The reaction between potassium dichromate and sodium thiosulfate is a redox reaction in which potassium dichromate acts as an oxidizing agent and sodium thiosulfate acts as a reducing agent. The balanced chemical equation for this reaction is:

K2Cr2O7 + 6 Na2S2O3 + 7 H2SO4 → Cr2(SO4)3 + 3 Na2SO4 + 7 S + 7 H2O

In this reaction, potassium dichromate is reduced from the +6 oxidation state of chromium to the +3 oxidation state, while sodium thiosulfate is oxidized from the -2 oxidation state of sulfur to the 0 oxidation state. The reaction proceeds through a series of complex steps, involving the formation of intermediate species such as chromium(IV) and chromium(V) complexes.

The overall effect of the reaction is the conversion of the bright orange potassium dichromate solution to a green chromium(III) sulfate solution, accompanied by the evolution of elemental sulfur as a yellow precipitate. The reaction is commonly used in analytical chemistry for the quantitative determination of chromium or thiosulfate, as well as in various industrial processes such as metal finishing and wastewater treatment.