It necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test because sulphuric acid will react with lead acetate To form a precipitate of lead sulphate, thus interfering in the test of sulphur.
Pb (CH3COO) 2 + H2SO4 → PbSO4↓ + 2CH3COOH