USGS Test Method I-2610-91

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Method Name:
Phosphorus in Water by Kjeldahl Digestion and Colorimetric Finish, Filtered. Official Name: Phosphorus, Dissolved

All forms of phosphorus, including organic phosphorus, are converted to orthophosphate ions using reagents and reaction parameters identical to those used in the block digester procedure for determination of organic nitrogen plus ammonia, that is, sulfuric acid, potassium sulfate, and mercury (II) at a temperature of 370 deg C. The digestion procedure was adapted from the method of Jirka and others (1976), which is identical to USEPA method 365.4 (U.S. Environmental Protection Agency, 1983, p. 365.4-1 to 365.4-3), except that sample and reagent volumes are halved, as is the time required for digestion. Orthophosphate ions originally present in samples along with those released by organophosphorus compounds during the digestion step, are determined colorimetrically by an automated version of the phosphoantimonylmolybdenum blue procedure of Murphy and Riley (1962). The hydrogen ion concentration, and acid to molybdate ratio, in the analytical stream are about 0.4 M, and about 74, respectively, in accordance with optimization experiments of Murphy and Riley (1962) and Pai and others (1990). Continuous flow dialysis, rather than the dilution loop technique of Jirka and others (1976), is used for automated digestion.

This method is used to determine total phosphorus in water, wastewater, brines, and water-suspended sediment. The suitability of this method for determination of total phosphorus in bottom materials has not been investigated. The analytical range of this method is 0.01 to 2.00 mg/L of phosphorus. Because a dialyzer is incorporated into the analytical cartridge of the continuous flow analyzer used for the colorimetric finish, resolvated digests containing suspended particulates (clays) do not require filtration prior to orthophosphate determinations.

Patton, C.J., and Gilroy, E.P., 1992, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory--Determination of total phosphorous by a Kjeldahl digetsion method and an automated colorimetric finish that includes dialysis: U.S. Geological Survey Open-File Report 92-146

Sodium chloride is added to the acidic molybdate/antimony reagent to inhibit reduction of Hg+2 in digests to Hg by ascorbic acid during the colorimetric finish. Turbidity resulting from this reaction otherwise would interfere. Barium, lead, and silver interfere by forming insoluble phosphates, but the effect is usually negligible in natural-water samples. Interference from silicate, which also can form reduced heteropoly acids with molybdate, is negligible under reaction conditions used here. Arsenic as arsenate ion (AsO4-3) produces a color similar to that of phosphate and might cause a positive interference. According to Murphy and Riley (1962), concentrations of As (V) up to 100 ug/L do not interfere.

QC Requirements:
Quality-control samples area analyzed at a minimum of one in every ten samples. These QC samples include at least one of each of the following: blanks, quality control samples, third party check solutions, replicates, and spikes. Correlation coefficients for calibration curves must be at least 0.99. QC samples must fall within 1.5 standard deviations of the mean value. If all of the data-acceptance criteria in the SOPs are met, then the analytical data are acceptable.

Maximum Holding Time:
30 days from sampling



0.01 - 2.00 (undiluted)

Within-run precision (repeatability) for methods I-2610/4610-91 on the basis of 55 replicate determinations of a sample high in suspended solids (Heidelberg) was 0.70 percent. The average phosphorus concentration determined for this run was 1.55 mg/L. Between-day precision on the basis of 26 replicate determinations of the same sample between April 8 and April 26, 1991, was 5.00 percent. The average phosphorus concentration determined for these replicates was 1.60 mg/L. Paired t-test analysis of approximately 1,600 samples determined for TP by methods I-2600/4600-85 and methods I-2610/4610-91 revealed a positive bias in the range of 0.02 to 0.04 mg/L for the latter. Positive bias of this extent was expected and is in agreement with a previous study by Jirka and others (1976) who compared their Kjeldahl method with USEPA?s persulfate digestion method 365.1. These workers suggested that the bias was real and resulted from the more rigorous reaction conditions of the Kjeldahl digestion relative to the persulfate digestion. Complete details of the method comparison studies can be found in the Discussion of Results section of USGS OFR 92-146.

Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Revision Number:

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