Neutral persulfate digestion at sub-boiling temperature in an oven for total dissolved phosphorus determination in natural waters

A simplified, easily performed persulfate digestion method has been developed to process a large number of water samples for routine determination of total dissolved phosphorus. A neutral potassium persulfate solution (5%, w/v, pH ∼6.5) is added to the samples (at 10 mg potassium persulfate per mL of sample), which are then digested at 90 °C in an oven for 16 h. This method does not require pH adjustment after digestion because neither an acid nor a base is added to the samples prior to digestion. The full color of phosphoantimonylmolybdenum blue from the digested samples develops within 8 min. Compared with the autoclave method, digestion at sub-boiling temperatures in an oven is safer, and a large number of samples can be heated overnight requiring no constant monitoring. The apparent molar absorptivity (ɛ) of nine organic phosphorus compounds and two condensed inorganic phosphates ranged from 1.17 × 104 to 1.82 × 104 L mol−1 cm−1 in both distilled water and artificial seawater matrixes. The average recovery of these phosphorus compounds was 94 ± 11% for the DIW matrix and 90 ± 12% for the ASW matrix. No significant difference in molar absorptivity was observed between the undigested and digested phosphate, especially in the seawater matrix. It is, therefore, suggested that a phosphate solution be directly employed without digestion as the calibration standard for routine determination of total dissolved phosphorus. This method was used to study the spatial distribution of total dissolved phosphorus in the surface waters of Florida Bay.