Question

Titrating past the endpoint is a common error in titrations. How does adding excess Na2S2O3 affect the calculated moles of ClO?

Answer 1

Chlorine water cannot be titrated against thiosulfate solution because there is no clear end point. Therefore, ClO- is first titrated with KI solution which converts all of the ClO- to I2 which can be detected.

ClO^- + KI + 2H⁺ -----> I₂ + Cl^- + H₂O

As we cannot detect the end point of the above reaction, the I2 product willbe titrated with thiosulfate and starch as indicator. at the end point the color of the solution changes from violet to colorless.

2S₂O₃^2- + I₂ -----> 2I^- + S₄O₆^2-