The effects of post-persulfate-digestion procedures on total phosphorus analysis in water

There are differences between the EPA Method 365 and the APHA-AWWA-WEF's Standard Method 4500 with respect to the post-digestion treatment procedures of the persulfate-digested water. The effects on total phosphorus analysis of different post-digestion treatment procedures, such as neutralization and reacidification, and shaking/settling, were investigated in this study using the total phosphorus measurements of water samples from the Everglades Round Robin (ERR) study and comparing the results with the ERR study. The effects of the insoluble particles or phosphorus adsorption/precipitation on/with Al and Fe hydroxides in different post-digestion treatment procedures adequately accounted for the differences between the most probable value and the higher or lower total phosphorus measurements reported in the ERR study. Based on the results of this investigation we recommend that a clearly defined set of digestion and post-digestion treatment procedures be adopted as the standard for total phosphorus analysis using the ascorbic acid method.