Q.Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Concept: Inorganic Synthesis — extraction and conversion of chromite ore into dichromate via oxidative fusion.

Preparation from iron chromite ($FeCr_2O_4$):

1. Fusion: The ore is fused with sodium carbonate in the presence of air (or oxygen) at high temperature.

   $4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$

2. Leaching & Acidification: The fused mass is leached with water to extract soluble sodium chromate. The solution is then acidified with concentrated $H_2SO_4$ to convert chromate to dichromate:

   $2Na_2CrO_4 + H_2SO_4 \rightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O$

3. Conversion to potassium salt: Sodium dichromate is treated with KCl (or $K_2CO_3$) to precipitate the less soluble potassium dichromate:

   $Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7 + 2NaCl$

Effect of increasing pH on $K_2Cr_2O_7$ solution:

In water, the dichromate ion exists in equilibrium with chromate: …

Why this approach works

Chromite ore ($\text{FeCr}_2\text{O}_4$) is a mixed oxide of iron(II) and chromium(III). The key challenge is to oxidise chromium from the +3 state to the +6 state, then isolate it as a pure dichromate salt. Direct acid leaching won’t work because Cr(III) is stable in acid — you need a strong oxidising environment under alkaline conditions first. The fusion with sodium carbonate and air does exactly that: it converts insoluble Cr(III) into water-soluble sodium chromate, while iron remains behind as insoluble iron(III) oxide.

The second part — the effect of pH — is a classic example of Le Chatelier’s principle applied to the dichromate–chromate equilibrium. Understanding this equilibrium is essential for controlling the colour and oxidising power of dichromate solutions in volumetric analysis.

Step-by-step preparation

1. Fusion of the ore

Finely powdered chromite ore is mixed with sodium carbonate and a small amount of quicklime (CaO, to prevent caking) and roasted in a reverberatory furnace at about 1000–1200 °C with a current of air. The reaction is:

The chromium(III) in the ore is oxidised to chromium(VI) as yellow sodium chromate. Iron(III) oxide remains as a solid residue.

2. Leaching and filtration

The fused mass is cooled and treated with hot water. Sodium chromate dissolves, while $\text{Fe}_2\text{O}_3$ and other insoluble impurities are filtered off. The yellow filtrate contains $\text{Na}_2\text{CrO}_4$.

3. Conversion to dichromate

The sodium chromate solution is acidified with concentrated sulfuric acid. The chromate ions dimerise to dichromate:

The solution turns from yellow to orange, indicating the formation of dichromate. Sodium dichromate is obtained on crystallisation.

4. Metathesis to potassium dichromate

Sodium dichromate is more soluble than potassium dichromate. A concentrated solution of $\text{Na}_2\text{Cr}_2\text{O}_7$ is treated with potassium chloride (or potassium sulfate). The less soluble $\text{K}_2\text{Cr}_2\text{O}_7$ crystallises out:

The orange crystals are filtered, washed with a little cold water, and dried.

Effect of increasing pH on a solution of potassium dichromate

An aqueous solution of $\text{K}_2\text{Cr}_2\text{O}_7$ contains the equilibrium: …

Method: Alkaline Oxidative Fusion (followed by acidification)

This is the standard industrial route to convert chromite ore ($\text{FeCr}_2\text{O}_4$) into soluble dichromate.

Steps for Preparation

1. Fusion (Roasting) with Soda Ash and Air Finely powdered chromite ore is mixed with sodium carbonate ($\text{Na}_2\text{CO}_3$) and a small amount of lime (CaO, to keep the mixture porous). The mixture is roasted in a reverberatory furnace at 1100–1200 °C in a current of air. Reaction:

2. Leaching

   The fused mass is cooled and treated with hot water. Sodium chromate ($\text{Na}_2\text{CrO}_4$, yellow) dissolves, while iron(III) oxide ($\text{Fe}_2\text{O}_3$) remains as insoluble residue and is filtered off.

3. Conversion to Dichromate (Acidification)

   The yellow sodium chromate solution is treated with concentrated sulfuric acid to lower the pH. This shifts the chromate-dichromate equilibrium toward dichromate:

The solution turns **orange** due to formation of sodium dichromate ($\text{Na}_2\text{Cr}_2\text{O}_7$).

4. Double Decomposition (to get Potassium Salt)

Sodium dichromate is more soluble than potassium dichromate, so treating the sodium salt with a potassium salt precipitates the less soluble potassium dichromate. A hot concentrated solution of $\text{Na}_2\text{Cr}_2\text{O}_7$ is treated with KCl (or $\text{K}_2\text{CO}_3$):

$\text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{KCl} \rightarrow \text{K}_2\text{Cr}_2\text{O}_7 + 2\text{NaCl}$ …

Here is a breakdown of the common mistakes students make on this specific inorganic synthesis question, along with how to avoid them.

1. Mistake: Writing the wrong formula for the ore

Students often confuse chromite with other ores or write incorrect formulas (e.g., $\ce{FeCrO4}$ or $\ce{Cr2O3}$ alone).

• The correct formula: $\ce{FeCr2O4}$ (Iron(II) chromite). It is a mixed oxide of $\ce{FeO}$ and $\ce{Cr2O3}$.
• How to avoid: Memorize the ore formula as a spinel structure ($\ce{Fe^{2+}Cr^{3+}2O4}$). Remember it contains both iron and chromium in a 1:2 ratio.

2. Mistake: Forgetting the role of air (oxygen) in the fusion step

Many students write the fusion reaction as just heating with $\ce{Na2CO3}$, forgetting that oxygen from air is the actual oxidizing agent.

• The correct reaction:

• How to avoid: Always include $\ce{O2}$ as a reactant. The purpose of heating in a reverberatory furnace is to maximize contact with air. Without oxygen, the chromium(III) cannot be oxidized to chromium(VI).

3. Mistake: Skipping the leaching and filtration step

Students jump directly from fusion to acidification, forgetting that the yellow sodium chromate ($\ce{Na2CrO4}$) must be separated from the insoluble red-brown iron(III) oxide ($\ce{Fe2O3}$).

• The correct sequence: Fusion → Cool → Leach with water → Filter → Acidify the yellow filtrate.
• How to avoid: Remember that $\ce{Fe2O3}$ is insoluble in water, while $\ce{Na2CrO4}$ is soluble. Filtration is essential to remove the iron impurity before converting to dichromate.

4. Mistake: Confusing the conversion of chromate to dichromate

Students often write the acidification step incorrectly, either forgetting the equilibrium or writing the wrong products.

• The correct equilibrium:

(Yellow) $\rightarrow$ (Orange)

• How to avoid: Understand this is a pH-dependent equilibrium, not a simple precipitation. Adding acid ($\ce{H+}$) shifts the equilibrium to the right (orange dichromate). Adding base (increasing pH) shifts it left (yellow chromate).

5. Mistake: Getting the effect of increasing pH wrong

This is the most common conceptual error. Students often think increasing pH makes the solution more orange (dichromate) because they associate dichromate with "stronger" conditions.

• The correct effect: Increasing pH (adding base) removes $\ce{H+}$ from the solution. The equilibrium shifts left, converting orange dichromate back to yellow chromate.

• How to avoid: Remember the Le Chatelier's principle logic:
  • Acidic pH → favors $\ce{Cr2O7^{2-}}$ (orange)
  • Basic/neutral pH → favors $\ce{CrO4^{2-}}$ (yellow)
  • Increasing pH = more basic = yellow color dominates. …