Determining phosphorus in glacial meltwaters

Phosphorus is present in a number of phases that are operationally defined. Dissolved P is usually that which passes though a 0.45 |m grade filter and so colloidal forms may be included inadvertently (these have yet to be assessed in glacial settings). The dissolved P pool is partitioned into reactive and non-reactive phases following the quantitative measurement of a reduced phosphomolybdate complex in filtrate and digested filtrate respectively (e.g. Tockner et al., 2002). Digestion of an unfiltered sample may also enable estimation of particulate P, which may be further characterized using sequential extraction. These methods are not without their problems, the most important being that concentrations in glacial meltwaters are often below conventional detection limits (usually 1-3|gPL-1). Recent work by the author has therefore developed a novel analytical procedure (pioneered for marine analyses by Karl & Tien, 1992) for detecting dissolved reactive phosphorus at nanomolar concentrations. This involves the co-precipitation of dissolved P with brucite (Mg(OH)2) after spiking water samples with MgCl2 and NaOH. The dissolved P content of large samples thus becomes adsorbed to a small quantity of precipitate, which is then separated by centrifugation and dissolved during the acidification step of the phosphomolybdate colorimetric method. In this way detection limits may be decreased to levels 25 times lower than those achieved with conventional methods.

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