Principle

Most phosphorus is present in some combined form in aquatic systems, often as dissolved or particulate organic forms. Owing to its rapid recycling, phosphorus in these bound forms is often recycled rapidly, so analysis of total phosphorus (or total phosphate) is appropriate to measure the real availability of this nutrient.

Method

An aliquot of unfiltered water is oxidized in a sodium hydroxide-boric acid buffer solution by potassium persulfate in an autoclave. Soluble reactive phosphate resulting from this hot oxidation is then analyzed by the standard molybdenum-blue method.

Procedure

Take a water sample from a natural body of water without introducing bulky foreign matter (twigs, sediment, etc.). The sample may be kept cool and in the dark unless analysis is to be delayed substantially (>24 hours), in which case it may be frozen.

Pour 30 ml of stirred sample into a large, clean (!) screw-cap test tube with a Teflon cap liner. Add 4 mls of the oxidation reagent and 6 mls of distilled water. Prepare blanks in the same way, substituting 30 mls of distilled water for the sample. Cap the tubes loosely and autoclave at 121 oC and 15 PSI for 20 minutes, then remove and cool to room temperature. Using clean graduated cylinders or centrifuge tubes, make the volume to 40 mls with distilled water. Mix, then pipet 10 mls of the solution into a clean 15 ml centrifuge tube, and add 1 ml of the mixed reagent for phosphate analysis. Complete the phosphate analysis as in the standard phosphate method. Calculate total phosphate as follows:

where the absorbance values of the samples and blanks are measured against a distilled water zero, F is a factor that converts corrected absorbance into uM P, 1.33 corrects for the 30:40 ml dilution, and 31 (the atomic weight of P) converts uM P into ug P/l.

Reagents

The oxidation reagent is prepared from reagent grade chemicals. It is critical that the potassium persulfate used be of the highest quality. Dissolve 30 g boric acid and 50 g potassum persulfate in 350 mls of 1 M NaOH with gentle stirring and then dilute to 1000 ml with distilled water. Oxidation reagent is good for several months.

Other reagents are prepared as for soluble reactive phosphate measurement.

Reference: Valderrama, J.G. 1981. The simultaneous analysis of total nitrogen and phosphorus in natural waters. Mar. Chem. 10:109-122.

Name/Pond....................SRP (ug/l)............................... TP (ug/l)

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