Procedures for the storage and digestion of natural waters for the determination of filterable reactive phosphorus, total filterable phosphorus and total phosphorus Review Article

An overview of the forms of phosphorus species likely to be encountered in natural waters and the implications for the measurement of filterable and total phosphorus is given. Procedures reported in the literature for the storage and digestion of water samples for filterable reactive phosphorus (FRP), total filterable phosphorus (TFP) and total phosphorus (TP) measurements are summarised and the advantages and limitations of methods discussed. Water samples for FRP and TFP measurements need to be filtered immediately on collection as exchange on and off particles may occur in the sample container. Slow freezing of filtered or turbid water samples in acid washed low density polyethylene bottles appears to be satisfactory for the long term storage (years) of a variety of water sample types. Storage of water samples at room temperature or refrigeration (1–5°C) with a preservative is suitable only for short term storage (days-months). If water samples contain < 20 μg P/l, adsorption to containers may be significant. Batch digestion of samples with alkaline or acid peroxodisulphate using autoclave or microwave heating offers the advantages of ease, simplicity and precision. Good recoveries of phosphorus from a range of phosphorus compounds containing P–O–P, C–O–P and C–P bonds expected in natural waters have been reported. If turbid samples are to be analysed, caution must be exercised to ensure that the carbon or suspended solids concentration does not exceed the capacity of the digestion procedure to oxidise the carbon present and release occluded phosphorus from particulate materials. Better recoveries of phosphorus from turbid water samples are achieved using microwave heating with closed vessels, probably because of the higher temperatures and pressures generated. The use of on-line heating (microwave, thermal induced) coupled with flow injection analysis and using peroxodisulphate or an oxidising acid mixture should also allow the automation of TFP and TP measurements. Reliable procedures for the removal of unwanted particulate material prior to or after the digestion step need to be developed.