Potassium Dichromate (K2Cr2O7)

Potassium Dichromate — Preparation

Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) is a bright orange crystalline solid, a very important industrial oxidising agent used in leather tanning, dyeing and analytical chemistry. Chromium is in the +6 oxidation state.

It is prepared from chromite ore, $\text{FeCr}_2\text{O}_4$, in three stages:

1. Roast chromite with molten alkali and air to get sodium chromate: $$4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \rightarrow 8\text{Na}_2\text{CrO}_4 + 2\text{Fe}_2\text{O}_3 + 8\text{CO}_2$$
2. Acidify the yellow sodium chromate solution to get sodium dichromate: $$2\text{Na}_2\text{CrO}_4 + 2\text{H}^+ \rightarrow \text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{Na}^+ + \text{H}_2\text{O}$$
3. Treat with KCl — potassium dichromate (less soluble) crystallises out as orange crystals: $$\text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{KCl} \rightarrow \text{K}_2\text{Cr}_2\text{O}_7 + 2\text{NaCl}$$

Chromate-Dichromate Equilibrium and Structure

In solution, chromate ($\text{CrO}_4^{2-}$, yellow) and dichromate ($\text{Cr}_2\text{O}_7^{2-}$, orange) are interconvertible and the equilibrium depends on pH:

$$2\text{CrO}_4^{2-} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}$$

• Acidic solution → dichromate (orange) dominates.
• Alkaline solution → chromate (yellow) dominates.

In both ions chromium is in the +6 state. Their structures:

• Chromate $\text{CrO}_4^{2-}$: a single tetrahedral unit.
• Dichromate $\text{Cr}_2\text{O}_7^{2-}$: two tetrahedra sharing one oxygen corner (a Cr-O-Cr bridge).

[NEET Important] Remember: acid → orange dichromate; base → yellow chromate. The Cr oxidation state stays +6 throughout; only the structure (and colour) changes.

Oxidising Action of Acidified Dichromate

In acidic solution, dichromate is a powerful oxidising agent. Its half-reaction (with $E^\circ = +1.33$ V) is:

$$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$$

Note: 6 electrons are gained, and Cr goes from +6 to +3 (orange → green).

Important oxidations by acidified $\text{K}_2\text{Cr}_2\text{O}_7$:

• Oxidises iron(II) to iron(III): $$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{Fe}^{2+} \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$$
• Oxidises iodide to iodine: $$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{I}^- \rightarrow 2\text{Cr}^{3+} + 3\text{I}_2 + 7\text{H}_2\text{O}$$
• Oxidises H$_2$S to sulphur: $$\text{Cr}_2\text{O}_7^{2-} + 8\text{H}^+ + 3\text{H}_2\text{S} \rightarrow 2\text{Cr}^{3+} + 3\text{S} + 7\text{H}_2\text{O}$$

Key Point: Acidified dichromate gains 6 electrons per ion (Cr$^{6+}$ → Cr$^{3+}$). This is why its equivalent mass is (molar mass)/6 — important for volumetric analysis.

Solved Examples

Example 1: Oxidation state of Cr in dichromate

What is the oxidation state of chromium in $\text{Cr}_2\text{O}_7^{2-}$?

Solution: Let it be $x$. $2x + 7(-2) = -2 \Rightarrow 2x = 12 \Rightarrow x = +6$. Chromium is in the +6 state.

Example 2: Effect of pH

What happens to a dichromate solution when made alkaline?

Solution: It turns yellow as dichromate converts to chromate: $\text{Cr}_2\text{O}_7^{2-} + 2\text{OH}^- \rightarrow 2\text{CrO}_4^{2-} + \text{H}_2\text{O}$. (Adding acid reverses it to orange dichromate.)

Example 3: Electrons gained

How many electrons does one dichromate ion gain when it acts as an oxidising agent in acid?

Solution: From $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$, it gains 6 electrons (each of the two Cr goes +6 → +3).

Example 4: Colour change

What colour change accompanies the reduction of acidified dichromate?

Solution: Orange to green — orange $\text{Cr}_2\text{O}_7^{2-}$ (Cr +6) is reduced to green $\text{Cr}^{3+}$ ions.

Example 5: Equivalent mass of K2Cr2O7

The molar mass of $\text{K}_2\text{Cr}_2\text{O}_7$ is 294 g mol$^{-1}$. Find its equivalent mass as an oxidising agent in acid.

Solution: It gains 6 electrons per formula unit, so equivalent mass $= \dfrac{294}{6} = 49$ g equiv$^{-1}$.

Example 6: Reaction with Fe2+

Write the ionic equation for the oxidation of Fe$^{2+}$ by acidified dichromate.

Solution: $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{Fe}^{2+} \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$.

Example 7: Structure of dichromate

Describe the structure of the dichromate ion.

Solution: Two CrO$_4$ tetrahedra joined by sharing one oxygen corner (a Cr-O-Cr bridge). Each chromium is tetrahedrally surrounded by oxygen, in the +6 state.

Example 8: Why dichromate is used as a primary standard

Why is $\text{K}_2\text{Cr}_2\text{O}_7$ preferred as a primary standard in titrations?

Solution: It can be obtained pure, is stable (non-hygroscopic), has a definite composition, and its solution does not change concentration on standing — making it an excellent primary standard for redox titrations.

Example 9: Oxidation of iodide

Write the equation for the oxidation of iodide ions by acidified dichromate.

Solution: $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{I}^- \rightarrow 2\text{Cr}^{3+} + 3\text{I}_2 + 7\text{H}_2\text{O}$.

Example 10: Acidifying chromate

What is observed when dilute sulphuric acid is added to a yellow potassium chromate solution?

Solution: The solution turns orange as chromate converts to dichromate: $2\text{CrO}_4^{2-} + 2\text{H}^+ \rightarrow \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}$.