Ultraviolet-Activated Persulfate Digestion for Total Phosphorus Determination

Total phosphorus (TP) measurement requires converting every phosphorus species—dissolved or particulate, inorganic or organic—into a single detectable form, orthophosphate. Potassium persulfate digestion accomplishes this conversion, and when coupled with ultraviolet irradiation, the oxidation becomes faster and more efficient than thermal activation alone.

The chemistry hinges on the peroxodisulfate ion. Ultraviolet light cleaves the O–O bond in persulfate, generating sulfate radical anions (SO₄·⁻, E ≈ 2.5–3.1 V) and hydroxyl radicals (·OH) as reactive intermediates. 

These radicals attack condensed phosphates and organophosphorus compounds through chain reactions, ultimately releasing phosphorus as orthophosphate. UV irradiation offers a practical advantage: it activates persulfate at mild temperatures, avoiding the high-pressure autoclave step required in conventional thermal digestion.

Once digestion is complete, the determination step is colorimetric rather than direct UV absorbance. Orthophosphate reacts with ammonium molybdate in an acidic medium to form phosphomolybdic acid, which is then reduced by ascorbic acid to a molybdenum blue complex. The absorbance of this blue species is measured at 700 nm or 880 nm and is proportional to the original TP concentration. The ultraviolet light in this method serves the digestion, not the detection; the analytical signal itself lies in the visible region.

Two practical issues deserve attention. Turbidity and natural color can elevate the apparent absorbance, so a sample blank—digested but without chromogenic reagents—is subtracted from each measurement; alternatively, a turbidity–color compensation solution is used. 

Arsenic, chromium, and sulfur may also interfere under acidic conditions, and samples with high concentrations of these species require dilution before digestion. With these corrections in place, UV-activated persulfate digestion provides a reliable route from complex phosphorus matrices to a stable, quantifiable blue end-point.