Peroxodisulphate digestion and filtration as sources of inaccuracy in determinations of total phosphorus and dissolved orthophosphate phosphorus in water samples containing suspended soil particles

Determinations of phosphorus fractions in water samples may be unreliable if an appreciable amount of phosphorus is attached to suspended soil particles. The inaccuracies in peroxodisulphate digestion for determination of total phosphorus were studied on the basis of surface runoff and drainflow samples from a clay soil. The results obtained with peroxodisulphate digestion tended to be lower than those obtained with a HF digestion method. The results of peroxodisulphate digestion indicated incomplete extraction of phosphorus, especially in samples with large amounts of suspended soil particles. More sulphuric acid, or dilution of the samples before the digestion, improved the results. Error in dissolved orthophosphate determination due to colloidal material was investigated by removing variable amounts of the material from samples before the analysis. The amounts of dissolved orthophosphate that were determined increased with the amount of suspended material in the filtrate, indicating release of orthophosphate from colloids. The described inaccuracies in total and orthophosphate phosphorus determinations may result in underestimation of the particulate phosphorus fraction.