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The KMnO4 titration is one of the Class 12 practicals where small habits decide the result: warming the flask, reading the right meniscus, stopping at the first permanent pink. This post walks through the NCERT procedure, explains the chemistry behind each step, and works a full calculation with sample readings.
Where is this in the syllabus?
The CBSE chemistry curriculum for 2026-27 lists, among Class 12 practicals, determining the molarity of KMnO4 by titrating it against a standard solution of oxalic acid or ferrous ammonium sulphate, and says students "will be required to prepare standard solutions by weighing themselves". The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus include the chemistry of oxalic acid versus KMnO4 in their practical sections. The procedure is Experiment 6.1 of the NCERT Class 12 chemistry laboratory manual. Check your school's practical list and your exam's current syllabus.
What reaction happens in the flask?
In acid, permanganate gains five electrons: MnO4⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H2O. Each oxalate ion loses two: C2O4²⁻ → 2CO2 + 2e⁻, as carbon goes from +3 to +4. Multiplying by 2 and 5 to balance electrons gives the ionic equation in the manual:
2MnO4⁻ + 5C2O4²⁻ + 16H⁺ → 2Mn²⁺ + 10CO2 + 8H2O
Check the charge: on the left, −2 − 10 + 16 = +4; on the right, 2 × (+2) = +4. So 2 mol of permanganate react with 5 mol of oxalic acid.
Why does each step of the procedure matter?
The NCERT manual gives a reason for almost every step:
Dilute sulphuric acid only. Nitric acid is itself an oxidising agent, and hydrochloric acid reacts with KMnO4 to give chlorine. The acid also prevents brown MnO2 forming during the titration.
Warm to 50 to 60°C. The reaction needs the higher temperature. The manual adds that the Mn²⁺ formed acts as a catalyst, so the first drops decolourise slowly and later ones quickly.
No indicator. KMnO4 is a self-indicator: the end point is the first permanent light pink from a slight excess of permanganate.
Read the upper meniscus. The solution is too dark to see the lower one.
Glass stopcock. Rubber is attacked by permanganate, so a pinch-cock burette is not used.
Three concordant readings. Repeat until they agree.
The Mohr's salt titration (Experiment 6.2) differs in one step: it is not heated, because that reaction is fast at room temperature and hot Fe²⁺ may be oxidised by air.
How do you prepare the standard oxalic acid?
The Class 11 laboratory manual prepares 250 mL of 0.1 M oxalic acid from hydrated crystals, H2C2O4·2H2O, molar mass 126 g mol⁻¹:
mass = 0.1 mol L⁻¹ × 0.250 L × 126 g mol⁻¹ = 3.15 g
The manual weighs this as 3.1500 g. Because you know this solution's molarity exactly, it can fix the molarity of the KMnO4.
Worked example with sample readings
These are sample numbers. You pipette 10.0 mL of 0.100 M oxalic acid into the flask, add about 5 mL of 1.0 M H2SO4 as NCERT directs, warm, and titrate:
Rough titration: 20.4 mL
Second titration: 0.5 to 20.5 mL, so 20.0 mL
Third titration: 20.5 to 40.5 mL, so 20.0 mL
The concordant volume is 20.0 mL. NCERT uses a1M1V1 = a2M2V2, with a1 = 2 (electrons lost per oxalic acid) and a2 = 5 (electrons gained per KMnO4):
2 × 0.100 × 10.0 = 5 × M2 × 20.0, so M2 = 2.00/100 = 0.0200 M
Check with moles: oxalic acid = 0.100 × 0.0100 = 1.00 × 10⁻³ mol, so KMnO4 = (2/5) × 1.00 × 10⁻³ = 4.00 × 10⁻⁴ mol, and 4.00 × 10⁻⁴/0.0200 L = 0.0200 M.
Strength = molarity × molar mass. With K = 39, Mn = 55 and O = 16, KMnO4 is 158 g mol⁻¹, so the strength is 0.0200 × 158 = 3.16 g L⁻¹.
Which mistakes cost marks?
Swapping a1 and a2. Above, that gives 0.125 M instead of 0.0200 M.
Not warming, so the colour lingers and you overshoot.
Going past the first permanent pink. A deep purple flask means several drops too many.
Reading the lower meniscus.
Rinsing the conical flask with the experimental solution. NCERT says rinse the burette and pipette with their solutions, but never the flask.
Hanging drops at the burette tip when you read the volume.
For the wider chemistry, see our posts on permanganate and dichromate reactions and the n-factor in redox titrations.
How should you prepare before the practical?
Preparation before a lab matters. Agustian and Seery (2017) reviewed over 60 reports and articles on pre-laboratory activities in university chemistry, grouped them into those that introduce concepts, those that introduce techniques and those that build confidence, and proposed a design framework based on cognitive load theory.
The night before: write the ionic equation and both half-reactions from memory.
Predict the volume: for the molarities you expect, work out the burette reading before you start.
Learn the reasons: for each precaution, write one line on why.
Practise the calculation with three sets of made-up readings, including a Mohr's salt one.
If a titration calculation step still does not make sense, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Tonight, work out the KMnO4 volume needed for 10.0 mL of 0.0500 M oxalic acid against 0.0200 M KMnO4. You should get 10.0 mL.
