Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory-Determination of ammonium plus organic nitrogen by a Kjeldahl digestion method and an automated photometric finish that includes digest cleanup by gas diffusion

Abstract

The National Water Quality Laboratory (NWQL) determined ammonium plus organic nitrogen (Kjeldahl nitrogen) by using semiautomated, block digester methods for filtered and whole-water samples from 1986 until October 1, 1991. During that time, phosphorus was determined by a persulfate digestion method. In 1991, projected increases in demand for both tests by the U.S. Geological Survey's National Water-Quality Assessment Program led the NWQL to develop and validate methods for determining both analytes in a common digest. This report describes a rapid and accurate method to determine Kjeldahl nitrogen. The batch, high-temperature (block digester), Hg (II)-catalyzed digestion step used in the new methods I-2515-91/4515-91 is similar to U.S. Geological Survey methods I-2552-85/4552-85 and U.S. Environmental Protection Agency method 351.2 except that sample and reagent volumes are halved. Prepared digests are desolvated at 220 degrees Celsius ( o C) and digested at 370 o C in separate block digesters set at these temperatures, rather than in a single, temperatureprogrammed block digester. This approach permits 40 calibrants, reference materials, and samples to be digested and resolvated in about an hour. Ammonium ions originally present in samples, along with those released during the digestion step, are determined photometrically by an automated, salicylatehypochlorite Berthelot reaction procedure at a methods were observed for a subset of about 350 samples with nitrate plus nitrite concentrations greater than 1 mg-N/L.