Paragraph 3 PRINCIPLE

The cupric ions are reduced in an acidic medium with potassium iodide:

$$2\mathrm{Cu}+++\mathrm{4I}-\mathrm{\to }\mathrm{2CuI}+\mathrm{I}2\mathrm{.}$$

The iodine released in this way is titrated with a standard sodium thiosulfate solution in the presence of starch as an indicator in accordance with:

$$\mathrm{I}2+\mathrm{2Na}2\mathrm{S}2\mathrm{O}3\mathrm{\to }2\mathrm{NaI}+\mathrm{Na}2\mathrm{S}4\mathrm{O}6\mathrm{.}$$