Iodometric Titration of Iron (II) Sulfate

This blog post describes the procedure for carrying out an iodometric titration of iron (II) sulfate. The iodometric method of analysis is based on the oxidation-reduction properties of the system [I3]–/3 I–. The procedure involves dissolving a sample of iron (II) sulfate in sulfuric acid and phosphoric acid, and then titrating the solution with a standard solution of potassium dichromate. The percentage of Fe2+-ions in the iron (II) sulfate can be calculated from the results of the titration.

Questions

• What is the purpose of adding phosphoric acid to the solution?
• What is the oxidation-reduction potential of the system [I3]–/3 I–?
• What is the semi-reaction that proceeds in iodometric determinations?

Answers

• Phosphoric acid is added to the solution to fix the Fe3+-ions formed, so that they do not interfere with the titration.
• The oxidation-reduction potential of the system [I3]–/3 I– is 0.545 V. This means that there are a number of reducers (E0 < 0.545 V) that can be oxidized by free iodine, and a number of oxidants (E0 > 0.545 V) that can be reduced by iodide-ions.
• The semi-reaction that proceeds in iodometric determinations is: [I3]– + 2 e → 3 I–