Iodometry

Iodometry or iodometric titration is a family of analytical techniques involving the redox chemistry of iodine. Unlike in other types of titrations, no indicator is required, as the appearance or disappearance of elementary iodine indicates the end point.

Procedure

For a sample of fixed volume, an excess but known amount of iodide is added, which the oxidizing agent then oxidizes 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 thiosulfate solution to give iodide again using starch indicator:

I₃⁻ + 2 e⁻ ⇌ 3 I⁻ (E^o = + 0.5355 V)

Together with reduction potential of thiosulfate:

S₄O₆²⁻ + 2 e⁻ ⇌ 2 S₂O₃²⁻ (E^o = + 0.08 V)

The overall reaction is thus:

I₃⁻ + 2 S₂O₃²⁻ → S₄O₆²⁻ + 3 I⁻ (E_reaction = + 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 the deep blue color is due to the decomposition of the iodine-starch clathrate marks the end point.

The reducing agent used does not necessarily need to be thiosulfate; tin(II) chloride, sulfites, sulfides, arsenic(III), and antimony(III) salts are commonly used alternatives.

S₂O₃²⁻ + 2 H⁺ → SO₂ + S + H₂O

Some reactions involving certain 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 below is reversible at pH < 4:

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

The volatility of iodine is often considered a source of error for the titration, though this can be effectively prevented by ensuring an excess iodide is present and cooling the titration mixture. Strong light, ass well as certain ions like nitrite and copper ions catalyzes the conversion of iodide to iodine, so these interferences 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

References

Relevant Sciencemadness threads

• Iodometry titrating for Fe3+ content
• iodometric titration confusion