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Potassium permanganate and potassium dichromate are the two oxidising agents every chemistry student must know: how they are made, what they turn into, and how to use them in a titration. Questions on them mix inorganic facts with redox arithmetic, which is why they cost marks. This post covers both preparations, the reactions NCERT lists, and one titration worked end to end.
Where is this in the syllabus?
Both compounds are under "Some Important Compounds of Transition Elements" in NCERT Class 12 chemistry, Unit 4, The d- and f-Block Elements. The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus ask for "preparation, properties and uses of K2Cr2O7 and KMnO4", and both list permanganate titrations against oxalic acid and Mohr's salt in the practical section. The JEE (Advanced) 2026 brochure asks for "preparation, structure, and reactions of oxoanions of chromium and manganese". The CBSE 2026-27 chemistry curriculum includes the same titrations as practicals. Check your exam's current bulletin.
How is potassium dichromate made?
NCERT gives three steps:
Fuse chromite ore with sodium carbonate in air: 4FeCr2O4 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2. Chromium goes from +3 to +6; oxygen from the air is the oxidant.
Acidify the yellow sodium chromate to get orange sodium dichromate: 2Na2CrO4 + 2H⁺ → Na2Cr2O7 + 2Na⁺ + H2O.
Add potassium chloride. Sodium dichromate is more soluble, so orange crystals of potassium dichromate come out: Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl.
Chromate and dichromate interconvert with pH: 2CrO4²⁻ + 2H⁺ → Cr2O7²⁻ + H2O in acid, and back in alkali. NCERT stresses that chromium is +6 in both, so this is not a redox reaction. The chromate ion is tetrahedral; dichromate is two tetrahedra sharing a corner, with a Cr–O–Cr angle of 126°.
How is potassium permanganate made?
MnO2 is fused with an alkali metal hydroxide and an oxidising agent such as KNO3 or air, giving dark green potassium manganate: 2MnO2 + 4KOH + O2 → 2K2MnO4 + 2H2O. In neutral or acidic solution, manganate disproportionates: 3MnO4²⁻ + 4H⁺ → 2MnO4⁻ + MnO2 + 2H2O. Check the electrons: two manganese atoms go from +6 to +7 (losing 2 electrons in total) while one goes from +6 to +4 (gaining 2).
Commercially, the manganate is oxidised electrolytically in alkaline solution instead. In the laboratory, a manganese(II) salt is oxidised by peroxodisulphate. NCERT also notes that green manganate is paramagnetic with one unpaired electron (Mn +6, d¹) while permanganate is diamagnetic (Mn +7, d⁰), and that KMnO4 decomposes at 513 K.
Why does the medium change what permanganate becomes?
NCERT lists three reductions:
Strongly alkaline: MnO4⁻ + e⁻ → MnO4²⁻ (E° = +0.56 V), 1 electron
Neutral or faintly alkaline: MnO4⁻ + 4H⁺ + 3e⁻ → MnO2 + 2H2O (E° = +1.69 V), 3 electrons
Acidic: MnO4⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H2O (E° = +1.52 V), 5 electrons
So the hydrogen ion concentration decides the product and the number of electrons. Dichromate in acid is simpler: Cr2O7²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H2O (E° = 1.33 V), 6 electrons per ion.
Worked example: one iron(II) sample, two oxidants
These are sample numbers. 25.0 mL of an iron(II) solution needs 20.0 mL of 0.0200 M K2Cr2O7 in acid. Find the iron(II) concentration, then the volume of 0.0200 M KMnO4 the same sample would need.
The ionic equation, from NCERT, is Cr2O7²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H2O.
Moles of dichromate = 0.0200 × 20.0 / 1000 = 4.00 × 10⁻⁴ mol
Moles of Fe²⁺ = 6 × 4.00 × 10⁻⁴ = 2.40 × 10⁻³ mol
[Fe²⁺] = 2.40 × 10⁻³ / 0.0250 L = 0.0960 M
With permanganate: 5Fe²⁺ + MnO4⁻ + 8H⁺ → Mn²⁺ + 4H2O + 5Fe³⁺.
Moles of MnO4⁻ = 2.40 × 10⁻³ / 5 = 4.80 × 10⁻⁴ mol
Volume = 4.80 × 10⁻⁴ / 0.0200 = 0.0240 L = 24.0 mL
Same concentration of oxidant, different volumes, because each dichromate ion takes six electrons and each permanganate ion five.
Which mistakes cost marks?
Calling chromate to dichromate a redox reaction. Chromium stays at +6.
Using 5 electrons for permanganate in neutral solution. It is 3, and the product is MnO2.
Acidifying a permanganate titration with HCl. NCERT notes that HCl is itself oxidised to chlorine, so the result is wrong; dilute sulphuric acid is used.
Forgetting to warm oxalate. NCERT gives the oxalate oxidation at 333 K.
Mixing up the colours: chromate yellow, dichromate orange, manganate green, permanganate purple.
Redox equations are a known weak point. In interviews with final-year school students, Garnett and Treagust (1992) found that many had trouble identifying oxidation-reduction equations, often by applying definitions of oxidation and reduction in the wrong place. Our guide to balancing redox reactions covers the half-reaction method in detail. If a step in a titration problem still does not make sense, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
A practice routine
Write both preparations from memory, with the oxidation state of the metal under each species.
Write the three permanganate half-reactions and the dichromate one, with electron counts.
Do three titration problems: oxalic acid with KMnO4, Mohr's salt with KMnO4 and Mohr's salt with K2Cr2O7.
Mix these with other redox problems a week later instead of practising them in a block; in Rohrer and Taylor's experiments, mixed practice gave much better results on a test a week later.
Tonight, write the balanced equation for permanganate oxidising oxalate in acid and check the electrons on both sides.
