Chemistry · General Principles Of Organic Chemistry · NEET
Both estimate nitrogen, but the chemistry is different. In the Dumas method the compound is heated with CuO in a CO2 stream and nitrogen comes out as free N2 gas, which is measured as a volume. In Kjeldahl's method the compound is heated with concentrated H2SO4 so nitrogen becomes ammonium sulphate (NH4)2SO4, and the NH3 released is measured by titration. Key NEET point: Dumas works for almost every nitrogen compound, while Kjeldahl fails for nitro (-NO2), azo (-N=N-) and ring nitrogen (like pyridine) because those do not turn into ammonium sulphate.
CO2 is passed to sweep out all the air from the apparatus before and during heating. If air stayed inside, its nitrogen would mix with the N2 from the compound and give a wrongly high reading. CO2 is chosen because it is later removed easily by KOH solution, leaving only the nitrogen from the sample.
The gas leaving the tube is a mixture of N2 and CO2. KOH (a strong base) absorbs the acidic CO2 completely, so only the pure nitrogen gas is left and collected in the graduated tube. This is how you measure the true volume of N2 from the compound.
During combustion, small amounts of oxides of nitrogen (like NO, NO2) can form. Passing the gas over hot copper gauze reduces these oxides back to N2. This makes sure all the nitrogen is counted as N2 and none is lost as an oxide.
You collect N2 at the lab temperature and pressure, not at standard conditions. To use the fact that 22400 mL of N2 at STP weighs 28 g, you first correct the measured volume to STP using P1V1/T1 = P2V2/T2. Also, the gas is collected over water/aqueous KOH, so you subtract the aqueous tension (water vapour pressure) from the pressure reading before using it.
Almost. Dumas is the general method and works even for nitro, azo and ring-nitrogen compounds where Kjeldahl fails. That is exactly why NEET asks you to pick Dumas for compounds like nitrobenzene, azobenzene or pyridine, and Kjeldahl only for simple -NH2 / amide type nitrogen.
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 estimates nitrogen in an organic compound by heating it with CuO in a CO2 atmosphere so all nitrogen becomes free N2 gas, which is collected over KOH and measured.
Percentage of N = (28 x V x 100) / (22400 x m), where V is the volume of N2 in mL corrected to STP and m is the mass of the organic compound in grams.
CuO acts as the oxidising agent that burns the compound completely, converting carbon to CO2, hydrogen to water and nitrogen to N2.
Dumas is more general and works for all types of nitrogen, including nitro, azo and ring nitrogen. Kjeldahl is simpler and used in industry for proteins/food, but it fails for those special nitrogen types.
Subtract aqueous tension from the observed pressure to get the pressure of dry N2, then apply P1V1/T1 = P2V2/T2 with STP values (273 K, 760 mm) to find the volume at STP.
