A comparison of methods for the determination of dissolved and particulate phosphorus in natural waters

Abstract

A comparison of five methods used for the determination of dissolved total phosphorus (DTP) in natural waters is reported. The methods compared different approaches to oxidizing organic-P compounds, and destroying polyphosphates to give forms available to standard colorimetric methods. The methods used were ultraviolet (UV) irradiation, high-temperature treatment with magnesium persulphate, autoclaving under acidic or alkaline conditions, and magnesium nitrate oxidation. One series of experiments measured the oxidation efficiency of the methods when used with model organic-P compounds (phosphomonoesters, P-nucleosides, polyphosphates, metaphosphates, and phosphonates) which were diluted in fresh and sea waters. The methods were also applied to four different marine and fresh water samples containing naturally occurring dissolved organic-P and polymeric-P compounds. The five methods had on average a greater than 90% oxidation efficiency for phosphomonoesters, including phospho-sugars and phosphonates. However, only the methods that include a hydrolysis step were able to satisfactorily depolymerize polyphosphates and metaphosphates as well as destroy the nucleosides adenosine-5-triphosphate and guanosine-5 diphosphate. The highest DTP concentrations in the natural water samples were obtained by the magnesium nitrate oxidation method, whilst the lowest values were given by the UV irradiation technique, implying that this latter commonly used technique can seriously underestimate DTP when resistant compounds represent a significant fraction of the DOP. Overall, the magnesium nitrate method was chosen as the best technique of those tested. In a slightly modified form the technique was used to determine particulate phosphorus in natural waters.