Determination of phosphorus by sample combustion followed by non-suppressed ion chromatography

The determination of phosphorus using sample combustion in oxygen followed by quantification of orthophosphate using non-suppressed ion chromatography (IC) was studied using phenylphosphonic acid as a model sample compound. Even under optimal conditions, the recovery of phosphorus (measured as orthophosphate) was < 70% when dilute aqueous hydrogen peroxide was used as the absorbing solution employed to collect the combustion products. However, use of inductively coupled plasma atomic emission spectroscopy and spectrophotometry using heteropoly blue formation showed that all of the phosphorus in the original sample was in fact also present in the absorbing solution, but suggested that some of this phosphorus was present as forms other than orthophosphate. 31P Nuclear magnetic resonance was used to study the composition of the combustion solution and confirmed the presence of orthophosphate, pyrophosphate and trimetaphosphate. Hydrolysis of the combustion sample solution by boiling for at least 20 min at pH < 3 was shown to convert all of these forms of phosphorus to orthophosphate, thereby enabling their determination by IC. Quantitative recoveries of phosphorus were achieved when this post-combustion hydrolysis step was incorporated into the analytical procedure.