Evaluation of Methods for Preserving the Levels of Soluble Inorganic Phosphorus and Nitrogen in Unfiltered Water Samples

Several methods for preserving the levels of soluble inorganic phosphorus (SIP), ammonium, and nitrate in surface runoff, tile drainage water, and river water were evaluated. The best overall technique for preservation of water samples for P and N analysis appeared to be storage at subzero temperatures. The SIP concentration in samples was also stabilized by addition of HgCl 2 (40 mg/liter) and storage at 4C; however, the ammonium and nitrate levels in samples were not preserved for long periods of time (12 weeks) by this technique. The soluble inorganic N levels in water samples were preserved by addition of phenylmercuric acetate (20 mg/liter) and storage at 4C, but use of this procedure increased the SIP concentration in some samples during storage. Storage of samples at 4C or 23C without addition of a chemical preservative resulted in large changes in soluble inorganic P and N contents.