Question

Why do we need acedic condition for iodometric titration?

Answer 1

Iodometric titration needs to be done in a weak acid environment which is why we need to remember that:  1. The iodine solution used needs to be at pH < 8.5 because at a base pH iodine disproportionates (a particular kind of oxidoreduction reaction where one substance partly oxidises and partly reduces);  2. Sodium thiosulphate needs a neutral or weak acid environment to oxidise with tetrathionate (in an alkaline solution we would get sulphate oxidation);  3. In a strong acid environment thiosulphate decomposes to S2;  4. In acid environments the iodide is oxidised to iodine as in the reaction below:  O2 + 4I- + 4H+ ↔2I2 + 2H2O