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Describe the preparation of potassium dichromate. |
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Answer» Solution :Preparation of potassium dichromate: (i) Potassium dichromate is prepared from chromite ore. The ore `FeO.Cr_(2)O_(3)` is concentrated by gravity separation process. (ii) The concentrated ore 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 - 1000^(@)C)to8 Na_(2) CrO_(4) + 2Fe_(2)O_(3) +8CO_(2)uarr` (iii) 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. `underset(underset("(yellow)")("sodium chromate"))(2Na_(2)CrO_(4)) + H_(2)SO_(4) tounderset(underset("(orange red)")("sodium dichromate"))(Na_(2)Cr_(2)O_(7)) + Na_(2)SO_(4) +H_(2)O` (iv) The above solution is concentrated to remove less soluble sodium sulphate. The RESULTING solution is filtered and concentrated. It is cooled to get the CRYSTALS of `Na_(2)SO_(4)2H_(2)O`. (v) 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 `K_(2)Cr_(2)O_(7)` crystals. `underset(underset("(orange red)")("sodium dichromate"))(Na_(2)Cr_(2)O_(7))+ 2KCI to underset(underset("(orange red)")("potassium dichromate"))(K_(2)Cr_(2)O_(7))+ 2NaCl` |
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