EVALUATION OF AN ENZYMATIC METHOD FOR ORTHO-PHOSPHATE DETERMINATION IN FRESHWATERS

An automated enzymatic method for orthophosphate determination was applied to freshwaters. The basis of the assay was the reaction of glyceraldehyde-3-phosphate with orthophosphate to form 1,3-diphosphoglycerate in the presence of glyceraldehyde-3-phosphate dehydrogenase and oxidized nicotinamide-adenine dinucleotide. Nicotinamide-adenine dinucleotide, reduced form, was produced in equimolar amounts to orthophosphate and was determined colorimetrically. The method was tested up to 200 μg P l-1 and had a limit of detection of 5 μg P l-1. Silicate or arsenite did not interfere but arsenate caused a positive interference. The results of the analysis of 100 freshwater samples by the enzymatic orthophosphate method and by the well-established soluble reactive phosphorus method indicated that a form of phosphorus was present which was determined as soluble reactive phosphorus but not as enzymatic orthophosphate. The mean soluble reactive phosphorus concentration was 71 μg P l-1 compared with a mean enzymatic orthophosphate concentration of 56 μg P l-1. A hydrous ferric oxide-orthophosphate complex was postulated to explain why soluble reactive phosphorus was greater than enzymatic orthophosphate. © 1979.