USGS-NWQL: I-2650-03: Nitrogen and Phosphorus, Dissolved, in water by Alkaline Persulfate Digestion

Summary
Analytes
Revision
Data and Sites

Official Method Name

Evaluation of Alkaline Persulfate Digestion as an Alternative to Kjeldahl Digestion for Determination of Total and Dissolved Nitrogen and Phosphorus in Water

Current Revision

2003

Media

WATER

Instrumentation

Colorimeter

Method Subcategory

Inorganic

Method Source

USGS-NWQL

Citation

Patton, C.J., and Kryskalla, J.R., Methods of Analysis of the U.S. Geological Survey National Water Quality Laboratory -- Evaluation of Alkaline Persulfate Digestion as an Alternative to Kjeldahl Digestion for Determination of Total and Dissolved Nitrogen and Phosphorus in Water, U.S. Geological Survey Water-Resources Investigations Report 03-4174, 40 p.

Brief Method Summary

Filtered samples are dispensed into glass culture tubes, dosed with alkaline persulfate reagent, capped tightly, and digested in an autoclave at 250^oF (121^oC) and 17 lb/in² (117.2 kPa) for 1 hour. The alkaline persulfate digestion procedure oxidizes all forms of inorganic and organic nitrogen to nitrate and hydrolyzes all forms of inorganic and organic phosphorus to orthophosphate. Nitrate and orthophosphate in alkaline persulfate digests are determined in parallel with a 2-channel photometric, air-segmented continuous flow analyzer.
Orthophosphate was determined by a phosphoantimonylmolybdenum blue method, which is equivalent to the 2-reagent variants of USEPA method 365.1 and USGS method I-2601-90.

Scope and Application

This method was validated for determination of total dissolved nitrogen and phosphorus in drinking water, wastewater, and water-suspended sediment. Their applicability to bottom materials was not investigated.

Applicable Concentration Range

Interferences

Chloride concentrations higher than 10,000 mg/L are likely to interfere because of reaction with persulfate to form oxychlorides or chlorine that might deplete persulfate required to oxidize inorganic and organic nitrogen species to nitrate. Resulting active chlorine species can also interfere in colorimetric reactions used to determine nitrate and orthophosphate in digests.
Organic carbon concentrations greater than 150 mg/L interfere because of reaction with persulfate to form carbon dioxide, depleting persulfate required to oxidize inorganic and organic nitrogen species to nitrate.
Overacidification of samples at collection sites can result in low recovery of inorganic and organic nitrogen, which can be avoided by exclusive use of the sulfuric acid field-amendment solution (see text for details).
Nitrate and nitrite do not contribute to Kjeldahl dissolved and total nitrogen concentrations in principle, but in practice, positive and negative interferences by these ions are well known. This interference can confound comparison of Kjeldahl nitrogen and alkaline persulfate digestion nitrogen when dissolved nitrate concentrations are greater than about 0.1 mg NO₃^--N/L.
Suspended particles remaining in digests must be removed by sedimentation and decantation or filtration prior to colorimetric analyses.
Barium, lead, and silver can interfere by forming insoluble phosphates, but their concentrations in natural-water samples usually are less than the interference threshold.
Arsenate, AsO₄^3-, (but not arsenite, AsO₃^3-) can interfere by forming reduced heteropoly acids analogous to those found by orthophosphate. Because of the possibility that arsenite might be oxidized to arsenate by persulfate, both species at concentrations up to 20 mg-As/L in deionized water were digested and analyzed, which determined that interference by either species up to 1 mg-As/L is negligible.

Quality Control Requirements

Calibrate instrument using calibration standards (CAL); quality control samples (QCS); and laboratory blanks (LB) analyzed at a minimum of 1 for every 10 samples, as described in OFR 95-443. The method report (WRIR 03-4174) provides detailed instructions for preparing calibrants, matrix spike solution, quality-control check solutions, and digestion check solution.

Sample Handling

Filter through 0.45-um filter. Field rinse and fill sample bottles directly from the filter assembly. Add 1 mL of 4.5 N H₂SO₄ per 120 mL of sample. Chill sample immediately and maintain at 4^oC or below without freezing.

Maximum Holding Time

30 days

Relative Cost

Less than $50

Sample Preparation Methods

This method has 2 analytes associated with it.

Analyte	Detection Level	Bias	Precision	Pct False Positive	Pct False Negative	Spiking Level
Nitrogen(1333-74-0) Nitrogen	0.015 mg/L	N/A	7.40 RSD			0.05 mg/L
Phosphorus(7723-14-0) Black phosphorus P Phosphorous, Red Phosphorus Phosphorus (yellow or white) Red phosphorus Violet phosphorus White phosphorus Yellow phosphorus	0.007 mg/L	N/A	8.00 RSD			0.02 mg/L

Precision Descriptor Notes:	The precision was assessed using statistical and graphical analysis of data for about 2,100 dissolved and whole-water samples that were collected during nominally high- and low-flow conditions and analyzed by alkaline persulfate and Kjeldahl disgestion methods.
Detection Level Note:	The MDL was determined using the methodology described in USEPA report Part 136, Appendix B.

Revision	PDF/Link
2003

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USGS-NWQL: I-2650-03: Nitrogen and Phosphorus, Dissolved, in water by Alkaline Persulfate Digestion

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