INTERFERENCE OF SOLUBLE SILICA IN THE DETERMINATION OF ORTHOPHOSPHATE-PHOSPHORUS

Determination of P concentration in dilute solutions is currently used to assess P loadings to fresh and marine waters from nonpoint sources. The aim was to ascertain possible P overestimates from interferences caused by the presence of silica. The commonly used methods of Murphy and Riley, Dick and Tabatabai, and the Italian ''Instituto di Ricerca Sulle Acque'' (IRSA) were tested for possible interferences caused by Si/P ratios (1000-5000) similar to those reported to occur in soil solutions and in groundwaters. The three methods overestimated P concentration due to the presence of Si in the solutions, especially if the spectrophotometric readings were taken more than 10 min from the addition of the reagents. The least interference occurred with the procedure of Murphy and Riley, which is therefore recommended.