Determination of Total Nitrogen in Aqueous Samples Containing Suspended Solids Using a Modified Alkaline Persulfate Digestion.
Tony Provin, Texas A&M Univ System, 343 Heep Center Tamu 2474, College Station, TX 77843-2474 and John Pitt, Texas Cooperative Extension, 3296 Meadow Lark Cir, Bryan, TX 77808.
The determination of low concentrations of total nitrogen in runoff and effluent samples containing suspended solids can require multiple analytical determinations and/or a reduction step to convert nitrate-N into Kjeldhal recoverable nitrogen (N) forms. The utilization of soluble total nitrogen analyzers is often limited by the non-homogenizable suspended solids plugging the sample introduction system. High temperature thermal combustions systems utilizing the Dumas procedure are either limited by sample size and/or inability to provide adequately low detection limits. Alkaline persulfate digestions are commonly used for the determination of total nitrogen and phosphorus in marine waters and freshwaters. Modification of this method have been used for the determination of N inn plant and soil samples; however, limited research has been reported on the use of alkaline persulfate digestions in samples with varying levels of suspended solids. The alkaline persulfate digestion, using an autoclave, allows for larger heterogeneous sample sizes and eliminates all hazardous wastes normally associated with Kjeldahl digests. This laboratory study investigated the modifications required to insure complete oxidation of all nitrogen forms present in manure effluents to nitrate-N. The visual and physical categorization of samples based on required modifications in reagent requirements will be presents. Additionally, methods of reagent additions and impact on oxidation efficiency and total N recovery will be discussed. Using good analytical procedures, laboratories using this methodology can achieve detection limits of 0.5 mg N L<sup>-1</sup>.