Answer to Question #20398 in Inorganic Chemistry for Bob

Question

How might you determine the concentration of a strong acid using a standard potassium thiosulfate solution, potassium iodide, and a standardized solution of sodium thiosulfate?

Answer 1

Thisis example of Redox Titration and connection between the reactantsis below
A standard reaction 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 + 2S₂O₃^2- = S₄O₆^2- + 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).