Digestors
Typically performed on foodstuffs, the Kjeldahl method or test is designed to determine (or confirm) the amount of protein in soil, wastewater, fertilizer, and other materials. They are used in agricultural and environmental testing to measure trace metals, nitrates, and ammonium.
The method converts proteins, amines, organic compounds and other nitrogen compounds into ammonia compounds. A caustic is added to release the ammonia, which is then distilled and titrated. Samples are heated from 360o to 410°C in the presence of concentrated sulfuric acid and a catalyst. This decomposes or digests the sample and frees the nitrogen. The digestion is complete when the liquid is clarified and the fumes are released. The samples are cooled, and the liquid is distilled and titrated.
Several catalysts are available (called Kjeldahl tablets), and usually contain a mixture of copper and titanium. Some catalysts work better with specific materials, and others are more environmentally friendly. Digestions can be performed manually, but are somewhat hazardous and should always be performed under a chemical hood.
Automated or semi-automated digestors reduce the risk to the person performing the Kjeldahl testing. Some equipment is designed to heat to the maximum temperature in an hour and cool down faster for more efficient digestions. Some perform only the digestion portion of the procedure, and the distillation and titration must occur separately. More complete digestion systems incorporate all of the necessary steps and deliver a final result. The choice of digestor type, including rapid digestors, depends on the workload, speed required, and sample volume.