permanganate will be present. The excess is shown when the reacting mixture turns
pale pink. The half-reaction for the reduction in acid solutionis:
MnO 4 (aq)8H(aq)5eMn^2 (aq)4H 2 O(l)
pink colourlessTwo common reactions involving oxidation by potassium permanganate are:
1.Oxidation of iron(II) to iron(III):
Fe^2 (aq)Fe^3 (aq) + e2.Oxidation of oxalates to carbon dioxide. This reaction needs to be carried out at
70 C, because it is too slow at room temperature:
C 2 O 42 (aq)2CO 2 (g) + 2ePotassium dichromate (or potassium dichromate(VI), K 2 Cr 2 O 7 ) is also used as an
oxidizing agent in similar reactions to those described for potassium permanganate.
In acid solution, it reacts according to the following half-reaction:
Cr 2 O 72 (aq)14H(aq)6e2Cr^3 (aq)7H 2 O(l)
orange greenThe green colour of the chromium(III) ion ‘masks’ the orange colour of the dichro-
mate ion, so the oxidizing agent cannot act as a self-indicator. The solution is used
together with a few drops of a redox indicator, such as diphenylamine. This indicator is
oxidized to an intense blue compound in the presence of a slight excess of dichromate.
VOLUMETRIC ANALYSIS 143Example 9.5
Iron(II) sulfate is oxidized by potassium permanganate in acid
solution. The overall ionic equation is5Fe^2 (aq) + MnO 4 (aq) + 8H(aq)Mn^2 (aq) + 4H 2 O(I) + 5Fe^3 (aq)What volume of 0.010 mol dm^3 iron(II) sulfate will be oxidized
by 25.00 cm^3 of 0.020 mol dm^3 permanganate solution?Answer
Work out the molar relationship between the two reactants:5 mol Fe^2 1 mol MnO 4 Express this ratio as a fraction:amount of Fe^2 in moles
5
amount of MnO 4 in moles 1Since the amount of substance in molesvolumemolar concentration, we can
substitute:volmolar conc Fe^2 5
volmolar conc MnO 4
1Substituting values into the equation:vol0.010 5
25/10000.020
1Therefore, the volume of Fe^2 0.250 dm^3 250 cm^3.