The measurement of dissolved phosphorus compounds: Evidence for hydrolysis during storage and implications for analytical definitions in environmental analysis

Labile organophosphorus and inorganic polyphosphate compounds have been identified as a potential source of error in the measurement of inorganic monophosphate in natural samples. To test this, a range of synthetic phosphorus containing compounds were used to prepare solutions of known composition, which were measured by three commonly used analytical methods for the determination of monophosphate in natural waters. The solutions were also measured after acidic persulphate digestion, to assess the overall recovery of phosphorus by a standard total dissolved phosphorus analysis. No significant concentration of phosphorus was detected in any of the solutions by the three monophosphate methods, when analysed within 4 h of preparation. However, when analysed after 72 h storage, about half of the selected compounds showed an increase in the monophosphate concentration. The phosphorus concentrations measured after acidic digestion were in close agreement with the formulated values for the compounds. These results indicate that the selected compounds are included in the total dissolved fraction, but may also contribute to the monophosphate fraction by hydrolysis to monophosphate in solution after storage.