Iodometric Titrations and Iodimetric
The reduction of free iodine to iodide ions and oxidation of iodide ions to free iodine takes place in these titrations.
I2 + 2e- → 2I- (reduction)
2I- → I2 + 2e- (oxidation)
These are divided into two types:
(a) Iodimetric titrations: These are the titrations where free iodine is used. Since it is hard to create the solution of iodine that is volatile and less soluble in water and it is reacted in potassium iodide solution.
KI + I2 → KI3 (Potassium tri-iodide)
This solution is first standardised previous to use. Along with the standard solution of I2. Substances like sulphite, arsenite, thiosulphate, are calculated.
(b) Iodometric titrations : In it, an oxidising agent is permitted to react in neutral medium or in acidic medium, along with excess of potassium iodide to liberate free iodine.
KI + oxidising agent → I2
Free iodine is titrated against a standard reducing agent generally along with sodium thiosulphate. Halogens, dichromates, cupric ion, peroxides, etc., can be calculated by this method.
I2 + Na2S2O3 → 2 NaI + Na2S4O6
2CuSO4 + 4KI → Cu2I2 + 2K2SO4 + I2
K2Cr2O7 + 6 KI + 7 H2So4 →
Cr2(SO4)3 + 4K2SO4 + 7H2O + 3I2
In iodimetric and iodometric titrations, starch solution is employed as an indicator. Starch solution gives blue or violet colour with free iodine. Last point the violet or blue colour disappears while iodine is totally changed to iodide.