Potassium dichromate

Potassium dichromate K₂Cr₂O₇

Preparation:

Potassium dichromate is prepared from chromate ore. The ore is concentrated by gravity separation. It is then mixed with excess sodium carbonate and lime and roasted in a reverbratory furnace.

4 FeCr₂ O₄ + 8 Na₂ CO₃ + 7 O₂ →^{900 - 10000C}→ 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂ ↑

The roasted mass is treated with water to separate soluble sodium chromate from insoluble iron oxide. The yellow solution of sodium chromate is treated with concentrated sulphuric acid which converts sodium chromate into sodium dichromate.

2 Na₂ CrO₄ + H₂SO₄ →Na₂Cr₂O₇ + Na₂SO₄ + H₂O

The above solution is concentrated to remove less soluble sodium sulphate. The resulting solution is filtered and further concentrated. It is cooled to get the crystals of Na₂SO₄.2H₂O.

The saturated solution of sodium dichromate in water is mixed with KCl and then concentrated to get crystals of NaCl. It is filtered while hot and the filtrate is cooled to obtain K₂Cr₂O₇ crystals.

Na₂ Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

physical properties:

Potassium dichromate is an orange red crystalline solid which melts at 671K and it is moderately soluble in cold water, but very much soluble in hot water. On heating it decomposes and forms Cr₂O₃ and molecular oxygen. As it emits toxic chromium fumes upon heating, it is mainly replaced by sodium dichromate.

Structure of dichromate ion:

Both chromate and dichromate ion are oxo anions of chromium and they are moderately strong oxidizing agents. In these ions chromium is in +6 oxidation state. In an aqueous solution, chromate and dichromate ions can be interconvertible, and in an alkaline solution chromate ion is predominant, whereas dichromate ion becomes predominant in acidic solutions. Structures of these ions are shown in the figure.

Chemical properties:

1. Oxidation

Potassium dichromate is a powerful oxidising agent in acidic medium. Its oxidising action in the presence of H+ ions is shown below. You can note that the change in the oxidation state of chromium from Cr⁶⁺ to Cr³⁺.Its oxidising action is shown below.

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → Cr³⁺ + 7 H₂O

The oxidising nature of potassium dichromate (dichromate ion) is illustrated in the following examples.

(i) It oxidises ferrous salts to ferric salts.

Cr₂ O₇^{2 −} + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

(ii) It oxidises iodide ions to iodine

Cr₂O₇²⁻ + 6I⁻ + 14H⁺ → 2Cr³⁺ + 3I₂ + 7H₂O

(iii) It oxidises sulphide ion to sulphur

Cr₂O₇²⁻ + 3S²⁻ + 14H⁺ → 2Cr³⁺ + 3S + 7H₂O

(iv) It oxidises sulphur dioxide to sulphate ion

Cr₂O₇²⁻ + 3SO₂ + 2H⁺ → 2Cr³⁺ + 3SO₄²⁻ + H₂O

(v) It oxidises stannous salts to stannic salt

Cr₂O₇²⁻ + 3Sn²⁺ + 14H⁺ 2Cr³⁺ + 3Sn⁴⁺ + 7H₂O

(vi) It oxidises alcohols to acids.

2K₂Cr₂O₇ + 8H₂SO₄ + 3CH₃CH₂OH → 2K₂SO₄ + 2Cr₂ ( SO₄ )₃ + 3CH₃COOH + 11H₂O

2. Chromyl chloride test:

When potassium dichromate is heated with any chloride salt in the presence of Conc H₂SO₄, orange red vapours of chromyl chloride (CrO₂Cl₂) is evolved. This reaction is used to confirm the presence of chloride ion in inorganic qualitative analysis.

K₂Cr₂O₇ + 4NaCl + 6H₂SO₄ → 2KHSO₄ + 4NaHSO₄ + 2CrO₂Cl₂ ↑ Chromyl chloride + 3H₂O

The chromyl chloride vapours are dissolved in sodium hydroxide solution and then acidified with acetic acid and treated with lead acetate. A yellow precipitate of lead chromate is obtained.

CrO₂Cl₂ + 4NaOH → Na₂CrO₄ + 2NaCl + 2H₂O

Na₂CrO₄ + ( CH₃COO)₂ Pb → PbCrO₄ ↓ (Leadchromate(Yellowprecipitate) + 2CH₃COONa

Uses of potassium dichromate:

Some important uses of potassium dichromate are listed below.

·           It is used as a strong oxidizing agent.

·           It is used in dyeing and printing.

·           It used in leather tanneries for chrome tanning.

·           It is used in quantitative analysis for the estimation of iron compounds and iodides.