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Concept
Kjeldahl method
Summary
The Kjeldahl method or Kjeldahl digestion (ˈkhelˌtɛˀl) in analytical chemistry is a method for the quantitative determination of nitrogen contained in organic substances plus the nitrogen contained in the inorganic compounds ammonia and ammonium (NH3/NH4+). Without modification, other forms of inorganic nitrogen, for instance nitrate, are not included in this measurement. Using an empirical relation between Kjeldahl nitrogen content and protein content it is an important method for analyzing proteins. This method was developed by Johan Kjeldahl in 1883.
The method consists of heating a sample to 360–410 °C with concentrated sulfuric acid (H2SO4), which decomposes ("digests" or "destructs") the organic sample by oxidation to liberate the reduced nitrogen as ammonium sulfate. Hot concentrated sulfuric acid oxidizes carbon (as bituminous coal) and sulfur (see sulfuric acid's reactions with carbon):
C + 2 H2SO4 → CO2 + 2 SO2 + 2 H2O
S + 2 H2SO4 → 3 SO2 + 2 H2O
Catalysts like selenium, Hg2SO4 or CuSO4 are often added to make the digestion go faster. Na2SO4 or K2SO4 is also added to increase the boiling point of H2SO4. Digestion is complete when the liquor clarifies with the release of fumes. A distillation system depicted below is built.
The end of the condenser is dipped into a known volume of standard acid (i.e. acid of known concentration). A weak acid like boric acid (H3BO3) in excess of ammonia is often used. Standardized HCl, H2SO4 or some other strong acid can be used instead, but this is less commonplace. The sample solution is then distilled with a small amount of sodium hydroxide (NaOH). NaOH can also be added with a dropping funnel. NaOH reacts the ammonium (NH4+) to ammonia (NH3), which boils off the sample solution. Ammonia bubbles through the standard acid solution and reacts back to ammonium salts with the weak or strong acid.
Ammonium ion concentration in the acid solution, and thus the amount of nitrogen in the sample, is measured via titration.
Official source
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