ChemistryClass 12ICSE

Why Orange Dichromate Turns Green When It Oxidises Something

Learn how potassium dichromate and potassium permanganate are prepared from their ores, their structures and properties, how their oxidising action changes with the medium, and how to find their equivalent masses in titrations.

Why are permanganate and dichromate such common oxidising agents?

A few purple crystals of potassium permanganate turn a whole bucket of water pink, and orange potassium dichromate turns green the moment it oxidises something. Both owe their power to a transition metal in a very high oxidation state — manganese at +7 and chromium at +6.

This lesson covers the preparation, properties and oxidising action of potassium dichromate and potassium permanganate, and how to find their equivalent masses in titrations.

How are potassium dichromate and potassium permanganate prepared, and how do they act as oxidising agents?

Potassium dichromate is made from chromite ore and potassium permanganate from pyrolusite, and both are strong oxidising agents because chromium(VI) and manganese(VII) readily accept electrons, especially in acid.

Preparing potassium dichromate:

- Chromite ore is fused with sodium carbonate in air:

4FeC r_{2} O_{4} + 8N a_{2} C O_{3} + 7 O_{2} \to 8N a_{2} Cr O_{4} + 2F e_{2} O_{3} + 8C O_{2}

- The yellow chromate solution is acidified to orange dichromate:

2N a_{2} Cr O_{4} + 2 H^{+} \to N a_{2} C r_{2} O_{7} + 2N a^{+} + H_{2} O

- Adding potassium chloride crystallises the less soluble

K_{2} C r_{2} O_{7}

Properties of dichromate:

- Chromate and dichromate interconvert with pH:

2Cr O_{4}^{2 -} + 2 H^{+} ⇌ C r_{2} O_{7}^{2 -} + H_{2} O

, yellow in base and orange in acid
- In acid it is reduced to green chromium(III):

C r_{2} O_{7}^{2 -} + 14 H^{+} + 6 e^{-} \to 2C r^{3 +} + 7 H_{2} O

- It oxidises iron(II) to iron(III), iodide to iodine and hydrogen sulphide to sulphur

Preparing potassium permanganate:

- Pyrolusite,

Mn O_{2}

, is fused with potassium hydroxide and an oxidising agent to give green potassium manganate:

2Mn O_{2} + 4KOH + O_{2} \to 2 K_{2} Mn O_{4} + 2 H_{2} O

- Manganate is then oxidised electrolytically, or disproportionates in neutral or acid solution:

3Mn O_{4}^{2 -} + 4 H^{+} \to 2Mn O_{4}^{-} + Mn O_{2} + 2 H_{2} O

- On heating, the purple crystals release oxygen:

2KMn O_{4} \to K_{2} Mn O_{4} + Mn O_{2} + O_{2}

Permanganate's oxidising action depends on the medium:

- Acidic:

Mn O_{4}^{-} + 8 H^{+} + 5 e^{-} \to M n^{2 +} + 4 H_{2} O

— purple to colourless
- Neutral or faintly alkaline:

Mn O_{4}^{-} + 2 H_{2} O + 3 e^{-} \to Mn O_{2} + 4O H^{-}

— a brown precipitate
- Strongly alkaline:

Mn O_{4}^{-} + e^{-} \to Mn O_{4}^{2 -}

— green manganate

Structures. The permanganate ion is tetrahedral; the dichromate ion is two

Cr O_{4}

tetrahedra sharing one oxygen atom.

Why hydrochloric acid is avoided. Permanganate would oxidise chloride ions to chlorine, using up extra titrant, so dilute sulphuric acid is used instead.

An everyday example. Pale pink potassium permanganate solution is used in many Indian homes to rinse fruit and vegetables, and to disinfect wells, because it oxidises and kills microbes.

The substance. Potassium dichromate makes a good primary standard — unlike permanganate, it can be obtained very pure and its solution does not decompose on standing.

Formula

How do you find the equivalent mass of KMnO4 and K2Cr2O7 in titrations?

**The equivalent mass of an oxidising agent is its molar mass divided by the number of electrons each formula unit gains, so it depends on the reaction:

E = \frac{M}{n}

.**

E = \frac{M}{n}

Worked example 1 — potassium permanganate (

M = 158

g mol

^{- 1}

):

- Acidic medium, n = 5:

E = \frac{158}{5} = 31.6

g
- Neutral or faintly alkaline, n = 3:

E = \frac{158}{3} = 52.7

g
- Strongly alkaline, n = 1:

E = 158

g

Worked example 2 — potassium dichromate in acid (

M = 294

g mol

^{- 1}

). Each chromium falls from +6 to +3, and there are two chromium atoms, so n = 6:

E = \frac{294}{6} = 49 g

Worked example 3 — a titration. 25.0 mL of iron(II) solution needs 20.0 mL of 0.020 M

KMn O_{4}

in acid. From

Mn O_{4}^{-} + 5F e^{2 +} + 8 H^{+} \to M n^{2 +} + 5F e^{3 +} + 4 H_{2} O

[F e^{2 +}] = \frac{5 \times 0.020 \times 20.0}{25.0} = 0.080 M

An everyday example. Laboratories checking iron supplement tablets can find their iron(II) content by a permanganate titration of exactly this kind.

The substance. A substance has no single equivalent mass — permanganate's value changes fivefold between acidic and strongly alkaline media, because it gains a different number of electrons.

Exam tip

What earns full marks on permanganate and dichromate?

State the medium before writing any permanganate half-equation — acidic, neutral and strongly alkaline conditions give three different products.

- Dichromate: chromite, fusion with

N a_{2} C O_{3}

, acidification, then KCl
- Permanganate: pyrolusite, fusion with KOH, then oxidation of manganate
- Acidic

KMn O_{4}

: n = 5, gives

M n^{2 +}

; acidic

K_{2} C r_{2} O_{7}

: n = 6, gives

C r^{3 +}

- Chromate is yellow in base; dichromate is orange in acid

The trap. Acidifying a permanganate titration with hydrochloric acid. Permanganate oxidises chloride to chlorine, so dilute sulphuric acid must be used.

Did you know

Why does a permanganate titration need no indicator?

Most titrations need an indicator, but a permanganate titration signals its own end point.

As purple permanganate runs into an acidified iron(II) solution, it is reduced at once to almost colourless

M n^{2 +}

, so each drop loses its colour. The moment all the iron(II) is used up, the next drop has nothing left to react with, and its intense purple turns the whole flask a faint, lasting pink.

A single extra drop is visible, which makes permanganate its own indicator.

Exam relevance

How do JEE Main and NEET test potassium permanganate and dichromate?

The d- and f-Block Elements is a recurring chapter in both JEE Main and NEET, and permanganate and dichromate run through its reaction-based questions.

What gets asked. Steps and equations for preparing both compounds, products of permanganate in acidic, neutral and alkaline media, the chromate-dichromate equilibrium, the structures of both ions, and n-factors and equivalent masses in redox titrations.

Question types. Mostly single-correct and match-the-column questions pairing reactions with products, plus numericals on titration volumes.

Why it matters later. Both oxidising agents return in Alcohols, Phenols and Ethers and Aldehydes, Ketones and Carboxylic Acids for oxidising organic compounds, and in the redox titrations of the practical course.

The trap that costs marks. Using n = 3 for dichromate — each chromium gains three electrons, but there are two chromium atoms, so n = 6.

Key takeaways

What must you be able to do from this lesson?

- Potassium dichromate: made from chromite by fusion, acidification and reaction with KCl; orange in acid and reduced to green

C r^{3 +}

- Potassium permanganate: made from pyrolusite via green manganate; reduced to

M n^{2 +}

Mn O_{2}

Mn O_{4}^{2 -}

depending on the medium
- Equivalent mass: molar mass divided by electrons gained — 31.6 g for acidic permanganate and 49 g for acidic dichromate

How many millilitres of 0.020 M

K_{2} C r_{2} O_{7}

are needed to oxidise 30.0 mL of 0.10 M iron(II) solution in acid?