Kjeldahl method
Without modification, other forms of inorganic nitrogen, for instance nitrate, are not included in this measurement.[3] Ammonium ion concentration in the acid solution, and thus the amount of nitrogen in the sample, is measured via titration.Cupric sulfate was not as efficient as mercuric oxide, and yielded lower protein results.This is evidenced by the 2007 pet food incident and the 2008 Chinese milk powder scandal, when melamine, a nitrogen-rich chemical, was added to raw materials to fake high protein contents.Additional disadvantages, such as the need to use concentrated sulfuric acid at high temperature and the relatively long testing time (an hour or more), compare unfavorably with the Dumas method for measuring crude protein content.Other detection methods have been used to quantify NH4+ after mineralisation and distillation, achieving improved sensitivity: in-line generator of hydride coupled to a plasma atomic emission spectrometer (ICP-AES-HG, 10–25 mg/L),[10] potentiometric titration (>0.1 mg of nitrogen), zone capillary electrophoresis (1.5 μg/mL of nitrogen),[11] and ion chromatography (0.5 μg/mL).