Redox Titration Calculations (Edexcel International A Level Chemistry)

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

   Answers:

   Balance the electrons:

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

5Fe²⁺ (aq) → 5Fe³⁺ (aq) + 5e^-

   Add the two half equations:

MnO₄^- (aq) + 8H⁺ (aq) + 5Fe²⁺ (aq) → Mn²⁺ (aq) + 4H₂O (l) + 5Fe³⁺ (aq)

• Manganate(VII) titrations can be used to determine:
  • The percentage purity of iron supplements
• Percentage purity =  
  • The formula of a sample of hydrated ethanedioic acid

   Answer:

• • MnO₄^- (aq) + 8H⁺ (aq) + 5Fe²⁺ → Mn²⁺ (aq) + 5Fe³⁺ (aq) + 4H₂O (l)
  • 1 : 5 ratio of MnO₄^- : Fe²⁺
  • Number of moles of MnO₄^- (aq)5.13 x 10^-4 moles 
  • Moles of iron(II) = 5 x 5.13 x 10^-4 = 2.565 x 10^-3 moles
  • Mass of iron(II) = 55.8 x 2.565 x 10^-3 = 0.143127 g
  • Percentage by mass = 14.9%

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

   Answer:

• • One mole of ClO^- (aq) produces one mole of I₂ (aq) which reacts with two moles of 2S₂O₃^2- (aq)
    • Therefore, 1 : 2 ratio of ClO^- (aq) : S₂O₃^2- (aq)
  • Number of moles of S₂O₃^2- (aq) 1.26 x 10^-3 moles 
  • Number of moles of I₂ (aq) and ClO^- (aq) in 25.0 cm³ 6.30 x 10^-4 moles 
  • Number of moles of ClO^- (aq) in 250.0 cm³ = 6.30 x 10^-4 x 10 = 6.30 x 10^-3 moles 
  • The 250.0 cm³ was prepared from 10.0 cm³ bleach
    • 10 cm³ bleach = 6.30 x 10^-3 moles of ClO^- ions
    • 1.0 dm³ bleach = 0.630 moles of ClO^- ions
    • Therefore, the concentration of ClO^- ions in the bleach is 0.630 mol dm^-3 

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