(a) Describe the preparation of potassium dichromate.

Explanation

Potassium dichromate is prepared from chromate ore. The ore is concentrated by gravity separation . It is mixed with excess sodium carbonate and lime and roasted in a reverbratory furnace .
`4FeCr_(2) O_(4) + 8Na_(2) CO_(3) + 7O_(2) overset(900 - 100^(@)C(to)`
The roasted mass is treated with water to separate soluble sodium chromate from insoluble iron oxide. They yellos solution of sodium chromate is treated with conc. sulphuric acid which converts sodium chromate into sodium dichromate .
`2Na_(2) CrO_(4) + H_(2) SO_(4) to Na_(2) Cr_(2) O_(3) + Na_(2) SO_(4) + H_(2) O`
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 `Na_(2) SO_(4) 2H_(2)O`.
The saturated solution of sodium dichromate in water in mixed with KCI and then con.c entreated to get crystal of NaCI. It is filtered while hot and filrate cooled to obtain `K_(2) Cr_(2) O_(7)` crystals.