General Discussion

  Alkali persulphates (peroxydisulphates) can be evaluated by adding a known excess of an acidified iron (II) salt solution, and determining the excess of iron (II) by titration with standard potassium permanganate solution.

S₂O₈^2- + 2Fe²⁺ + 2H⁺ = 2Fe³⁺+ 2HSO₄^-

The reduction is complete in a few minutes at room temperature by adding phosphoric acid or hydrofluoric acid. Many organic compounds interfere.

Potassium permanganate solutions may be standardised using arsenic(III) oxide or sodium oxalate as primary standards: secondary standards include metallic iron, and iron(II) ethylenediammonium sulphate (or ethylenediamine iron(II) sulphate), FeSO₄,C₂H₄(NH₃)₂SO₄,4H₂O.

 

Preparation of 0.1N potassium permanganate

Weigh out about 3.2-3.25 g potassium permanganate, transfer it to a 1000mL beaker, add 1 L water, cover the beaker with a clockglass, heat the solution to boiling, boil gently for 15-30 minutes and allow the solution to cool to the laboratory temperature. Filter the solution through a funnel containing a plug of purified glass wool, or, more simply, through a sintered-glass or porcelain filtering crucible or funnel. Collect the filtrate in a vessel which has been cleaned with chromic acid mixture and then thoroughly washed with distilled water. The filtered solution should be stored in a clean, glass-stoppered bottle, and kept in the dark or in diffuse light except when in use: alternatively, it may be kept in a dark brown glass bottle.

Standardisation of permanganate solution

Sodium oxalate is obtained pure and anhydrous, and the ordinary material has a purity of at least 99.9 per cent. The best way is to make a more rapid addition of 90-95 per cent of the permanganate solution (about 25-35 mL.min ^-1) to a solution of sodium oxalate in 1 M sulphuric acid at 25-30 °C, the solution is then warmed to 55-60 °C and the titration completed, the last 0.5- 1 mL portion being added dropwise. The method is accurate to 0.06 per cent. Experimental details are given below.

 

2MnO₄^- +5H₂C₂O₄ + 6H⁺ = 2Mn²⁺ + 10CO₂ + 8H₂O

 

It should be mentioned that if oxalate is to be determined it is often not convenient to use the room temperature technique for unknown amounts of oxalate. The permanganate solution may then be standardised against sodium oxalate at about 80 °C using the same procedure in the standardisation as in the analysis.

Procedure

Dry some analytical grade sodium oxalate at 105-110 °C for 2 hours, and allow it to cool in a covered vessel in a desiccator. Weigh out accurately from a weighing bottle about 0.3 g of the dry sodium oxalate into a 600 mL beaker, add 240 mL of recently prepared distilled water, and 250 mL of 1M sulphuric acid. Cool to 25-30 °C and stir until the oxalate has dissolved. Add 90-95 per cent of the required quantity of permanganate solution from a burette at a rate of 25-35 mLmin^-1 while stirring slowly. Heat to 55-60 °C (use a thermometer as stirring rod), and complete the titration by adding permanganate solution until a faint pink colour persists for 30 seconds. Add the last 0.5-1 mL dropwise, with particular care to allow each drop to become decolorised before the next is introduced. For the most exact work, it is necessary to determine the excess of permanganate solution required to impart a pink colour to the solution. This is done by matching the colour produced by adding permanganate solution to the same volume of boiled and cooled dilute sulphuric acid at 55-60 °C. This correction usually amounts to 0.03-0.05 mL. Repeat the determination with two other similar quantities of sodium oxalate.

 

Procedure

Prepare an approximately 0.1 M solution of ammonium iron(II) sulphate by dissolving about 9.8 g of the solid in 200 mL of sulphuric acid (0.5 M) in a 250 mL graduated flask, and then making up to the mark with freshly boiled and cooled distilled water. Standardise the solution by titrating 25 mL portions with standard potassium permanganate solution (0.1N) after the addition of 25 mL sulphuric acid (0.5M).

Weigh out accurately about 0.3 g potassium persulphate into a conical flask and dissolve it in 50 mL of water. Add 5mL phosphoric (ortho) acid or 2.5 mL 35-40 per cent hydrofluoric acid (CARE!), 10 mL 2.5M sulphuric acid, and 50.0 mL of the 0.1 M iron (II) solution. After 5 minutes, titrate the excess of Fe²⁺ ion with standard 0.02 M potassium permanganate.

From the difference between the volume of 0.02 M permanganate required to oxidise 50 mL of the iron(II) solution and that required to oxidise the iron(II) salt remaining after the addition of the persulphate, calculate the percentage purity of the sample.

Method 2

Another procedure utilises standard oxalic acid solution. When a sulphuric acid solution of a persulphate is treated with excess of standard oxalic acid solution in the presence of a little silver sulphate as catalyst, the following reaction occurs:

H₂S₂O₈ + H₂C₂O₄ = 2H₂SO₄ + 2CO₂

The excess of oxalic acid is titrated with standard permanganate solution.

Prepare an approximately 0.05 M solution of oxalic acid by dissolving about 6.4 g of the compound and making up to 1000 mL in a graduated flask. Standardise the solution with standard (0.1N) potassium permanganate standard solution.

Weigh out accurately 0.3-0.4 g potassium persulphate into a 500 mL conical flask, add 50 mL of 0.05M oxalic acid, followed by 0.2 g of silver sulphate dissolved in 20 mL of 10 per cent sulphuric acid. Heat the mixture in a water bath until no more carbon dioxide is evolved (15-20 minutes), dilute the solution to about 100 mL with water at about 40 °C, and titrate the excess of oxalic acid with standard 0.1N potassium permanganate.