Question

Question 12.25 Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?

Class XI Organic Chemistry : Some Basic Principles and Techniques Page 363

Answer 1

Hi

Here is your answer,

Sodium extract is boiled  with nitric acid to decompose NaCN and Na₂S if present.

                                NaCN + HNO₃ ------> NaNO₃ + HCN ↑

                                Na₂S + 2HNO₃ -------> 2NaNO₃ + H₂S ↑

If cyanide and sulphide are not removed, they will react with AgNO₃ and hence, will interfere with the silver nitrate test for halogens.

            NaCN + AgNO₃ --------> AgCN + NaNO₃
                                                  (white ppt.)
            Na₂S + 2AgNO₃ ---------> Ag₂S + 2NaNO ₃
                                                   ( black ppt.)

Answer 2

If organic compound contains nitrogen and sulphur , the sodium extract on boiling with dilute HNO3 ( nitric acid) decomposes sodium cyanide or Sodium sulphide formed during fusion . NaCN + HNO3 ----> NaNO3 + HCN↑
Na2S + 2HNO3 -------> 2NaNO3 + H2S↑

if cyanide and sulphide ions are not decomposed , they will react with silver nitrate and hence, will interfere with the test .
NaCN + AgNO3 -----> AgCN(white ppt) + NaNO3
Na2S + 2AgNO3 -----> Ag2S (black ppt) + 2NaNO3
That's why nitric acid is added to sodium extract before adding of silver nitrate for testing halogens .