Why is acid added to sodium extract before adding silver nitrate for testing halogens ?

Explanation

Problem can arise in case the organic compound which also contains nitrogen and sulphur in addition to halogens. Both will form `NaCN` and `Na_(2)S` on fusion with sodium metal and will react with silver nitrate solution to give precipitates.
`NaCN+AgNO_(3) rarr underset("White ppt.")(AgCN)+NaNO_(3)`
`Na_(2)S+2AgNO_(3) rarr underset("Black ppt.")(Ag_(2)S)+2NaNO_(3)`
These precipitates will interfere with the precipitate of silver of silver halide formed due to halogens. Nitric acid is added to destroy both these compounds to form volatile gases.
`NaCN+HNO_(3) rarr NaNO_(3)+HCN uarr`
`Na_(2)S+2HNO_(3) rarr 2NaNO_(3)+H_(2)S uarr`