test for halides

if we use sulphuric acid, some compound like calcium salt will
form ppt. then it will affect the result. it is because calcium sulphate
is insoluble. so we have to use nitric acid because all nitrate
compound are soluble.

Fristly, we have to know why acidified is required.

Because many anions (e.g. carbonate and sulphate) can from ppt with Silver(I) ion.

2Ag (aq) (CO3)^2- (aq) →Ag2CO3(s)

But after acidified.

Ag2CO3 (s) 2H (aq) → 2Ag (aq) CO2(g) H2O(l)

If H2SO4 is used, inaccurate result will appear.

e.g. Ba2 , Ca2 , Pb2 they can form ppt with (SO4)^2-

Ba2 (aq) (SO4)^2- (aq) →BaSO4(s)
Ca2 (aq) (SO4)^2- (aq) →CaSO4(s)
Pb2 (aq) (SO4)^2- (aq) →PbSO4(s)

(Very clearly, HCl cannot be used)