Overestimation of Orthophosphorus Concentrations in Surface Waters of Southern Lake Michigan: Effects of Acid and Ammonium Molybdate

Water from southern Lake Michigan was analyzed to determine if orthophosphorus (PO 4 -P) concentrations are overestimated by molybdenum blue methods and to assess the potential effects of ammonium molybdate on soluble reactive phosphorus (SRP) determinations. Time-course "hydrolysis" tests, based on the Chamberlain–Shapiro extraction method, showed that large amounts of PO 4 -P were released from bound sources into solution within seconds after acid molybdate was added, and that molybdate-reactive PO 4 -P could be retained by filters or released from particulate material into filtrates during filtration. PO 4 -P concentrations obtained by using the prescribed exposure of 30 s were overestimated minimally by up to 86%, and ammonium molybdate dramatically accelerated PO 4 -P release in filtered samples. Tests using up to a fourfold range in ammonium molybdate concentrations also showed that the extraction method and Harvey's method yielded SRP values that varied two- to three-fold or more, and that most of the PO 4 -P was released from "particles" in filtrates. Analytical methods, therefore, can yield substantially different SRP estimates because of differences in the length of time samples are exposed to acid molybdate and because of differences in molybdate concentrations.Key words: ammonium molybdate, hydrolysis, phosphorus, soluble reactive phosphorus, orthophosphorus