Estimation of Nitrogen -- Dumas Method

The Dumas method estimates nitrogen directly, as a gas, and rests on one core fact: heating a nitrogenous organic compound together with cupric oxide, IN AN ATMOSPHERE OF CARBON DIOXIDE, converts every nitrogen atom present into free, elemental nitrogen gas -- CxHyNz + (2x + y/2) CuO -> x CO2 + (y/2) H2O + (z/2) N2 + (2x + y/2) Cu. (Any oxides of nitrogen that happen to form as a side product along the way are reduced back to elemental N2 by passing them over a heated copper spiral before they leave the apparatus, so the gas finally collected is genuinely pure N2.)

The apparatus has three parts. A CO2 GENERATOR supplies the carrier gas -- CO2 is produced either by heating magnetite or sodium bicarbonate in a hard-glass tube, or by reacting dilute HCl with marble in a Kipp's apparatus, and is dried by bubbling it through concentrated sulphuric acid before use. The COMBUSTION TUBE, heated in a furnace, is charged (from the inlet end) with a roll of oxidised copper gauze (to prevent back-diffusion of the combustion products and to help heat the sample by radiation), a weighed portion of the organic substance mixed with excess copper oxide, a length of coarse copper oxide packed in place with asbestos plugs (which oxidises any organic vapour passing through), and finally a reduced-copper spiral (which reduces any lingering nitrogen oxides to elemental N2). Finally, SCHIFF'S NITROMETER collects the gas leaving the combustion tube -- a mixture of the liberated N2 with a considerable excess of CO2 -- by passing it through a body of KOH solution, which absorbs every trace of the CO2 while letting the pure, insoluble N2 collect and rise into the upper, graduated part of the nitrometer tube.

Practically: with the nitrometer's tap left open, CO2 is first passed through the whole system to sweep out the air originally trapped in the combustion tube -- this is confirmed complete once the gas bubbling up through the potash solution is seen to be absorbed completely (proving only CO2, and no residual air, is coming through). The nitrometer is then filled with KOH solution by lowering its reservoir, and its tap is closed. The combustion tube is then heated in the furnace, and as the temperature rises the nitrogen freed from the sample collects steadily in the nitrometer. Once combustion is judged complete, a further strong current of CO2 is passed through to sweep the very last traces of nitrogen out of the tube and into the nitrometer. The volume of gas finally collected is then read off (after adjusting the reservoir so the liquid level inside and outside the graduated tube match, to avoid a pressure-head error), together with the atmospheric pressure and room temperature at the time. …