Why must thiosulfate solution be standarized every couple of days? Please explain with an equation. Thanks.

Answer 1

I quote from Fundamentals of Analytical Chemistry by Skoog/West/Holler, sixth edition (1992) page 374-5--
"Although sodium thiosulfate solutions are resistant to air-oxidation, they do tend to decompose to give sulfur and hydrogen sulfite ion:
S2O3^2- + H^+ ==> HSO3^- + S(s)

Variables that influence the rate of this reaction include pH, presence of microorganisms, solution concentration, presence of copper(II) ions, and exposure to sunlight. These variables may cause the concentration of a thiosulfate solution to change by several percent over a period of a few weeks. On the other hand, proper attention to detail will yield solutions that need only occasional restandardization."

My biggest problem through the years has been the microorganism problem and I always advised my students to restadardize if the solution was more than a week old.

Answer 2

Thank you DrBob222