Equation between potassium iodate and sodium thiosulphate?

Answer 1

This reaction may be misunderstood as a direct reaction between the thiosulphate and iodate ions , however, in practice an iodide and acid mediated production of iodine from the iodate is used to react with the thiosulphate.

A standard reaction used to calibrate a solution of sodium thiosulphate is as follows:

Acid and potassium iodide are added to a solution of potassium iodate getting the following reaction:

KIO3 + 5KI + 3H2SO4 = 3I2 + 3K2SO4 + 3H2O

represented by the following ionic equation:

IO3- + 5I- + 6H+ = 3I2 + 3H2O

Thiosulpathe is titrated against this solution (effectively against iodine):

I2 + 2Na2S2O3 = Na2S4O6 + 2NaI

represented by the following ionic equation:

I2 + 2S2O32- = S4O62- + 2I-

where the dark brown coloured solution of iodine turns pale yellow and finally colourless as the reaction proceeds (starch is used as indicator after the pale yellow transition forming a black solution due to an iodine-starch complex which turns colourless upon further addition of thiosulphate).