4500-N A. NITROGEN
Introduction

In waters and wastewaters the forms of nitrogen of greatest interest are, in order of decreasing oxidation state, nitrate, nitrite, ammonia, and organic nitrogen. All these forms of nitrogen, as well as nitrogen gas (N2), are biochemically interconvertible and are components of the nitrogen cycle. They are of interest for many reasons.
Organic nitrogen is defined functionally as organically bound nitrogen in the trinegative oxidation state. It does not include all organic nitrogen compounds. Analytically, organic nitrogen and ammonia can be determined together and have been referred to as “kjeldahl nitrogen,” a term that reflects the technique used in their determination. Organic nitrogen includes …ver más…

If suspended organic matter is dissolved by the persulfate digestion reagent, yields comparable to those from true solutions are obtained; if it is not dissolved, the results are unreliable and probably reflect a negative interference. The persulfate method is not effective in wastes with high organic loadings. Dilute such samples and re-analyze until results from two dilutions agree. A conductimetric method (4500-N.D) for the determination of inorganic nitrogen also is presented.

4500-N C. PERSULFATE METHOD

1- General Discussion
The persulfate method determines total nitrogen by oxidation of all nitrogenous compounds to nitrate. Should ammonia, nitrate, and nitrite be determined individually, “organic nitrogen” can be obtained by difference. a. Principle: Alkaline oxidation at 100 to 110°C converts organic and inorganic nitrogen to nitrate. Total nitrogen is termined by analyzing the nitrate in the digestate. b. Selection of nitrate measurement method: Automated or manual cadmium reduction may be used to determine total nitrogen levels below 2.9 mg N/L. c. Quality control (QC): the QC practices considered to be an integral part of each method are summarized.

2- Apparatus

a. Autoclave, or horplate and pressure cooker capable of developing 100 to 110°C for 30 min. b. Glass culture tubes:* 30-ml screw-capped (polypropylene linerless caps), 20 mm OD X 150 mm