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Analysis of Group VI

Radical      : Mg²⁺

Group reagent   : Na₂HPO₄ in presence of NH₄Cl/NH₄OH

Precipitate         : White

Principle: Filtrate of gp-V is heated with (NH₄)₂SO₄ and ammonium oxalate and filtered. Filtrate is cooled and NH₄OH is added followed by addition of disodium hydrogen phosphate (Na₂HPO₄).

A white ppt. is formed slowly after scratching the sides of test tube.

Sky blue colour also confirms Mg²⁺ if formed on addition of magneson reagent (p-nitroazoresorcinol) and NaOH. 

 

Reactions and Explanations:

MgCl₂ + Na₂HPO₄ + NH₄OH → Mg(NH₄)PO₄ + 2NaCl + H­₂O

Magnesium ammonium Phosphate                        

(white ppt.)