1. Selection of Method
The Kjeldahl methods (4500-Norg B and C) determine nitrogen in the trinegative state. They fail to account for nitrogen in the form of azide, azine, azo, hydrazone, nitrate, nitrite, nitrile, nitro, nitroso, oxime, and semi-carbazone. Kjeldahl nitrogen is the sum of organic nitrogen and ammonia nitrogen.
The major factor that influences the selection of a macro- or semi-micro-Kjeldahl method to determine organic nitrogen is its concentration. The macro-Kjeldahl method applies to samples containing either low or high concentrations of organic nitrogen but requires a relatively large sample volume for low concentrations. In the semi-micro-Kjeldahl method, which applies to samples containing high concentrations of organic nitrogen, choose a sample volume that contains organic plus ammonia nitrogen in the range of 0.2 to 2 mg.
The block digestion method (4500-Norg D) is a micro method with an automated analysis step capable of measuring organic nitrogen as low as 0.1 mg/L when blanks are carefully controlled.