The Determination of Micro Amounts of Polythionates. IV. A Photometric Method for the Determination of Tetrathionate by Decomposition with Sodium Sulfite

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Abstract
A new photometric method for the determination of tetrathionate by means of its decomposition with sulfite has been developed. It is based on the formation of thiosulfate equivalent to the tetrathionate, and on the photometric determination of thiosulfate by measuring its decoloration of an iodine solution. 5×10−6–1×10−3 m (1 mg./l.–0.2 g./l.) of tetrathionate can be determined easily and rapidly within a range of error of ±2×10−6–2×10−5 m (0.4 mg./l.–4 mg./l.) by using 20 ml. of a sample. The tetrathionate solution is decomposed with sodium sulfite in a slightly alkaline medium. After the completion of the decomposition reaction, a formaldehyde solution is added to mask the interference of the excess sulfite remaining and this solution is acidified with acetic acid. Then the standard iodine solution is added. The absorbancy of the solution is measured at the wavelength of 372 or 440 mμ 20 min. after the addition of an iodine solution. By this method tetrathionate can be determined, even when it is coexistent with thiosulfate and sulfite in up to similar amounts. Many substances, such as aluminum, calcium, cadmium, magnesium, zinc, silicate, nitrate, sulfate, phosphate, chloride and fluoride, do not interfere, but iron(II, III), manganese(II), copper(II) and sulfide do interfere.