what is kjeldahl's method describe with example (chemistry class 11)
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The Kjeldahl method or Kjeldahl digestion (Danish pronunciation: [ˈkʰɛlˌtæːˀl]) 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. This method was developed by Johan Kjeldahl in 1883.[1][2]
Method Edit
The method consists of heating a sample at 360–410°C with concentrated sulfuric acid (H2SO4), which decomposes ("digests") the organic sample by oxidation to liberate the reduced nitrogen as ammonium sulfate. Catalysts like selenium, Hg2SO4 or CuSO4 are often added to hasten the digestion. Na2SO4 is also added to increase the boiling point of H2SO4. Digestion is complete when the liquor clarifies with the release of fumes.[3] A distillation system depicted below is built.
Kjeldahl digestion Kjeldahl distillation
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).[3] NaOH can also be added with a dropping funnel.[4] 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.[3]
Ammonium ion concentration in the acid solution, and thus the amount of nitrogen in the sample, is measured via titration. If boric acid (or some other weak acid) was used, direct acid-base titration is done with a strong acid of known concentration. HCl or H2SO4 can be used. Indirect back titration is used instead if strong acids were used to make the standard acid solution: strong base of known concentration (like NaOH) is used to neutralize the solution. In this case, the amount of ammonia is calculated as the difference between the amount of HCl and NaOH. In the case of direct titration, it is not necessary to know the exact amount of weak acid (e.g. boric acid) because it does not interfere with the titration (it does have to be in excess of ammonia to efficiently trap it). Thus, one standard solution is needed (e.g. HCl) in the direct titration, while two are needed (e.g. HCl and NaOH) in the back-titration. One of the suitable indicators for these titration reactions is Tashiro's indicator.[3]
In practice, this analysis is largely automated; specific catalysts accelerate the decomposition. Originally, the catalyst of choice was mercuric oxide. However, while it was very effective, health concerns resulted in it being replaced by cupric sulfate. Cupric sulfate was not as efficient as mercuric oxide, and yielded lower protein results. It was soon supplemented with titanium dioxide, which is currently the approved catalyst in all of the methods of analysis for protein in the Official Methods and Recommended Practices of AOAC International.[5]
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