Experiment: oxalic acid and Mohr’s salt against KMnO₄

What is a titration with potassium permanganate?

In plain words: purple permanganate loses its colour as fast as it reacts. Add it drop by drop, and the moment there is nothing left for it to oxidise, one extra drop leaves the whole flask faintly pink. That pink is the end point, so no separate indicator is needed.

The NCERT Class 12 textbook gives the chemistry (NCERT Section 4.4, pages 106–108), and the Class 11 book the idea of redox titrations, but neither describes the method. The JEE Main 2027 and NEET (UG) 2026 syllabuses list these titrations among the practical chemistry experiments of Unit 20. The method below follows the NCERT Laboratory Manual (Chemistry, Class 12), Unit 6, Experiments 6.1 and 6.2 (pages 40–48).

KMnO_₄ as its own indicator

• The reaction. In acid, permanganate takes five electrons and is reduced to Mn²⁺ (Lab Manual page 40):

• Self-indicator. Each drop of permanganate is decolourised as it reacts. Once the reducing agent is used up, a little excess permanganate gives a faint pink colour that persists for the observation interval: that is the end point (Lab Manual pages 41 and 43).

• Dilute sulphuric acid for these experiments. Nitric acid is itself an oxidising agent, and hydrochloric acid reacts with permanganate to give chlorine, which also oxidises (Lab Manual page 40; NCERT textbook page 108):

  • 2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O A practical endpoint criterion is a faint pink persisting for about 30 seconds while swirling, or the interval specified by the laboratory protocol (Royal Society of Chemistry schools’ titration method). This observed endpoint is just beyond the stoichiometric equivalence point; “permanent” does not mean the colour must last forever.

• Enough acid. About 5 mL of 1.0 M H₂SO₄ is added to the flask to stop manganese dioxide forming during the titration (Lab Manual page 42); in neutral or faintly alkaline solution permanganate commonly gives MnO₂ in the textbook reactions (reasoned from NCERT textbook page 108).

• Why it is standardised. Potassium permanganate is a secondary standard (see the Class 11 page on standard solutions): it is not weighed straight into a standard solution, so its exact concentration is found by titrating it against oxalic acid or Mohr’s salt; oxalic acid is the usual choice (Wikipedia, “Potassium permanganate”). Its solutions are not indefinitely stable: NCERT notes that permanganate should oxidise water, though only extremely slowly (page 107; reasoned). Titrations with permanganate are called permanganometry (Wikipedia, “Potassium permanganate”).

• Working out the molarity. The Lab Manual uses (NCERT Laboratory Manual, Eq. (6.1), page 43):

Subscript 1 is the standard reducing agent in the flask (oxalic acid or Mohr’s salt) and subscript 2 the permanganate in the burette (Lab Manual pages 43 and 47). Here a is the number of electrons lost or gained per formula unit in the half-reaction: a₂ = 5 for KMnO₄; M is the molarity and V the volume. The strength in g L⁻¹ is the molarity × molar mass (Lab Manual pages 43–44).

Oxalic acid against KMnO_₄ at 333 K

• The equations (Lab Manual page 41):
  • ionic: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O
  • molecular: 2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂
• Electrons. Carbon goes from +3 in C₂O₄²⁻ to +4 in CO₂, so each oxalate gives 2 electrons: a₁ = 2, and 2M₁V₁ = 5M₂V₂ (Lab Manual pages 41 and 43–44).
• Lab Manual slip (page 41): the oxalate half-reaction is printed C₂O₄⁻ → 2CO₂ + 2e⁻. The ion is C₂O₄²⁻, as the ionic equation just below it shows; with a single minus the charges would not balance (−1 against −2). Write C₂O₄²⁻ → 2CO₂ + 2e⁻.
• The standard solution. For 0.1 M oxalic acid, weigh the dihydrate, H₂C₂O₄·2H₂O (126 g mol⁻¹): 0.1 × 0.250 × 126 = 3.1500 g, made up to 250 mL in a measuring flask (Lab Manual page 42 refers to the Class 11 manual for the method).
• Heat to 50–60 °C. The oxalic acid and sulphuric acid in the flask are warmed to 50–60 °C before titrating, because the reaction needs a higher temperature (Lab Manual pages 41–42). The NCERT textbook gives 333 K, that is 60 °C (NCERT page 108).
• Slow, then fast. The manganous sulphate formed in the titration catalyses the reduction of permanganate by oxalic acid, so the first drops decolourise slowly and later ones quickly (Lab Manual page 41). The textbook makes a related point: at [H⁺] = 1 permanganate should oxidise water, but the reaction is extremely slow unless manganese(II) ions are present or the temperature is raised (NCERT page 107).

Mohr’s salt against KMnO_₄

• Mohr’s salt is ferrous ammonium sulphate, FeSO₄(NH₄)₂SO₄·6H₂O, molar mass 392 g mol⁻¹. Iron is +2 in it and is oxidised to +3 (Lab Manual pages 45–46).
• The equations (Lab Manual page 45):
  • ionic: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
  • molecular: 2KMnO₄ + 8H₂SO₄ + 10FeSO₄(NH₄)₂SO₄·6H₂O → K₂SO₄ + 2MnSO₄ + 5Fe₂(SO₄)₃ + 10(NH₄)₂SO₄ + 68H₂O
• Electrons. Each Fe²⁺ gives one electron: a₁ = 1, and M₁V₁ = 5M₂V₂ (Lab Manual page 47).
• No heating. The reaction is fast at room temperature, and when hot, iron(II) can be oxidised to iron(III) by the oxygen of the air, which would introduce an error (Lab Manual page 45).
• The standard solution. For 0.05 M, weigh 4.9000 g into a 250 mL measuring flask, add dilute sulphuric acid drop by drop to get a clear solution, and make up to the mark (Lab Manual page 46). Use a fresh sample of the salt (Lab Manual page 47).
• Why the acid. Acid suppresses hydrolysis and helps preserve iron(II). Any Fe(III) formed by air oxidation hydrolyses particularly readily and can contribute to turbidity; it is too simple to attribute all cloudiness just to Fe(II) in water (reasoned; the Lab Manual asks this on page 48).

Apparatus and procedure

Apparatus (Lab Manual pages 42 and 46): a 250 mL measuring flask, a 50 mL burette and stand, a pipette, a conical flask, a funnel, a weighing bottle, a white glazed tile, a burner and wire gauze to heat the oxalic acid, and a chemical balance for either standard (the Mohr’s salt list leaves the balance out).

Procedure (Lab Manual pages 42–43 and 46):

1. Rinse the burette with the permanganate solution, fill it, and run a little out to remove any air bubble from the nozzle. NCERT specifies a glass stopcock because permanganate attacks rubber; a compatible modern PTFE stopcock also avoids the rubber-contact problem.
2. Pipette 10 mL of the standard solution (0.1 M oxalic acid, or 0.05 M Mohr’s salt) into a conical flask and add about 5 mL of 1.0 M H₂SO₄.
3. Oxalic acid only: heat the flask to 50–60 °C.
4. Stand the flask on the white tile, note the initial burette reading, and add permanganate in small amounts while swirling, until a light pink colour persists.
5. Read the upper meniscus: the permanganate solution is too dark to see the bottom of the meniscus. Repeat until three readings agree (concordant readings).

Precautions

From the Lab Manual (pages 44 and 47):

• Rinse the burette and pipette with the solutions that go in them, but never rinse the conical flask with them.
• Remove any air gap from the burette, and take the funnel out before the initial reading; no drop should hang from the burette tip at the end point.
• Do not blow out the last drop from the pipette.
• For the manual’s reporting format, calculate strength to four decimal places using unrounded intermediate values. This display convention does not improve the precision of the measurements.
• Heat the oxalic acid and sulphuric acid mixture to 50–60 °C while titrating; do not heat Mohr’s salt.

Solved examples

Common mistakes to avoid

Quick revision summary

• KMnO_₄ is its own indicator in acid: the first lasting light pink is the end point.
• Use dilute H_₂SO_₄ in these school experiments; HCl and HNO_₃ can introduce side reactions.
• Oxalic acid: titrate at 50–60 °C (333 K in the textbook); Mn^²⁺ formed catalyses the reaction.
• Mohr’s salt: titrate at room temperature; air oxidises hot Fe^²⁺.
• Molarity from a_₁M_₁V_₁ = a_₂M_₂V_₂, with a = 2 for oxalic acid, 1 for Mohr’s salt and 5 for KMnO_₄; strength = molarity × molar mass.

Questions students ask

How do you prepare a KMnO₄ solution for titration?

A KMnO₄ solution is made up only roughly, then heated or aged and filtered before it is standardised, because in neutral solution permanganate slowly turns into brown MnO₂ (beyond the NCERT text; Wikipedia, “Potassium permanganate”). The NCERT Laboratory Manual (Chemistry, Class 11) dissolves 10.0 g in 1 litre of distilled water for its 0.06 M solution, heats it and filters it through glass wool (Appendix III, page 141). The US National Bureau of Standards aged its solutions for at least two weeks, filtered them into a bottle kept from light and drew them off by siphon, cleaning the incoming air of dust and acid fumes (Fowler and Bright, 1935).

Why is Mohr’s salt used in the KMnO₄ titration instead of ferrous sulphate?

Mohr’s salt is used instead of plain ferrous sulphate because a fresh, pure sample is more resistant to air oxidation than ordinary ferrous sulphate. This relative stability makes it useful as a standard, but does not guarantee unchanged iron(II) content after arbitrary storage (beyond the NCERT text; Wikipedia, “Ammonium iron(II) sulfate”). The NCERT Laboratory Manual (Chemistry, Class 11) rules out unstable hydrated salts as primary standards but accepts ferrous ammonium sulphate “because of their sufficient stabilities” (page 67). Air oxidises Fe²⁺ faster as the pH rises, and the NH₄⁺ ions keep a Mohr’s salt solution slightly acidic (same source).

Why is 0.1 N KMnO₄ only 0.02 M, and how is normality used in KMnO₄ titrations?

A 0.1 N KMnO₄ solution is only 0.02 M because each MnO₄⁻ takes 5 electrons in acid and normality is a times molarity, N = aM; the NCERT Laboratory Manual (Chemistry, Class 11) writes its stock as “0.06 M (0.3 N)” (page 141). So N₁V₁ = N₂V₂ is the Lab Manual’s a₁M₁V₁ = a₂M₂V₂ (NCERT Lab Manual, Class 12, Eq. (6.1), page 43). Thus 0.0172 M KMnO₄ and 0.0430 M oxalic acid (a = 2) are both 0.0860 N and react volume for volume (reasoned).

How is hydrogen peroxide titrated with KMnO₄?

Hydrogen peroxide is titrated directly with acidified KMnO₄, the first lasting pink again marking the end point (reasoned): 2MnO₄⁻ + 5H₂O₂ + 6H⁺ → 2Mn²⁺ + 5O₂ + 8H₂O (beyond the Lab Manual; Wikipedia, “Permanganometry”, lists hydrogen peroxide among the substances it estimates). Oxygen goes from −1 to 0, so each H₂O₂ gives 2 electrons and a = 2, as for oxalic acid. If 10.0 mL of a peroxide solution needs 14.4 mL of 0.0175 M KMnO₄, its molarity is 5 × 0.0175 × 14.4 ÷ (2 × 10.0) = 0.0630 M (reasoned).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is a burette with a rubber pinch-cock not used for permanganate titrations?

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   Rubber is attacked by permanganate ions, so NCERT specifies a glass stopcock; compatible PTFE equipment is another modern option (Lab Manual page 42).

2. Q2. Why is oxalic acid warmed to 50–60 °C before titrating it with permanganate, but Mohr’s salt is not?

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   The reaction with oxalic acid is slow and needs a higher temperature (Lab Manual page 41). The reaction with iron(II) is already fast at room temperature, and hot iron(II) would be partly oxidised by the air (Lab Manual page 45).

3. Q3. What volume of 0.100 M oxalic acid reacts with 16.0 mL of 0.0250 M KMnO₄?

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   2M₁V₁ = 5M₂V₂: V₁ = 5 × 0.0250 × 16.0 ÷ (2 × 0.100) = 10.0 mL.