SPECTROPHOTOMETRIC DETERMINATION OF MICRO-AMOUNTS OF TETRATHIONATE IN MIXTURES WITH THIOSULFATE AND SULFITE BY MEANS OF ITS SULFITOLYSIS

The reaction of tetrathionate with sulfite was investigated, and an optinal 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 thiosuifate equivalent to the tetrathionate by means of its sulfitolysis, and the spectrophotometric measurement' of an excess iodine for the thiosuifate. A method for the determination of tetrathionate, thiosuifate and sulfite mixture also is devised. This method consists of three procedures, and the three sulfur compounds in the mixtures give the following equivalents: [formula omitted] The absorbance obtained by Procedure I corresponds to the sum of tetrathionate and thiosuifate 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 thiosuifate in the mixtures. The absorbance obtained by Procedure W corresponds to the sum of the amounts of thiosuifate and twice as much as sulfite in the mixtures. Thus, the following equations can be derived : (S4O62-)=I-II, (S2O62-) = II. and (SO32-) = (III-II)/2, where I, II and III denote the molar concentration of thiosuifate which was determined from the calibration graph in Fig. 1, using the absorbance obtained by Procedure I, II and III respectively. Table 2 reveals that the proposed method can be successfully applied to the determination of mixtures of tetrathionate, thiosuifate and sulfite at various ratios. © 1979, The Chemical Society of Japan. All rights reserved.