For testing sulphur, the sodium extract is acidified with acetic acid because lead acetate is soluble and does not interfere with the test. If \(\mathrm{H}_2 \mathrm{SO}_4\) were used, lead acetate itself will react with \(\mathrm{H}_2 \mathrm{SO}_4\) to form white ppt. of lead sulphate which will interfere with the test.
\(\underset{\text{Lead acetate}}{\mathrm{Pb}\left(\mathrm{OCOCH}_3\right)_2}+\mathrm{H}_2 \mathrm{SO}_4 \longrightarrow \mathrm{PbSO}_4 \downarrow+\underset{(White ppt.)}{2 \mathrm{CH}_3 \mathrm{COOH}}\)