the function of conc . HNO3in the detection of halogen is--------------?

(A)Silver Nitrate Test

(a) When nitrogen and / or sulphur are absent

Take 1-2 ml of sodium extract in test tube and acidify it with dilute nitric acid. Add silver nitrate solution (0.5ml). A white precipitate soluble in Ammonium hydroxide indicates the presence of chlorine. A pale yellow precipitate soluble in excess of Ammonium hydroxide solution indicates the presence of bromine. And a yellow precipitate insoluble in Ammonium hydroxide indicates the presence of iodine.

Reaction of Halogens:

Sodium halide (ionic form of halogen in extract) reacts with silver nitrate to give the precipitate of silver halide.

NaX+AgNO₃?AgX?+NaNO₃

(ppt)

If X=Cl, then AgCl (silver chloride) will be formed which is white colour precipitate soluble in Ammonium hydroxide.

If X=Br, then AgBr (silver bromide) will be formed which is pale yellow colour precipitate soluble in excess of Ammonium hydroxide.

If X=I, then AgI (silver iodide) will be formed which is yellow colour precipitate insoluble in Ammonium hydroxide.

(b) When nitrogen and / or sulphur are present

Take 1 ml of sodium extract in test tube and acidify it with conc. nitric acid (2-3ml). Boil the mixture to reduce the original volume to half to get rid of HCN and / or H₂S. And now perform the silver nitrate test.

When the silver nitrate test is positive (precipitate is obtained) then the layer test is to be performed.

The positive silver nitrate test only indicates the presence of halogens. And which of the halogen is present is confirmed by the Layer test.

Thank You
Ruchi
Askiitians Faculty