Potassium Dichromate

Potassium Dichromate

Properties of Potassium Dichromate

1) Colour and melting point : It is an orange red crystalline solid having melting point of 670 K.

2) Solubility :  It is moderately soluble in cold water but readily soluble in hot water.

3) Action of heat : It decomposes on heating to form potassium chromate , chromic acid and oxide.

4 K₂Cr₂O₇ → 4K₂CrO₄ + 2 Cr₂O₃ + 3 O₂

4) Action of alkalies : On heating with alkalies, the orange colour of dichromate solution changes to yellow due to the formation of chromate ion.

4 K₂Cr₂2O₇ + 2KOH → 2 K₂CrO₄  + H₂O

Cr₂O₇^2- + 2 OH¯ → 2 CrO₄^2- + H₂O

On acidifying the yellow solution , the colour again changes to orange red due to the reversible reaction.

2 K₂CrO₄  + H₂SO₄ → K₂Cr₂O₇ + K₂SO₄ + H₂O

2 CrO₄^2- + 2 H⁺ → Cr₂O₇^2- + H₂O

5) Chromyl chloride test

When potassium dichromate is heated with conc. sulphuric acid and a soluble metal chloride , orange red vapours of chromyl chloride are evolved.

4 K₂Cr₂2O₇ + 4 NaCl + 6 H₂SO₄ → 2 KHSO₄ + 4 NaHSO₄ + 2 CrO₂Cl₂ + 3 H₂O

6) Action with Hydrochloric acid

Potassium dichromate reacts with hydrochloric acid and evolve chlorine

K₂Cr₂2O₇ + 14 HCl → 2KCl + 2CrCl₃ + 7H₂O + 3 Cl₂

7) Reaction with hydrogen peroxide

Acidified potassium dichromate reacts with hydrogen peroxide to give a deep blue solution due to the formation of [CrO(O₂)₂] or CrO₅

Cr₂O₇^2- + 4 H₂O₂ + 2 H⁺ -→ 2CrO₅ + 5 H₂O

The blue colour fades away gradually due to the decomposition of CrO₅ to Cr³⁺ and oxygen.

8) Oxidising character

The dichromates act as a powerful oxidising agents in acidic medium. In the presence of dilute sulphuric acid , K₂Cr₂O₇ liberates nascent oxygen and therefore acts as an oxidising agent.

K₂Cr₂2O₇ + H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3(O)

The Cr₂O₇^2- ion takes up electron in the acidic medium and hence acts as an oxidising agent.

Cr₂O₇ ^2- + 14H⁺ + 6e- →  2Cr³⁺ + 7 H₂O

Cr₂O₇ ^2- ( Cr in +6 oxidation state) is itself reduced to Cr³⁺ (Cr in +3 oxidation state) and its equivalent weight will be :

Equivalent weight of K₂Cr₂2O₇ = Molecular weight /  6 = 294 /6 = 49

K₂Cr₂2O₇  is used as a primary standard in volumetric analysis because it is hygroscopic.

Oxidising reaction of K₂Cr₂2O₇

1) It liberates I₂ from KI

K₂Cr₂O₇  + H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3O

2KI + H₂SO₄ + O  → K₂SO₄ + H₂O + I₂ ] × 3

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6KI + K₂Cr₂O₇  +  7H₂SO₄  → 4 K₂SO₄ +Cr₂(SO₄)₃ + 7H₂O + 3 I₂

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Cr₂O₇ ^2- + 14 H⁺ + 6e^– → 2Cr³⁺ + 7 H₂O

6I^– → 3I₂  + 6 e^–

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Cr₂O₇ ^2- + 14 H⁺ + 6I^– → 2Cr³⁺ + 7 H₂O + 3I₂

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2) It oxidises ferrous salts to ferric salts 

K₂Cr₂O₇  + H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3O

2FeSO₄ + H₂SO₄ + O → Fe₂(SO₄)₃ + H₂O ] × 3

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6 2FeSO₄ + K₂Cr₂O₇  + 7 H₂SO₄  → 3 Fe₂(SO₄)₃  + K₂SO₄ + Cr₂(SO₄)₃ + 7 H₂O

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or ferrous to ferric ions

6 Fe²⁺ + Cr₂O₇ ^2- + 14 H⁺ —–> 6 Fe³⁺ + 2 Cr³⁺ + 7 H₂O

3) It oxidises hydrogen sulphide to sulphur

K₂Cr₂O₇  + 4 H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3O

H₂S + O → H₂O + S ] × 3

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3 H₂S + K₂Cr₂O₇  + 4 H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 3 S + 7 H₂O

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3 H₂S + Cr₂O₇ ^2- + 8 H ⁺ → 2 Cr³⁺ + 3 S + 7 H₂O

4) It oxidises sulphites to sulphates

K₂Cr₂O₇  + 4 H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3O

Na₂SO₃ + O -→ Na₂SO₄ ] × 3

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3 Na₂SO₃ + K₂Cr₂O₇  + 4 H₂SO₄ → 3 Na₂SO₃ + K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O

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3 SO₃^2- + Cr₂O₇ ^2- + 8 H⁺ → 2 Cr³⁺ + 3 SO₄^2- + 4H₂O

It also axidises thiosulphates to sulphates and sulphur

3 S₂O₃^2- + Cr₂O₇ ^2- + 8 H⁺ → 2 Cr³⁺ + 3 SO₄^2- + 4H₂O + 3S

It oxidises , chlorides to chlorine , nitrites to nitrates , sulphur dioxide to sulphuric acid , thiosulphates to sulphates and sulphur.

Cr₂O₇ ^2- + 6Cl¯ + 14H⁺ → 2 Cr³⁺ + 3 Cl₂ + 7 H₂O

Cr₂O₇ ^2- + 3 NO₂¯ +8 H⁺ → 2 Cr³⁺ + 3 NO₃¯ + 4 H₂O

Cr₂O₇ ^2- + 3 SO₂ + 2H⁺ → Cr₂(SO₄)₃  + H₂O

5) It oxidises stannous salt to Stannic salts

Cr₂O₇ ^2- + 14H⁺+ 3 Sn²⁺ → 2 Cr³⁺ + 3 Sn⁴⁺ + 7 H₂O

6) It oxidises ethyl alcohol to acetaldehyde and acetic acid

K₂Cr₂O₇  + 4 H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3 O

CH₃CH₂OH  + O → CH₃CHO + H₂O

CH₃CHO + O → CH₃COOH

Structure of Chromate and Dichromate Ions

1) The chromate ion CrO₄^2- has tetrahedral structure in which four atoms around Cr atoms are oriented in a tetrahedral arrangement. All the four Cr–O bonds are equivalent.

2) The dichromate ion consist of two tetrahedral units sharing one corner with Cr-O-Cr bond angle of 126°

3) These two Cr-O bonds are larger than the other six equivalent Cr-O bonds.

 Structure of Chromate ion

Structure of Dichromate ion

Uses of Potassium Dichromate

1) It is used in dyeing and calico printing

2) It is used in chrome tanning in leather industry.

3) For the preparation of a large number of chromium compounds such as chrome yellow , chrome red , chrome alum , zinc yellow etc.

4) It is used as a volumetric reagent in laboratory in redox titration for the estimation of ferrous ions , iodide ions.

5) In photography for hardening gelatin film.

6) A mixture of K₂Cr₂O₇ and conc H₂SO₄ is used as a cleaning mixture in the laboratories.

7) In organic chemistry as an oxidising agent.