A-Level Chemistry 9701 / Topic 28 / A2

Titrate with a colour change.

Because transition metals have several stable oxidation states, their compounds make excellent oxidising and reducing agents. Manganate(VII) and dichromate(VI) titrations, and the iodine-thiosulfate method for copper, turn those colour changes into numbers.

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The key idea

Transition elements have variable oxidation states because the 3d and 4s electrons are close in energy. Acidified manganate(VII), MnO₄⁻, is reduced to Mn²⁺ (purple to colourless) and is its own indicator: the end point is the first permanent pale pink. It reacts with Fe²⁺ in a 1 : 5 ratio and with ethanedioate in a 2 : 5 ratio. Acidified dichromate(VI), Cr₂O₇²⁻, is reduced to Cr³⁺ (orange to green) and reacts with Fe²⁺ in a 1 : 6 ratio. E^⊖ values predict feasibility: a reaction is feasible if E^⊖_cell is positive. Cu²⁺ oxidises I⁻ to I₂, which is titrated with thiosulfate: overall n(Cu²⁺) = n(S₂O₃²⁻).

Fig. 1: standard electrode potentials of the half-cells used in transition metal titrations

Section 01

Oxidation states and half-equations.

Work out the oxidation number of the metal in each species in the simulator, then build the half-equations by balancing O with H₂O, H with H⁺ and charge with electrons.

Species	Oxidation state	Colour	Half-equation
MnO₄⁻ / Mn²⁺	+7 / +2	purple / very pale pink (colourless)	MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Cr₂O₇²⁻ / Cr³⁺	+6 / +3	orange / green	Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Fe³⁺ / Fe²⁺	+3 / +2	yellow-brown / pale green	Fe²⁺ → Fe³⁺ + e⁻
CO₂ / C₂O₄²⁻	+4 / +3 (C)	colourless	C₂O₄²⁻ → 2CO₂ + 2e⁻

With Fe²⁺: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O (ratio 1 : 5).
With ethanedioate: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O (ratio 2 : 5; warm to about 60 °C; Mn²⁺ formed autocatalyses the reaction).
Dichromate with Fe²⁺: Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O (ratio 1 : 6).

Stage 1 · Learn

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What is the oxidation state of manganese in MnO₄⁻?

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What is the mole ratio MnO₄⁻ : Fe²⁺ in an acidified titration?

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What colour change is seen at the end point of a potassium manganate(VII) titration?

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Why is hydrochloric acid not used to acidify potassium manganate(VII) titrations?

Section 02

Titration calculations.

Practise combining half-equations into full ionic equations in the simulator, then use the mole ratios in the worked titrations below.

Titration	Data	Working	Answer
Fe²⁺ by MnO₄⁻	25.0 cm³ Fe²⁺ needs 22.50 cm³ of 0.0200 mol dm⁻³ KMnO₄	n(MnO₄⁻) = 4.50 × 10⁻⁴; n(Fe²⁺) = 5 × that = 2.25 × 10⁻³ mol	0.0900 mol dm⁻³
ethanedioate by MnO₄⁻	25.0 cm³ of 0.0500 mol dm⁻³ C₂O₄²⁻, 0.0200 mol dm⁻³ KMnO₄	n(C₂O₄²⁻) = 1.25 × 10⁻³; n(MnO₄⁻) = 2/5 × that = 5.00 × 10⁻⁴ mol	25.0 cm³
Fe²⁺ by Cr₂O₇²⁻	25.0 cm³ Fe²⁺ needs 15.00 cm³ of 0.0200 mol dm⁻³ K₂Cr₂O₇	n(Cr₂O₇²⁻) = 3.00 × 10⁻⁴; n(Fe²⁺) = 6 × that = 1.80 × 10⁻³ mol	0.0720 mol dm⁻³
Cu²⁺ by I⁻ then S₂O₃²⁻	25.0 cm³ Cu²⁺ + excess KI; I₂ needs 21.20 cm³ of 0.100 mol dm⁻³ Na₂S₂O₃	n(S₂O₃²⁻) = 2.12 × 10⁻³ = n(Cu²⁺)	0.0848 mol dm⁻³

Copper method: 2Cu²⁺ + 4I⁻ → 2CuI + I₂ (off-white CuI in a brown iodine solution); I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. So 2Cu²⁺ ≡ I₂ ≡ 2S₂O₃²⁻. Add starch near the end point: blue-black to colourless.
Choosing the acid with E^⊖: MnO₄⁻/Mn²⁺ (+1.52 V) is more positive than Cl₂/Cl⁻ (+1.36 V), so manganate(VII) would oxidise Cl⁻ from HCl; use dilute H₂SO₄. Cr₂O₇²⁻/Cr³⁺ (+1.33 V) is just below, so dichromate does not oxidise dilute HCl under standard conditions.
Fe³⁺ and I⁻: E^⊖_cell = 0.77 − 0.54 = +0.23 V, so 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ is feasible.

Examiner trap

Get the ratios right: MnO₄⁻ : Fe²⁺ = 1 : 5, MnO₄⁻ : C₂O₄²⁻ = 2 : 5, Cr₂O₇²⁻ : Fe²⁺ = 1 : 6, Cu²⁺ : S₂O₃²⁻ = 1 : 1. Acidify manganate(VII) with sulfuric acid, never hydrochloric. The manganate(VII) end point is the first permanent pale pink, not colourless.

Stage 2 · Exam

Exam-style questions

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25.0 cm³ of acidified ethanedioate solution needs 20.00 cm³ of 0.0200 mol dm⁻³ KMnO₄. What is the concentration of ethanedioate?

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A 1.00 g sample of brass is dissolved and made up to 250 cm³. A 25.0 cm³ portion with excess KI needs 23.60 cm³ of 0.0500 mol dm⁻³ Na₂S₂O₃. What is the percentage of copper in the brass? (A_r Cu = 63.5)

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Using E^⊖ values (Fe³⁺/Fe²⁺ +0.77 V, I₂/I⁻ +0.54 V), what is E^⊖_cell for 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂?

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An iron tablet is dissolved in dilute sulfuric acid and titrated with 0.0100 mol dm⁻³ KMnO₄; 18.60 cm³ is needed. What mass of iron does the tablet contain? (A_r Fe = 55.8)

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Six original Cambridge-style questions

Oxidation states of transition metals, half-equations, using E^⊖ to predict reactions, and manganate(VII), dichromate(VI) and iodine-thiosulfate titrations.