Spectrophotometric Determination of Tetrathionate by Means of Its Alkaline Decomposition

Abstract

A method has been proposed for the determination of tetrathionate. It is based on the formation of thiosulfate equivalent to tetrathionate by means of its alkaline decompositi on and then the spectrophotometric measurement of an excess iodine as triiodide for the reaction with the thiosulfate, after removing the sulfite and the sulfide yielded by the decomposition of tetrathionate. The recommended procedure is as follows. Place 10ml of a sample solution containing tetrathionate up to 3.7 x 10-4mol/l and 2m1 of 1 mol/l sodium carbonate into a 50ml beaker, and then allow the mixture solution to stand at 40°C for 60 min for driving the alkaline decomposition of tetrathionate to completion. To this mixture, add 6 drops of zinc carbonate hydroxide solution prepared by mixing 1.5% (w/v) sodium carbonate with 1%(w/v) zinc acetate in equal volume ratio, and 2ml of 1mol/l acetic acid. Filter off the precipitate of zinc sulfide formed, wash the precipitate and the filter paper with redistilled water and then transfer the filtrate and the washings into a 50ml volumetric flask. After adding, 1ml of 0.05mol/l formaldehyde solution, allow the mixture to stand for 1min to mask completely the sulfite formed. Then, add 3ml of glacial acetic acid and 4.8m/l of 1.0x10^-3mol/l standard iodate in 0.87mol/l iodide solution. Fill the flask to the mark with redistilled water, and measure the absorbance of iodine-iodide complex against distilled water at 350nm. e absorbance for tetrathionate decreased with an increase in the concentratio n of tetrathionate, but the calibration graph for tetrathionate exhibited a straight line. In 11 results f or a 10ml aliquot of the sample solution containing 2.00/2mol of tetrathionate, the present method gave a mean value of 2.00, μmol of tetrathionate with a relative standard deviation of 0.59%.