When orange crystals of ammonium dichromate(VI),((NH4)2)Cr2O7 is heated,the crystals decompose to form chromium(III) oxide,nitrogen and water.
Describe a confirmatory test to confirm the presence of ammonium ion in ammonium dichromate(VI).
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When a solution of an ammonium salt is heated with NaOH, ammonia gas is liberated which is characterized by its pungent smell, or by the brown coloured precipitate that forms when a paper soaked with Nesseler’s reagent is placed into its path. This method eliminates the chance of interference in this extremely sensitive test in solution.
(NH4)2Cr2O7 + 2NaOH = 2NH3 + Na2Cr2O7 + 2H2O
NH3 + H2O = NH4OH
NH4+ + 2[HgI4]2− + 4OH− → HgO·Hg(NH2)I (brown) + 7I− + 3H2O
So, the test would be to boil a little aq. solution of ammonium dichromate with aq. NaOH and to put a paper soaked with Nesseler’s solution in the mouth of the test tube. A brown precipitate in the paper indicating a positive response.
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