Richard C. Sheridan

Simultaneous determination of fluoride, phosphate and sulfate in wet-process phosphoric acid by ion chromatgraphy

Abstract In the production of phosphoric acid, frequent, rapid and accurate determinations of sulfate and phosphate are needed to monitor the concentration of product acid, prepare recycle acid of the proper concentration, and maintin the desired amount of free sulfuric acid in the slurry. The density method for phosphate and the barium sulfate-centrifuge method for sulfate, along with various other methods, have been widely used. However, they are not as accurate and reliable as desired becasue they are affected by changes in the amounts of impurities in the acid. The reactor slurry also contains significant amounts of fluoride, which may affect both the composition and crystalline habit of the by-product calcium sulfate. Thus, a routine, yet acurate, method is needed for the simultaneous determination of anions in wet-process phosphoric acid. The use of ion chromatography (IC) has grown rapidly in recent years, and a few applications to fertilizer materials have been reported. Phosphate rock1–3, urea4–6, and granular fertilizers7 were successfully analyzed, but no reports have appeared on the analysis of wet-process phoshoric acid by IC. This note describes a simple, rapid, and reliable ion chromatographic method for the simultaneous determination of fluoride (F-), phosphate (HPS2-4), and sulfate (SO2-4 in wet-process phosphoric acid.