Iodometry

Iodometry or iodometric titration is a family of analytical techniques involving the redox chemistry of iodine. Starch is usually as indicator (however some techniques use disappearence of purple colouration of iodine in chlorinated solvent for end-point determination).

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.

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. If reaction with iodine take some time, reaction mixture is kept in closed ground joint flask to prevent iodine sublimation. 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 elemental iodine or from potassium iodate and potassium iodide.

IO₃⁻ + 8 I⁻ + 6 H⁺ → 3 I₃⁻ + 3 H₂O

Solutions

• 0,01-0,1M I₂
• 0,01-01M Na₂S₂O₃
• KI (as solution of appropriate concentration or as solid)

Standardization

Potassium dichromate, bromate or iodate are often used as primary standards for sodium thiosulfate. Standards should be predried at 105 °C, but it is not necessary, all three standards aren't hygroscopic.

Preweighed standard is dissolved in water and acidified with 10 ml of 1+4 H₂SO₄. Excees of solid KI is added to the solution, flask is stoppered and left in dark for 10 minutes. Liberated iodine is then titrated with sodium thiosulfate solution. Two titrations are performed. Exact concentration is calculated from volume of thiosulfate and weight of standard.

Ascorbic acid or aresnic trioxide are used as primary standards for iodine. Sodium thiosulfate of exact concentration is used as secondary standard. Sodium thiosulfate is tirated in neutral solution, ascorbic acid in strongly acidic solution (HCl, H₂SO₄), aresnic trioxide at pH 8-9 in NaHCO₃ solution.

Application

Potassium iodide + sodium thiosulfate is used for determination of strong oxidizing agents. Some mild oxidizing agents like I₂, Cu²⁺, Fe³⁺ or MoO₄^2- can be also determine using this method.

Iodine is used for determination of few reducing agents. pH is very important at these reactions. Ascorbic acid, sulfite and sulfide are determined at strongly acidic conditions. Because sulfite and sulfide form gases in acidic environment, they are added from burette, while known volume of iodine is in titration flask. Sb³⁺ and As³⁺ are deterimned bicarbonate solution at pH 8-9. To prevent hydrolysis of Sb³⁺, excess of tartrate is added to titration flask. Formaldehyde is oxidized by iodine to formate in alkaline solution. Excess of know volume of iodine is added to alkaline solution of formaldehyde. Solution is acidified after completion of the reaction and leftover iodine is titrated with thiosulfate. Alkenes can be determined by reaction with excess of iodine (or IBr). Leftover iodine is then titrated with thiosulfate.

References

• Analyticka Chemie I (Analytical Chemistry I)

Relevant Sciencemadness threads

• Iodometry titrating for Fe3+ content
• iodometric titration confusion