4. POTASSIUM DICHROMATE

POTASSIUM DICHROMATE (K₂Cr₂O₇)

Preparation :

The chromite ore is roasted with sodium carbonate in presence of air in a reverberatory furnace.

The roasted mass is extracted with water when Na₂CrO₄ goes into the solution leaving behind
insoluble Fe₂O³. The solution is then treated with calculated amount of H₂SO₄.

The solution is concentrated when less soluble Na₂SO₄ crystallises out. The solution is further concentrated when crystals of Na₂Cr₂O₇ are obtained. Hot saturated solution of Na₂Cr₂O₇ is then treated with KCl when orange red crystals of K₂Cr₂O₇ are obtained on crystallisation. Sodium dichromate is more soluble than potassium dichromate.

Na₂Cr₂O₇ + 2KCI  ¾® K₂Cr₂O₇ + 2 NaCl

K₂Cr₂O₇ is preferred over Na₂Cr₂O₇ as a primary standard in volumetric estimation because Na₂Cr₂O₇ is

hygroscopic in nature but K₂Cr₂O₇ is not.

Properties

(a) Physical :

It is orange-red coloured crystalline compound. It is moderately soluble in cold water but freely soluble in hot water. It melts at 398°C.

Structure of Chromate and Dichromate ion

(b) Chemical :

(i) Effect of heating :

On heating strongly, it decomposes liberating oxygen.

2K₂Cr₂O₇ ¾® 2K₂CrO₄ + Cr₂O₃ + O₂

On heating with alkalies, it is converted to chromate, i.e., the colour changes from orange to yellow. On acidifying, yellow colour again changes to orange.

K₂Cr₂O₇ + 2KOH  ¾® 2K₂CrO₄ + H₂O

Cr₂O₇^2-(Orange) + 2OH¯  ¾®  2CrO₄^2- (Yellow)+ H₂O

2CrO(Yellow) + 2H⁺ ¾® Cr₂O₇^2- (Orange) + H₂O

Thus CrO₄^2– and Cr₂O₇^2– exist in equilibrium and are interconvertable by altering the pH of solution.

2CrO₄^2-  + 2H⁺2HCrO₄^–Cr₂O₇^2- + H₂O

In alkaline solution, chromate ions are present while in acidic solution, dichromate ions are present.

(ii) K₂Cr₂O₇ + 2H₂SO₄ (conc. & cold)  ¾® 2CrO₃¯ (bright orange/red) + 2KHSO₄ + H₂O

2K₂Cr₂O₇ + 8H₂SO₄ (conc. & Hot) ¾® 2K₂SO₄ + 8H₂O + 2Cr₂(SO₄)₃ + 3O₂

(iii)   Acidified K₂Cr₂O₇ solution reacts with H₂O₂ to give a deep blue solution due to the formation of CrO₅.

Cr₂O₇^2–   +  2H+  + 4H₂O₂ ¾® 2CrO₅ + 5H₂O

Blue colour in aqueous solution fades away slowly due to the decomposition of CrO₅ to Cr³⁺ ions and oxygen. In less acidic solution K₂Cr₂O₇ and H₂O₂ give salt which is violet coloured and diamagnetic due to the formation of [CrO(O₂)(OH)]^–.

(iv)Potassium dichromate reacts with hydrochloric acid and evolves chlorine gas.

K₂Cr₂O₇ + 14HCl ¾® 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂

(v) It acts as a powerful oxidising agent in acidic medium (dilute H₂SO₄)

Cr₂O₇^2- + 14H⁺ + 6e^– ¾® 2Cr⁺³ + 7H₂O. (Eº = 1.33 V)

The oxidation state of Cr changes from + 6 to +3.

(a) Iodine is liberated from potassium iodide :

(b) Ferrous salts are oxidised to ferric salts :

6Fe²⁺ + Cr₂O₇^2- + 14H⁺ ¾® 6Fe³⁺ + 2Cr³⁺ + 7H₂O

(c) Sulphites are oxidised to sulphates :

 Cr₂O₇^2- + 3SO₃^2- + 8H⁺¾® 3SO₄^2- + 2Cr³⁺ + 4H₂O

(d) H2S is oxidised to sulphur :

Cr₂O₇^2- + 3H₂S + 8H⁺ ¾® 2Cr³⁺ + 7H₂O + 3S

(e) SO₂ is oxidised to H₂SO₄ :

Cr₂O₇^2– + 3SO₂ + 2H+  ¾® 2Cr³⁺ + 3SO₄^2– + H₂O ;

Chrome alum is obtained when acidified K2Cr2O7 solution is saturated with SO2.

K₂Cr₂O₇ + H₂SO₄ + 3SO₂ + 23H₂O   K₂SO₄ . Cr₂(SO₄)₃ . 24H₂O

(f) It oxidises ethylalcohol to acetaldehyde and acetaldehyde to acetic acid

C₂H₅OH   CH₃CHO  CH₃COOH

 ethyl alcohol          acetaldehyde         acetic acid

(g) It also oxidises nitrites to nitrates, arsenites to arsenates, HBr to Br₂. HI to I₂, etc.

(h)  K₂Cr₂O₇ + 2C (charcoal)   Cr₂O₃ + K₂CO₃ + CO ↑­

(vi)     Chromyl chloride test :   

4Cl^– + Cr₂O₇^2– + 6H⁺ ¾® 2CrO₂Cl₂ ↑ (deep red) + 3H₂O

CrO₂Cl₂ + 4OH^– ¾® CrO₄^2– (yellow) + 2Cl^– + 2H₂O

CrO₄2^– (yellow) + Pb²⁺ ¾® PbCrO₄ ↓ (yellow)

(vii) Cr₂O₇^2– (concentrated solution) + 2Ag⁺ ¾® Ag₂Cr₂O₇↓ (reddish brown)

 Ag₂C_r2O₇ ↓ + H₂O   Ag₂CrO₄  ↓+ CrO₄^2– + 2H⁺ .

(viii)  Cr₂O₇^2– + Ba²⁺  + H₂O  2BaCrO₄ ↓+ 2H⁺

As strong acid is produced, the precipitation is only partial. But if NaOH or CH₃COONa is added, precipitate becomes quantitative.

Uses : It is used :

(i) as a volumetric reagent in the estimation of reducing agents such as oxalic acid, ferrous ions, iodide ions, etc. It is used as a primary standard.

(ii)  for the preparation of several chromium compounds such as chrome alum, chrome yellow, chrome red, zinc yellow, etc.

(iii) in dyeing, chrome tanning, calico printing, photography etc.

(iv) as a cleansing agent for glass ware in the form of chromic acid.

in leather industry and as an oxidant for preparation of azo compounds.