High performance ion chromatography measurement of sulfate in 20 mM phosphate extracts of soil

Abstract

High Performance Ion Chromatography (HPIC) methods for measuring sulfate 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 phosphate and potassium or calcium, a diversity of solutes including organic compounds, and colloidal particles.

Anions were detected by electrical conductivity rather than indirect photometry, because interference from the high phosphate concentration on sulfate measurements was much less, particularly under acid conditions. Orthophosphate has a much lower limiting equivalent conductivity than sulfate, and H₂PO₄ ^‐ has only half the conductivity of HPO₄ ^2‐. Also, H₂PO₄ ^‐ is retained less, and in acid gives a wide peak separation from sulfate. Under the acid conditions proposed the phosphate peak is negative, and elutes just after the solvent front, thus eliminating interference on sulfate. By making the acid pH of the eluent that of the extractant, pH 4, baseline changes were also minimized.

Eluents of trimesic acid (TMA) and o‐phthalic acid (PA), were investigated. PA required a higher eluent concentration than TMA to elute sulfate rapidly but, with the instrumentation used, the stability of the higher baseline conductance was no problem and PA, unlike TMA, produced no interfering system peaks. The method using PA has been used commercially by the New Zealand AgResearch Soil Fertility Service for over three years on a variety of soils involving over 75,000 samples.