Total Phosphorus Determination in Colloid‐Containing Soil Solutions by Enhanced Persulfate Digestion

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It is unclear whether persulfate digestions completely liberate P associated with mineral colloids in soil solutions and aqueous soil extracts. We tested a modified persulfate digestion using aqueous soil extracts and suspensions of goethite, gibbsite, illite, and montmorillonite that were exposed to 24 μmol P L−1 as orthophosphate (ortho‐P) or myo‐inositol hexaphosphate (IHP). Digestion was performed by autoclaving (121°C, 60 min) 5 mL of extract or suspension after the addition of 1 mL of solution, which contained 150 mmol L−1 K2O8S2 and 180 mmol L−1 H2SO4 Subsequently, 0.7 mL of 188 mmol L−1 ascorbic acid was added and the sample was heated in a water bath at 95°C for 60 min. For the samples where P was sorbed onto goethite and gibbsite, recoveries of total P were 97.2 ± 3.7% (ortho‐P) and 102 ± 3.7% (IHP). For the samples where P was sorbed onto clay minerals, average total P recoveries were 99.2 ± 5.1% (ortho‐P) and 106 ± 3.0% (IHP). No differences between total P concentrations measured in persulfate‐digested soil extracts or with an additional digestion with HF and HNO3 were detected.