HIGH-PERFORMANCE ION CHROMATOGRAPHY MEASUREMENT OF SULFATE IN 20 MM PHOSPHATE EXTRACTS OF SOIL

High Performance Ion Chromatography (HPIC) methods for measuring sulfa te in 20 mM phosphate extracts of soil need to cope with low sulfate ( down to 0.1 ppm S) in up to 5000 times the molar concentration of phos phate and potassium or calcium, a diversity of solutes including organ ic compounds, and colloidal particles. Anions were detected by electri cal conductivity rather than indirect photometry, because interference from the high phosphate concentration on sulfate measurements was muc h less, particularly under acid conditions. Orthophosphate has a much lower limiting equivalent conductivity than sulfate, and H2PO4- has on ly half the conductivity of HPO42-. Also, H2PO4- is retained less, and in acid gives a wide peak separation from sulfate. Under the acid con ditions proposed the phosphate peak is negative, and elutes just after the solvent front, thus eliminating interference on sulfate. By makin g the acid pH of the eluent that of the extractant, pH 4, baseline cha nges were also minimized. Eluents of trimesic acid (TMA) and o-phthali c acid (PA), were investigated. PA required a higher eluent concentrat ion than TMA to elute sulfate rapidly but, with the instrumentation us ed, the stability of the higher baseline conductance was no problem an d PA, unlike TMA, produced no interfering system peaks. The method usi ng PA has been used commercially by the New Zealand AgResearch Soil Fe rtility Service for over three years on a variety of soils involving o ver 75,000 samples.