This document specifies a method for the determination of total bound nitrogen (ST-TNb) in water of various origins: groundwater, surface water, and wastewater, in a measuring range of concentration generally between 0,5 mg/l and 220 mg/l of ST-TNb using the small-scale sealed tube method. Different measuring ranges of small-scale sealed tube methods can be required.The measuring ranges can vary depending on the type of small-scale sealed tube method of different manufacturers. It is up to the user to choose the small-scale sealed tube with the appropriate application range or to adapt samples with concentrations exceeding the measuring range of a test by preliminary dilution.NOTE The results of a small-scale sealed tube are most precise in the middle of the application range of the test.All small-scale sealed tube methods are based on a heated alkaline potassium persulfate oxidation in a heating block. Different digestion temperatures, 100 °C or 120 °C or 170 °C, and different digestion times are applicable. Dimethylphenol colour reactions are applied, depending on the typical operating procedure of the small-scale sealed tube used, see Clause 9.