Determination of total phosphorus in natural waters with a simple neutral digestion method using sodium persulfate

Abstract

The determination of total phosphorus (TP) in an aqueous sample is based on digestion of the sample to convert phosphorus compounds into orthophosphate, which can then be determined based on spectrophotometry. The widely used oxidant, potassium persulfate, has poor aqueous solubility and requires careful handling with furnace, autoclave, or oven. Here, a very simple method for the determination of TP has been developed, using an inexpensive portable water boiler for neutral digestion and highly water soluble sodium persulfate instead of potassium persulfate. The key experimental factors have been evaluated and optimized. The optimal conditions for efficient TP determination were found to be 3 h of digestion time at sub-boiling temperature, followed by addition of ascorbic acid to eliminate free chlorine generated from the saline sample, and then addition of the ammonium molybdate reagent for detection by UV-Vis spectrophotometry. The detection limit is 0.02 μM using 5-cm cuvette. Under these optimized conditions, both artificial solutions containing model phosphorus compounds (n519) and natural water samples (n=14) were analyzed using both sodium persulfate and potassium persulfate. There is no significant difference between the digestion performances of these two oxidants. This new method was used to analyze natural water samples (n=28) in comparison with the standard autoclaving method using potassium persulfate. The analytical results using this method agreed very well with the standard method. Samples of different matrix were analyzed, showing the successful application of this method in both environmental monitoring (time series and spatial survey) and marine science (spatial and vertical distribution).