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Total Kjeldahl N is determined by converting the various organic nitrogen forms to NH4+ and then measuring the NH4+ concentration. To accomplish this, a measured weight of oven-dried ground sample material is digested in a mixture of conc. H2SO4, K2SO4, and selenium at about 380 °C for a certain period of time. The ammonium formed is determined colorimetrically by reacting it with sodium salicylate in the presence of sodium hypochlorite and nitroprusside. This will form an emerald-green complex (the Berthelot reaction) whose intensity is measured at 660 nm. This method only partially converts nitrate; thus, samples with high nitrate levels must be pretreated with salicylic acid-thiosulfate to ensure complete conversion (Bremner and Mulvaney, 1982; Bremner, 1996).
Note: To determine total Kjeldahl N, the sample weight required is at least 1 g of plant or grain sample.