Chemistry · General Principles Of Organic Chemistry · NEET
Kjeldahl's method only works if the nitrogen can be changed into ammonium sulphate when heated with conc. H2SO4. Nitrogen inside a nitro group (-NO2), inside an azo group (-N=N-), or locked in an aromatic ring (like pyridine) does not convert to ammonium sulphate. So these compounds cannot be estimated by this method. Nitrogen present as -NH2 or -NH- (like in aniline or amines) works fine. This exact idea was asked in NEET 2022.
Both estimate nitrogen, but the chemistry is different. In Dumas method the compound is heated with copper oxide (CuO) in a CO2 atmosphere and free N2 gas is collected and measured by volume. In Kjeldahl's method the compound is digested with conc. H2SO4 to form ammonium sulphate, ammonia is released with NaOH, and it is measured by titration. Dumas works for all nitrogen compounds; Kjeldahl fails for nitro, azo and ring nitrogen.
Percentage of N = (1.4 x M x volume of acid used by ammonia) / m, where M is the molarity of the acid, and m is the mass of the organic compound in grams. The acid used by ammonia is found by subtracting the leftover acid (found by back titration with standard NaOH) from the total acid taken. Learn the number 1.4 as a shortcut: it comes from 14 (atomic mass of N) times 2 divided by 1000, times 100.
The ammonia gas is passed into a known excess of standard sulphuric acid. Not all the acid reacts, so some acid is left over. We cannot directly measure how much acid the ammonia used. Instead we titrate the leftover acid with standard NaOH (back titration). The acid used by ammonia = total acid taken minus leftover acid. This tells us the amount of nitrogen.
The Kjeldahl's method for the estimation of nitrogen can be used to estimate the amount of nitrogen in which one of the following compounds?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is used to find the percentage of nitrogen in an organic compound. It is important in NEET and also used in food and fertilizer industries to measure protein content.
Concentrated sulphuric acid digests the compound and converts its nitrogen into ammonium sulphate. This is the key step. Copper sulphate or mercury is sometimes added as a catalyst to speed it up.
Excess sodium hydroxide is added to the ammonium sulphate to release ammonia gas. The reaction is (NH4)2SO4 + 2NaOH gives Na2SO4 + 2NH3 + 2H2O. The ammonia is then absorbed in standard acid and measured.
Yes. Proteins have nitrogen mostly as -NH- and -NH2 groups, which convert to ammonium sulphate. So Kjeldahl's method is widely used to estimate protein content in foods.
Dumas method is more general because it works for all nitrogen compounds, including nitro, azo and ring nitrogen. Kjeldahl is simpler and faster but fails for those. For a compound like aniline, both can be used.
