An organic compound contains C, H, N, S and Cl. For the detection of chlorine, the sodium extract of the compound is first heated with a few drops of concentrated HNO3 and then AgNO3 is added to get a white ppt of AgCl. The digestion with HNO3 before the addition of AgNO3 is

Na + C + N→∆NaCN 2Na+S→∆Na2SBoth NaCN and Na2S will react with AgNO3 to form AgCN (white ppt.) and Ag2S (black ppt.)They must be removed before performing AgNO3 test for halogensThus, on adding HNO3, NaCN and Na2SNaCN + HNO3 → NaNO3 + HCN ↑ Na2S + 2HNO3 →2NaNO3 + H2S ↑Hence, CN– and S2– are removed as HCN and H2S by adding HNO3.