Potassium dichromate K2Cr2O7
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
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.
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.
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 O72 − + 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
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
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.