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Total Kjeldahl nitrogen

Total Kjeldahl N is determined by converting the various organic nitrogen forms to NH4+3 and then measuring the NH4+3 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.

Anonymous Manual. 1977. Technicon AutoAnalyzer II Industrial Method No. 334-74W/B. Release January 1976/Revised March 1977. Individual/simultaneous determination of nitrogen and/or phosphorus in BD Acid Digest.
Bremner JM. 1996. Chapter 37- Nitrogen-Total: In Methods of Soil Analysis. Part 3. Chemical Methods. SSSA Book Series no. 5.
Bremner JM, Mulvaney CS. 1982. Nitrogen-Total. P. 595-624, In A.L. Page et al (eds). Methods of soil analysis. Part 2. 2nd ed. Agron. Monograph 9.
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