Iodometry - Basic Principles

Basic Principles

Iodometry is commonly used to analyse the concentration of oxidizing agents in water samples, such as oxygen saturation in ecological studies or active chlorine in swimming pool water analysis. To a known volume of sample, an excess but known amount of iodide is added, which the oxidizing agents oxidizes iodide to iodine. Iodine dissolves in the iodide-containing solution to give triiodide ions, which have a dark brown color.

The triiodide ion solution is then titrated against standard thiosulphate solution to give iodide again using starch indicator:

I₃- + 2 e- ⇌ 3 I- (Eo = + 0.5355 V)

Together with reduction potential of thiosulfate:

S₄O₆2- + 2 e- ⇌ 2 S₂O₃2- (Eo = + 0.08 V)

The overall reaction is thus:

I₃- + 2 S₂O₃2- → S₄O₆2- + 3 I- (Eo = + 0.4555 V)

For simplicity, the equations will usually be written in terms of aqueous molecular iodine rather than the triiodide ion, as the iodide ion did not participate in the reaction in terms of mole ratio analysis.

The disappearance of deep blue color due to the decomposition of the iodine-starch clathrate marks the end point

The reducing agent used does not necessarily be thiosulfate, stannous chloride, sulphites, sulfides, arsenic(III), and antimony(III) are commonly used alternatives. At higher pH (> 8)) At low pH would also react with the thiosulfate:: :S₂O₃2− + 2 H+ → SO₂ + S + H₂O

Some reactions involving certain some reductants are reversible at certain pH, thus the pH of the sample solution should be carefully adjusted before the performing the analysis. For example, the reaction:

H₃AsO₃ + I₂ + H₂O → H₃AsO₄ + 2 H+ + 2 I-

is reversible at pH < 4.

The volatility of iodine is also a source of error for the titration, this can be effectively prevented by ensuring an excess iodide is present and cooling the titration mixture. Strong light, nitrite and copper ions catalyzes the conversion of iodide to iodine, so these should be removed prior to the addition of iodide to the sample.

For prolonged titrations, it is advised to add dry ice to the titration mixture to displace air from the erlenmeyer flask so as to prevent the aerial oxidation of iodide to iodine. Standard iodine solution is prepared from potassium iodate and potassium iodide, which are both primary standards):

IO₃- + 8 I- + 6 H+ → 3 I₃- + 3 H₂O