Acetic acid and not sulphuric acid is essential for acidification of sodium extract for testing sulphur by lead acetate test because

Explanation

For testing sulphur, the sodium extract is acidified with acetic acid because lead acetate is soluble and does not interfere with the test. If `H_(2)SO_(4)` were used, lead acetate itself will react with `H_(2)SO_(4)` to form white ppt. of lead sulphate which will interfere with the test.
`underset("Lead acetate")(Pb(OCOCH_(3))_(2) )+H_(2)SO_(4) rarr underset("(White ppt.)")(PbSO_(4)darr) + 2CH_(3)COOH`