Preparation on `K_(2)Cr_(2)O_(7)` from chromite ore :
i) a) Potassiu dichromate is obtained by the fusion of chromite ore `(FeCr_(2)O_(4))` with sodium (or potassium) carbonate in the presence of excess of air.
`4FeCr_(2)O_(4)+8Na_(2)CO_(3)+7O_(2)rarr underset("(Yellow solution)")(8Na_(2)CrO_(4))+2Fe_(2)O_(3)+8CO_(2)`
b) The solution is filtered and treated with sulphuric acid.
`2Na_(2)CrO_(4)+underset(("From "H_(2)SO_(4)))(2H^(+)) rarr underset("Sodium dichromate (orange)")(Na_(2)Cr_(2)O_(7)(s)+2Na^(+)+H_(2)O)`
c) Now sodium dichromate is treated with potassium chloride. As a result, potassium dichromate is produced.
`Na_(2)Cr_(2)+2Clrarrunderset("Potassium dichromate crystals")(K_(2)Cr_(2)O_(7)+2NaCl)`
ii) Effect of increasing `pH" on "K_(2)Cr_(2)O_(7)` solution :
`underset("Orange")(Cr_(2)O_(7)^(2-))+2OH^(-)rarrunderset("Yellow")(2CrO_(4)^(2-))+H_(2)O`
On increasing `pH, K_(2)Cr_(2)O_(7)` changes into `K_(2)CrO_(4)` (orange to yellow)
