IIT JEE , AIEEE Forum - Chemistry , Inorganic Chemistry

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THESE ARE THE PRELIMINARY TESTS FOR THE THREE IONS YOU MENTIONED ABOVE

ACTION OF SODIUM HYDROXIDE SOLUTION

To a solution of the test substance, sodium hydroxide solution is added drop by drop until it is in excess (test with red litmus paper). Some solutions may be acidic to begin with (test the original solution with blue litmus paper); in such cases nothing will happen until the acid has all been neutralised

White precipitate soluble in excess NaOH: Al³⁺, Zn²⁺, Pb²⁺.

Dirty-green precipitate, turning brown on standing: Fe²⁺

Foxy red (rust coloured) precipitate: Fe³⁺.

The action of aqueous ammonia solution

To a solution of the test substance, ammonia solution is added drop by drop until it is in excess (test with red litmus paper). Some solutions may be acidic to begin with (test the original solution with blue litmus paper); in such cases nothing will happen until the acid has all been neutralised.

White precipitate insoluble in excess ammonia: Mg²⁺, Ca²⁺, Al³⁺, Pb²⁺.

Dirty-green precipitate, turning brown on standing: Fe²⁺.

Foxy red (rust coloured) precipitate: Fe³⁺.

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