand and leave the reacted solution for 20 minutes in order to convert the thiosulfate quantitatively to thiocyanate. To this mixture, add 1.5 ml of 0.2 M ammonium iron (III) sulfate in 0.33 N sulfuric acid solution, 1.2 ml of 2.0 × 10^-3 M Methylene Blue solution and 10.0 ml of 1, 2-dichloroethane. Shake the solutions for about 3 minutes to extract the ion-pair formed between Methylene Blue and thiocyanate. When the two layers have separated, transfer the organic layer to an approximately 15 ml glass tube equipped with a glass stopper, and add some amount of anhydrous sodium sulfate. Shake the mixture vigorously by hand until the solution becomes transparent, and measure the absorbance of the clear solution against dichloroethane at 657 nm.
The effects of such factors as pH, reaction time, amounts of cyanide and copper(II) and way of shaking, on the cyanolysis of thiosulfate in amounts such as 10^-6 M were discussed, and the conditions for thiosulfate to be completely converted into thiocyanate were established. The serious interference of copper(II) used as the catalyst, which may be caused by the formation of cyano-copper(I) complex anion, could be removed by an addition of iron(III). The method is suitable for the determination of 2.5 × 10 ^-712 × 10 ^-6 M thiosulfate.