Preparation and Standardization of Potassium Permanganate
Aim: To perform the standardization of potassium permanganate.
- Apparatus: Beaker, Funnel, Pipette, Burrette.
- Chemicals: Potassium bromate, Sodium thiosulphate, Sulphuric acid, Potassium iodide, Starch indicator.
Iodometry is one of the most important redox titration methods. Iodine reacts directly, fast, and quantitively with many organic and inorganic substances. Thanks to its relatively low, pH-independent redox potential, and reversibility of the iodine/iodide reaction, iodometry can be used both to determine the amount of reducing agents (by direct titration with iodine) and of oxidizing agents (by titration of iodine with thiosulfate). In all cases, the same simple and reliable method of endpoint detection, based on the blue starch complex, can be used. The reversible iodine/iodide reaction mentioned above is
2I− ←→ I2 + 2e−
and obviously, whether it should be treated as oxidation with iodine or reduction with iodides depends on the other redox system involved.
The second important reaction used in iodometry is the reduction of iodine with thiosulfate:
2S2O32−+ I2 → S4O62−+ 2I−
In the case of both reactions, it is better to avoid low pH. Thiosulfate is unstable in the presence of acids, and iodides at low pH can be oxidized by air oxygen to iodine. Both processes can be a source of titration errors.
Iodine is very weakly soluble in water and can be easily lost from the solution due to its volatility. However, in the presence of excess iodide, iodine creates I3 – ions. This lowers free iodine concentration and such solutions are stable enough to be used in lab practice. Still, we should remember that their shelf life is relatively short (they should be kept tightly closed in dark brown bottles, and standardized every few weeks). Iodine solutions are prepared by dissolving elemental iodine directly in the iodide solution. Elemental iodine can be prepared very pure through sublimation, but because of its high volatility, it is difficult to weigh. Thus use of iodine as a standard substance, although possible, is not easy nor recommended. Iodine solutions can be easily normalized against arsenic (III) oxide (As2O3) or sodium thiosulfate solution.
It is also possible to prepare iodine solutions by mixing potassium iodide with potassium iodate in the presence of strong acid:
5I− + IO3− + 6H+ → 3I2 + 3H2O
Potassium iodate is a primary substance, so the solution prepared in this way can have exactly a known concentration. However, this approach is not cost-effective and in lab practice, it is much better to use iodate as a primary substance to standardize thiosulfate, and then standardize iodine solution against thiosulfate.
Iodine in water solution is usually colored strong enough so that its presence can be detected visually. However, close to the endpoint, when the iodine concentration is very low, its yellowish color is very pale and can be easily overlooked. Thus for endpoint detection starch solutions are used.
Iodine gets adsorbed on the starch molecule surface and the product of adsorption has a strong, blue color.
In the presence of small amounts of iodine adsorption and desorption are fast and reversible. However, when the concentration of iodine is high, it gets bonded with starch relatively strong, and desorption becomes slow, which makes detection of the endpoint relatively difficult. Luckily high concentrations of iodine are easily visible, so if we are using thiosulfate to titrate a solution that initially contains high iodine concentration, we can titrate till the solution gets pale and add starch close to the endpoint. In the case of titration with iodine solution, we can add starch at the very beginning, as high iodine concentrations are not possible before we are long past the endpoint.
At elevated temperatures adsorption of the iodine on the starch surface decreases, so titrations should be done in the cold.
Finally, it is worthy of noting that starch solutions, containing natural carbohydrates, have to be either prepared fresh, or conserved with an antibacterial agent like mercuric iodide HgI2.
The two most important solutions used in iodometric titrations are the solution of iodine and the solution of sodium thiosulfate. Both substances can be easily obtained in a pure form, but their other characteristics (volatility, hard controlling the amount of water of crystallization) make them difficult to use as a primary standard.
It is also worth mentioning that both solutions are not quite stable and they cannot be stored for a prolonged period. Iodine can be lost from the solution due to its volatility, while thiosulfate slowly decomposes giving off elemental sulfur. The latter process is easily visible, as thiosulfate solutions become slightly cloudy with time.
(A) Potassium permanganate (0.02 M): Dissolve 3.2 g of potassium permanganate in 1000 ml of water; heat in a water bath for 1 hour to stand for 2 days and filter through glass wool.
(B) Standardization: To 25 ml of solution in a glass-stopper flask add 2 g of potassium iodide, followed by 10 ml of 1 M sulphuric acid.
Titrate the liberated iodine with 0.1 M sodium thiosulphate using 3 ml of starch solution, added towards the end of the titration, as an indicator. Perform a blank determination and make the necessary correction.
1 ml of 0.1 M sodium thiosulfate is equivalent to 0.003161 g of KMnO4.
Observation Table of Preparation and Standardization of Potassium Permanganate
|1.||Burette solution||Sodium thiosulphate solution|
|2.||Conical flask solution||Potassium iodide, sulphuric acid|
|4.||Endpoint||Yellow to blue|
|Burette reading B.R (ml)||Trial 1||Trial 2||Trial 3||Average (BR)|
Factor: 1 ml of 0.1 M sodium thiosulphate is equivalent to 0.003161 g of KMnO4.
Result: The molarity of potassium permanganate is …… M.
Make sure you also check our other amazing Article on : Standardization of Sodium Thiosulphate