Stability of sodium thiosulfate solution and constant-current coulometric iodometric titration of potassium iodate

The effective purity of potassium iodate was determined by adding it to a sufficiently acidified potassium iodide solution, followed by the addition of a measured excess of a standard sodium thiosulfate solution and back-titrating the excess with anodically generated iodine. First, the preparation and stability of a sodium thiosulfate solution were investigated. If pure, freshly boiled and cooled water was used to dissolve an analytical-grade sodium thiosulfate pentahydrate, the decomposition of the thiosulfate solution due to oxygen, carbon dioxide and thiobacteria was inhibited, and the concentration of ca. a 0.5 mol kg(-1) thiosulfate solution stored in a Teflon bottle was unchangeable, even immediately after preparation, and was stable for at least one month without the addition of any preservatives. Tightly stoppered plastic containers were appropriate for preserving a thiosulfate solution. The effective purity of a potassium iodate reference material (certified value 99.98% +/- 0.01%) was found to be 99.968% with a relative standard deviation of 0.018% (n = 10), and the assay of potassium iodate was successfully determined by coulometric iodometric titration with the standard thiosulfate solution.