Spectrophotometric Determination of Micro Amounts of Tetrathionate in Mixtures with Thiosulfate and Sulfite by Means of Its Sulfitolysist

Abstract

The reaction of tetrathionate with sulfite was investigated, and an optin. al pH for its sulfitolysis was found to be in the range 7.7, -12.5 when sulfitolyzed for 20min at room temperature. Under the same conditions as those for tetrathionate, the optimal pH for sulfitolysis was in the range 6.5-8.2 for pentathionate, and 6.9-9.5 for hexathionate. Therefore, each sulfitolysis of tetra-, penta-, and hexathionate could proceed to stoichiometric completion, so far as the pH of the reaction solution is kept in the range 7.7-8.2. Present method for the determination of tetrathionate is based on the formation of thiosulfate equivalent to the tetrathionate by means of its sulfitolysis, and the spectrophotometric measurement of an excess iodine for the thiosulfate. A method for the determination of tetrathionate, thiosulfate and sulfite mixture also is devised. This method consists of three procedures, and the three sulfur compounds in the mixtures give the following equivalents:
Procedure I S₄o₆^2-S₂o₈^2-
Procedure II S₂o₃^2-
Procedure III S₂o₈^2--2 So₈^2-
The absorbance obtained by Procedure I corresponds to the sum of tetrathionate and thiosulfate in the mixtures, because the sulfite is masked by the formaldehyde added for removal of the excess sulfite. The absorbance obtained by Procedure I, in which no sulfitolysis of tetrathionate was carried out, corresponds to the amount of thiosulfate in the mixtures. The absorbance obtained by Procedure III corresponds to the sum of the amounts of thiosulfate and twice as much as sulfite in the mixtures. Thus, the following equations can be derived: (S₄o₆^2-)=I-II, (S₂o₈^2-)=II, and (SO₃^2-)=(III-II)/2, where I, II, and III denote the molar concentration of thiosulfate which was determined from the calibration graph in Fig.1, using the absorbance obtained by Procedure I, II and DI, respectively. Table 2 reveals that the proposed method can be successfully applied to the determination of mixtures of tetra-thionate, thiosulfate and sulfite at various ratios.