Redox Titration Calculations (Edexcel International A Level Chemistry): Revision Note

Redox Titration - Calculations

Redox Titrations

• In a titration, the concentration of a solution is determined by titrating with a solution of known concentration.

• In redox titrations, an oxidising agent is titrated against a reducing agent

• Electrons are transferred from one species to the other

• Indicators are sometimes used to show the endpoint of the titration

• However, most transition metal ions naturally change colour when changing oxidation state

• There are two common redox titrations you should know about manganate(VII) titrations and iodine-thiosulfate titrations

Potassium manganate(VII) titrations

• In these redox titrations the manganate(VII) is the oxidising agent and is reduced to Mn²⁺(aq)

• The iron is the reducing agent and is oxidised to Fe³⁺(aq) and the reaction mixture must be acidified, to excess acid is added to the iron(II) ions before the reaction begins

• The choice of acid is important, as it must not react with the manganate(VII) ions, so the acid normally used is dilute sulfuric acid

  • As it does not oxidise under these conditions and does not react with the manganate(VII) ions

Iodine-Thiosulfate Titrations

• A redox reaction occurs between iodine and thiosulfate ions:

2S₂O₃^2– (aq) + I₂ (aq) → 2I^–(aq) + S₄O₆^2– (aq)

• The light brown/yellow colour of the iodine turns paler as it is converted to colourless iodide ions

• When the solution is a straw colour, starch is added to clarify the end point

• The solution turns blue/black until all the iodine reacts, at which point the colour disappears.

• This titration can be used to determine the concentration of an oxidizing agent, which oxidizes iodide ions to iodine molecules

• The amount of iodine is determined from titration against a known quantity of sodium thiosulfate solution

Redox Titration - Uncertainty

Percentage Uncertainties

• Percentage uncertainties are a way to compare the significance of an absolute uncertainty on a measurement

• This is not to be confused with percentage error, which is a comparison of a result to a literature value

• The formula for calculating percentage uncertainty is as follows:

Percentage uncertainty =

Adding or subtracting measurements

• When you are adding or subtracting two measurements then you add together the absolute measurement uncertainties

• For example,

  • Using a balance to measure the initial and final mass of a container

  • Using a thermometer for the measurement of the temperature at the start and the end

  • Using a burette to find the initial reading and final reading

• In all these examples you have to read the instrument twice to obtain the quantity

• If each time you read the instrument the measurement is ‘out’ by the stated uncertainty, then your final quantity is potentially ‘out’ by twice the uncertainty

Reducing uncertainty in titrations

• In titrations, uncertainty can be reduced by adjusting the concentration of the known liquid, which is usually in the burette

  • This is to make it so that the burette readings are larger

• For example, if a burette had an uncertainty of ±0.1 cm³, the initial reading was 0.0 cm³ and the final reading was 5.0 cm³ 

  • The titre is 5.0 cm³ 

  • The percentage uncertainty is, therefore,  = 4%

• If the concentration of the chemical in the burette was diluted so that the initial reading was 0.0 cm³ and the final reading was 30.0 cm³

  •  The titre is 30.0 cm³ 

  • The percentage uncertainty is, therefore, = 0.67%

• Overall, to reduce the uncertainty in burette readings, you:

  • Dilute the chemical in the burette

  • This means that more will be required for the titration

  • Therefore, the overall uncertainty in the burette readings is reduced