<P> When iodide is added to a solution of hexacyanoferrate (III), the following equilibrium exists: </P> <Dl> <Dd> 2 (Fe (CN)) + 2 I ⇌ 2 (Fe (CN)) + I </Dd> </Dl> <Dd> 2 (Fe (CN)) + 2 I ⇌ 2 (Fe (CN)) + I </Dd> <P> Under strongly acidic solution, the above equilibrium lies far to the right hand side, but is reversed in almost neutral solution . This makes analysis of hexacyanoferrate (III) troublesome as the iodide and thiosulfate decomposes in strongly acidic medium . To drive the reaction to completion, an excess amount of zinc salt can be added to the reaction mixture containing potassium ions, which precipitates the hexacyanoferrate (II) ion quantitatively: </P>

Why excess of ki is used in iodometric titration