Analytical ChemistryHard

Question

The Lassaigne′s extract is boiled with dil. HNO3 before testing for halogens because

Options

A.Silver halides are soluble in HNO3
B.Na2S and NaCN are decomposed by HNO3
C.Ag2S is soluble in HNO3
D.AgCN is soluble is HNO3

Solution

Na2S and NaCN, formed during fusion with metallic sodium, must be removed before adding AgNO3 otherwise black ppt. due to Na2S or white precipitate due to AgCN will be formed and thus white precipice of AgCl will not identified easily.
Na2S + 2AgNO3 → 2NaNO3 + Ag2S↓
                                                        Black
NaCN + AgNO3 → NaNO3 + AgCN ↓
                                                  White
NaCl + AgNO3 → NaNO3 + AgCl ↓
                                                  White
NaCN + HNO3 NaNO3 + HCN ↑
Na2S + 2HNO3 2NaNO3 + H2S ↑

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