COMPARISON OF SOME METHODS FOR DETERMINATION OF SULFATE IN SOILS

Sulfate (SO42-) is present in soils as salts of various metals, and the different metals associated with sulfate may influence adsorption of SO42- by soils. The analytical method used for determination of SO42- could be affected by the type of metal associated with the SO42-. Four analytical methods based on different principles were evaluated for determination of SO42- in different metal salts and in soil extracts obtained with three extractants {0.1M lithium chloride (LiCl), 0.15% calcium chloride (CaCl2), and 500 mg P/L as calcium phosphate (Ca(H2PO4)2]}. The analytical methods were: (i) a methylene blue (MB) colorimetric method after the reduction of SO42- to hyrogen sulfide (H2S), (ii) an ion chromatographic (IC) method, (iii) a turbidimetric (TD) method, and (iv) an indirect barium (Ba) atomic absorption spectrophotometric (SP) method. The recovery Of SO42- associated with various mono-, di-, and trivalent metals was quantitative by the MB method. But, trivalent metals, such as aluminum (Al), indium (In), lanthanum (La), and scandium (IC), decreased the recovery Of SO42- by the other three methods. The MB and IC methods gave similar values for SO42- in soils by using the three extractants. The TD and SP methods gave variable results and, in general, underestimated the amounts of SO42- in soils. Among the four methods, the MB and IC methods were the most accurate and precise.