Redox Titrations (AQA A Level Chemistry)

Redox Titrations

• Redox titration involves an oxidising agent being titrated against a reducing agent

• Electrons are transferred from one species to another

• In acid-base titrations indicators are used to show the endpoint of a reaction; however redox titrations using transition metal ions naturally change colour when changing oxidation state, so indicators are not always necessary

• They are said to be 'self-indicating'

• Two common transition metal ion redox titrations are manganate(VII) and dichromate(VI) titrations

Manganate(VII) Titrations

• Potassium manganate(VII) is an oxidising agent and is a deep purple colour

• In acidic solutions it is reduced to the almost colourless manganese(II) ion (the ion is actually pink, but in low concentrations it is effectively colourless)

• The reduction equation for the manganate(VII) ion is

MnO₄^- (aq) + 8H⁺ (aq)  + 5e^-   →   Mn²⁺ (aq)  + 4H₂O (aq)

purple                                            colourless

• The potassium ion is a spectator ion so can be left out of the equations

• By convention the potassium manganate(VII) solution is placed in the burette, so that as it reacts with the reducing agent the solution becomes colourless

• At the endpoint the manganate(VII) ion becomes in excess so the first appearance of a permanent colour change marks the endpoint

• The colour seen is pink; if the colour is purple then you have overshot the endpoint and there is too much manganate(VII) in excess

• Analysis of iron in iron(II)sulfate tablets is typical manganate(VII) titration

Dichromate(VI) Titrations

• Potassium dichromate(VI), K₂Cr₂O₇, is another oxidising agent used in redox titrations

• The oxidation state of the chromium changes from +6 to +3 in the reaction:

Cr₂O₇^- (aq) + 14H⁺ (aq)  + 6e^-   →   2Cr³⁺ (aq)  + 7H₂O (l)

          orange                                                  green

• From the half equation you can see that the reaction needs hydrogen ions so the solution must be acidified with excess dilute sulfuric acid

• The colour change is hard to see as the dichromate(VI) ion is pale orange and the chromium(III) ion is pale green

• To enhance the endpoint an indicator called diphenylaminesulfonate is used which turns from colourless to purple at the endpoint

• To obtain the balanced equation in the analysis of iron(II) first work out the redox changes in the half equations, then balance the number of electrons transferred:

The steps in producing a balanced redox equation between iron(II) and dichromate(VI)