Introduction Iodometric analysis is one of the most important volumetric procedures, because concent — Physical Chemistry — Thermodynamics Chemistry Question

Introduction
Iodometric analysis is one of the most important volumetric procedures, because concentrations of both oxidizing and reducing agents, can be accurately determined using this approach. The reaction between thiosulfate dianions and elemental iodine in a neutral or acidic solution is the basis of this method.

S₂O₃ 2– + I₂ → S₂O₆ 2– + 2 I –
blue colourless

For the determination of oxidizing agents an excess of potassium iodide and a small amount of an acid are added to the sample solution. The iodine formed in this reaction is titrated with sodium thiosulfate solution.
In contrast a back titration is typically performed for the determination of reducing agents in which a well defined excess of an iodine solution is added to the sample solution and the unreacted iodine is titrated with thiosulfate solution. Potassium iodate is used as a titrimetric standard for the standardization of the thiosulfate solution, because of its high stability and the fact that it can be produced in a very pure state. If an excess of potassium iodide is added to a well defined amount of potassium iodate in an acidic solution, an equivalent amount of iodine will be generated which can be titrated with sodium thiosulfate solution.
The practical exercise involves the preparation of potassium iodate and the determination of its purity by iodometric titration.

List of chemicals
* Acetic acid, aqueous sol., w(H₃CCOOH) = 0.05
* Ethanol, w(C₂H₅OH) = 0.96
* Hydrochloric acid, c(HCl) = 2 mol dm–3
* Potassium iodide, solid
* Potassium permanganate, solid
* Sodium thiosulfate, aqueous sol., c(Na₂S₂O₃) = 0.1 mol dm–3

Procedure 1: Preparation of potassium iodate
6 g of potassium permanganate are dissolved in 150 cm₃ of hot distilled water.
3 g of potassium iodide dissolved in a small amount of distilled water are added to the solution. The reaction mixture is heated on a boiling water bath for 30 min. The unreacted potassium permanganate is removed by the addition of ethanol. During this procedure, the supernatant liquid becomes colourless.
The resulting precipitate of manganese(IV) oxide is filtered off and the filtrate is acidified by the addition of acetic acid. The solution is concentrated by heating on a water bath until the product begins to crystallize. The solution is allowed to cool to room temperature. The crystalline product is filtered off and washed with a small amount of ethanol. More product can be isolated by further concentration of the mother liquor. The product can be recrystallized from water and dried at 110°C.
Record the yield of the product in g

Procedure 2: Iodometric determination of the purity of the isolated potassium iodate.
If a 25 mL burette is to be used in the determination take about 60 mg of the product prepared in procedure 1 (record the exact weight) and dissolve it in about 100 cm₃ of distilled water. Add 1 g of potassium iodide to the solution and slightly acidify with dilute hydrochloric acid. The solution is titrated with sodium thiosulfate solution (c(Na₂S₂O₃) = 0.1 mol dm–3) until it becomes colourless. Just before the end point 2 - 3 cm₃ of starch solution are added as an indicator.
Record the volume of the sodium thiosulfate solution used in cm₃

Preparation of the starch solution:
About 2 g of starch are suspended in 3 cm₃ of distilled water and the suspension vigorously stirred. The mixture is added to 300 cm₃ of boiling water and heated for about two min. Any undissolved starch should be removed by decanting.
The starch solution should be prepared as required, however, it can be kept for a longer period by the addition of a small amount of a mercury(II) iodide solution.