Catálogo General de Material para Laboratorio

1000 KNOWLEDGE Infrared quick digestion system (InKjel 1225 TCP) Manual titration station (STI) NITROGEN DITERMINATION ACC. TO KJELDAHL Digestion of sample By digestion, the nitrogen com- pounds (proteins, amines and or- ganic compounds) are converted into ammonium sulphate with boiling concentrated sulfuric acid with the addition of suitable catalysts (e.g. selenium reaction mixture). At the end of the digestion, the sam- ples are light-green or clear. For more than 120 years, nitrogen determination according to Kjeldahl has been an internationally accepted standard. In 1883, the Danish chemist Johan Kjeldahl developed this method for quantitative determination of the nitrogen content. This method is still widely used. It can be used for determining the nitrogen content in many substances, which shows in the wide range of use of the method: n Food industry n Environmental analysis n Pharmaceutical or chemical industry In the food industry, nitrogen determination according to Kjeldahl is used to determine the protein content. The procedure of a Kjeldahl determination is divided into three phases: Distillation In a second step, the ammonium ions are converted into gaseous am- monia (NH 3 ) by adding caustic soda (NaOH) until the reaction becomes basic (colour indicator). By means of steam distillation, the released am- monia is collected in a cooler in the receiver of aqueous boric acid solu- tion (concentration 2 – 4%). Absorb- ing the ammonia results in ammo- nium borate. Titration By means of titration with sulfuric acid 0.05 mol/l and the expulsion of the weaker boric acid by the strong- er sulphuric acid, ammonium borate becomes ammonium sulphate. Af- ter the ammonium borate has been completely converted into ammo- nium sulphate, the excessive acid leads to a change of the colour indi- cator from apple-green to magenta. Steam distiller (S 4) F rom the exhausted sulphuric acid, the user calculates the nitrogen content of the sample. behr Labor-Technik