Core Practical 11: Redox Titration of Iron Tablets

Reaction Equation

Fe²⁺(aq) + MnO₄⁻(aq) + H⁺(aq) → Fe³⁺(aq) + Mn²⁺(aq) + H₂O(l)

Full ionic equation:
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O

Safety

• Wear eye protection.
• 1.5 mol dm⁻³ sulfuric acid is an irritant.
• Perform a risk assessment using CLEAPSS Hazcards 81 and 98A.

Apparatus and Chemicals

• 5 iron tablets (containing iron(II) sulfate)
• 100 cm³ of 1.5 mol dm⁻³ sulfuric acid
• 100 cm³ of 0.005 mol dm⁻³ potassium manganate(VII)
• Pestle and mortar, filter paper and funnel
• 250 cm³ volumetric flask
• 25.0 cm³ pipette and 50.0 cm³ burette
• White tile, conical flask, beakers
• Balance (2 d.p.), glass rod

Procedure

1. Crush 5 iron tablets in a pestle and mortar.
2. Transfer powder into a weighing boat and record mass. Reweigh empty boat to find exact sample mass.
3. Add 100 cm³ of 1.5 mol dm⁻³ H₂SO₄ and stir to dissolve.

4. Filter into a 250 cm³ volumetric flask, rinsing beaker and filter paper.
5. Make up to 250 cm³ with distilled water and shake well.
6. Pipette 25.0 cm³ of this solution into a conical flask.
7. Titrate with KMnO₄ solution until the first permanent pink colour appears.

8. Repeat until two concordant titres are obtained.

Data Analysis

Key equations:
n = c × V / 1000
n = mass / M_r

Sample data:
Concentration of KMnO₄ = 0.005 mol dm⁻³
Average titre = 21.40 cm³

Step 1: Moles KMnO₄ = 0.005 × 21.40 / 1000 = 0.000107 mol
Step 2: Fe²⁺ : MnO₄⁻ ratio = 5 : 1
Moles Fe²⁺ = 0.000107 × 5 = 0.000535 mol (in 25.0 cm³)
Step 3: Moles Fe²⁺ in 250 cm³ = 0.000535 × 10 = 0.00535 mol
Step 4: Mass Fe = 0.00535 × 55.8 = 0.298 g ≈ 0.3 g
Mass per tablet: 0.3 ÷ 5 = 0.060 g = 60 mg
Published value = ~65 mg

Matt’s Exam Tips

• Pink colour at end point = unreacted KMnO₄ (self-indicating).
• Invert volumetric flask to ensure complete mixing.
• Wash all glassware with distilled water to avoid loss of reactant.
• Use a white tile to spot the colour change clearly.

Common Sources of Error

• Iron not fully dissolved – warm gently if needed.
• Solution not mixed properly – always invert the flask.
• Loss during filtering – rinse filter paper and funnel.
• End-point misjudged – swirl consistently and view against a white background.