Coordination compounds and double salts:
When two or more stable compounds in solution are mixed together and allowed to evaporate, in certain cases there is a possibility for the formation of double salts or coordination compounds. For example when an equimolar solution of ferrous sulphate and ammonium sulphate are mixed and allowed to crystallise, a double salt namely Mohr's salt (Ferrous ammonium sulphate, FeSO4.(NH4)2SO4.6H2O) is formed. Let us recall the blood red colour formation in the inorganic qualitative analysis of ferric ion, the reaction between ferric chloride and potassium thiocyanate solution gives a blood red coloured coordination compound, potassium ferrithiocyanate K3 [Fe(SCN)6]. If we perform a qualitative analysis to identify the constituent ions present in both the compounds, Mohr's salt answers the presence of Fe2+ ,NH4+ and SO42- ions, whereas the potassium ferrithiocyanate will not answer Fe3+ and SCN-ions. From this we can infer that the double salts loose their identity and dissociates into their constituent simple ions in solutions , whereas the complex ion in coordination compound, does not loose its identity and never dissociate to give simple ions.
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