Standardising potassium permanganate solution using oxalic acid

The oxalate ion, C2O42–, is colourless The oxalate ion, C2O42–, is colourless. It will reduce MnO4– to Mn2+ while itself being oxidised to CO2. C2O42– → 2CO2 + 2e– MnO4– + 8H+ + 5e– → Mn2+ + 4H2O 5C2O42– + 2MnO4– + 16H+ → 5CO2 + 2Mn2+ + 8H2O Oxalic acid is a weak acid, releasing few H+ ions into solution. It is necessary to add extra acid in the form of dilute sulfuric acid. Why is hydrochloric acid unsuitable? Because permanganate oxidises Cl– to Cl2. The reaction is slow, so we heat the oxalate mixture to around 70 °C before titrating in the permanganate.

A standard solution of oxalic acid is prepared. This solution was made by dissolving 3.578 g of (HCOO)2.2H2O in water and making it up to 1.000 L.

20.0 mL of oxalic acid is pipetted into each of 3 conical flasks.

About 20 mL of dilute sulfuric acid is added to each flask.

The flasks are heated until they are hot (above 70 °C)

Permanganate solution is added from the burette. At first the purple solution quickly turns colourless, but as the reaction proceeds it takes longer for the purple colour to completely disappear.

Just before the end-point, use a wash bottle to rinse any unreacted permanganate off the sides of the flask. The end-point is the first drop of pink permanganate that is not decolourised. It is very faint!

This has been overshot. This is correct.