Kjeldahl apparatus is used for quantiying the nitrogen present in organic compounds, as well as the nitrogen present in inorganic compounds, ammonia and ammonium (NH₃/NH₄⁺). Protein is a nitrogen containing organic compound. Briefly, the measured substances are initially heated and digested in the presence of sulfuric acid and catalyst. The decomposed ammonia combines with sulfuric acid to form ammonium sulfate. Then alkalization distillation makes ammonia free, ammonia is absorbed with boric acid and then titratied with sulfuric acid or hydrochloric acid standard solution, according to the consumption of acid multiplied by the conversion coefficient. Because the method of protein content measurement and calculation is called Kjeldahl nitrogen determination method, it is called Kjeldahl Apparatus. It is composed of a digester and a semi-automatic nitrogen determination apparatus distiller. Kjeldahl apparatus is widely used in detecting protein content of grain, food, dairy products, beverage, feed and so on.