Chemistry · General Principles Of Organic Chemistry · NEET
Both estimate nitrogen, but in different forms. In Dumas method, the compound is heated with copper oxide (CuO) in a stream of CO2, and nitrogen comes out as free N2 gas, which is collected and its volume measured. In Kjeldahl method, the compound is heated with concentrated H2SO4, which turns the nitrogen into ammonium sulphate; then NaOH releases ammonia (NH3) gas, which is absorbed in a known amount of acid. Simple rule: Dumas gives you a GAS VOLUME of N2; Kjeldahl gives you AMMONIA measured by titration.
Kjeldahl works only if all the nitrogen changes into ammonium sulphate when heated with concentrated H2SO4. Nitrogen in a nitro group (-NO2, like in nitrobenzene), in an azo group (-N=N-, like azobenzene), or inside a ring (like pyridine) does NOT convert into ammonium sulphate under these conditions. So Kjeldahl cannot estimate them. It works for nitrogen in -NH2 (amines like aniline) or amide groups. This is a very common NEET trap.
By the Liebig combustion method. A known mass of the compound is burnt fully with copper oxide (CuO) in excess oxygen. Carbon turns into CO2 and hydrogen turns into H2O. The water is absorbed in a weighed tube of anhydrous CaCl2, and the CO2 is absorbed in a weighed tube of concentrated KOH. The increase in mass of each tube tells us how much H2O and CO2 formed, and from that we calculate the percentage of C and H.
CO2 is an acidic gas (acidic oxide). KOH is a strong base, so it reacts with and traps CO2 completely: 2KOH + CO2 -> K2CO3 + H2O. The KOH tube is weighed before and after; the increase in mass equals the mass of CO2 absorbed. From the CO2 mass we find how much carbon was in the sample.
Kjeldahl estimates NITROGEN only, not carbon. Carbon (and hydrogen) are estimated by the Liebig combustion method. Students often mix these up. Remember: Dumas and Kjeldahl are both for nitrogen; Liebig is for carbon and hydrogen.
Estimation is QUANTITATIVE analysis: it measures the exact amount (percentage) of each element present in a compound. It is different from 'detection' (Lassaigne's test), which is QUALITATIVE and only tells you WHICH elements are present, not how much. Estimation gives numbers; detection gives yes/no.
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?
An organic compound contains 78% (by wt.) carbon and the remaining percentage of hydrogen. The empirical formula of this compound is: [Atomic wt. of C = 12, H = 1]
Try the real previous-year questions from this chapter — each with the answer and a full solution.
% N = (28 x volume of N2 at STP in mL) / (22400 x mass of compound in g) x 100. First convert the measured N2 volume to STP conditions, then apply this formula. 28 is the molar mass of N2 and 22400 mL is the molar volume at STP.
% N = (1.4 x M x V) / mass of compound, where M is the molarity of the acid and V is the volume of acid (in mL) actually neutralised by the ammonia. The factor 1.4 comes from (14 x 2 x 100)/(1000 x ... ) simplification for H2SO4. It uses the acid consumed by the released ammonia.
Carbon and hydrogen: Liebig combustion (CO2 in KOH, H2O in CaCl2). Nitrogen: Dumas (as N2 gas) or Kjeldahl (as ammonia). Halogens and sulphur are estimated by the Carius method.
Dumas is more general because it can estimate nitrogen in ALL types of nitrogen compounds, including pyridine, nitro and azo compounds. Kjeldahl is limited but is simpler and widely used for proteins and amines.
