Question

For the acidification of sodium extract for testing sulphur by lead acetate test, acetic acid was used in test tube A and sulphuric acid was used in test tube B. Which one of them will give the correct result?

• A. Test tube A
• B. Test tube B
• C. Both of them
• D. Neither of them

Answer 1

The correct option is A Test tube A

For testing of sulphur present in the organic compound, Lassaigne's extract is acidified with acetic acid $\left(C{H}_{3}COOH\right)$ and not sulphuric acid because lead acetate is soluble and does not interfere with the test. If sulphuric acid $\left(H_{2}S{O}_4\right)$ is used, lead acetate will react with $H_{2}S{O}_4$ itself to form white precipitate of $PbS{O}_4$ as follows: $Pb(C{H}_{3}COO{)}_2+H_{2}S{O}_4⟶PbS{O}_4\downarrow +2C{H}_{3}COOH$ The white precipitate of $PbS{O}_4$ so formed will interfere with this test of sulphur. $Pb(C{H}_{3}COO{)}_2+N{a}_{2}S⟶PbS\downarrow +2C{H}_{3}COONa$ (sulphur from Black ppt. the organic compound).