GYPSUM DETERMINATION IN SOILS BY CONVERSION TO WATER-SOLUBLE SODIUM-SULFATE

Precise and accurate determination of gypsum in soils has been difficult by the wet-chemical methods, based on its solubility in water. Zn this work, 2-g samples of three Typic Gypsiorthids were washed once with 25-mL portions of 50% (v/v) ethanol to selectively remove nongypsic soluble sulfates. Soil gypsum was subsequently converted to CaCO3(s) and Na2SO4 by ultrasonic dispersion of the samples twice with 25-mL increments of 0.5 M Na2CO3 solution. The SO42- in the supernatant solution was centrifuge separated and analyzed by the standard gravimetric BaSO4 (A) and turbidimetric (B) methods to obtain the gypsum content of the samples. The agreement between the data of Methods A and B was excellent where the best-fitted regression line coincided with the 1:1 line in a scattergram. Soil gypsun was also extracted with water, then determined by turbidimetric SO42- (C) and the Bower-Huss conductometric: (D) methods for comparison. Statistical analyses of the data showed higher values (P = 0.05) of gypsun in soils obtained by the proposed method than by water extraction even when the same method of SO42- analysis was applied, The efficiency of the proposed method was 25 to 31 times greater than that achieved by water. The proposed method of gypsum extraction and its determination in soils from turbidimetric SO(4)(2-)analysis is simple, rapid, and quite accurate for soil taxonomy, land reclamation, and other engineering purposes.