Principle: The organic compound containing nitrogen when heated with excess of copper oxide in the atmosphere of carbon dioxide, yields nitrogen in addition to carbon dioxide and water.
Traces of nitrogen oxides formed during combustion of organic compound are reduced to nitrogen by passing the gaseous mixture over a heated copper gauze. The percentage of nitrogen present in a given organic compound is calculated from the volume of nitrogen collected over potassium hydroxide solution from a known mass of organic compound.
Procedure: The apparatus used for the estimation of nitrogen by this method is shown in the figure.
A known mass of organic compound is mixed with copper oxide and placed in the combustion tube. The carbon dioxide gas is passed through the combustion tube to displace air present in the tube. The combustion tube is now heated in the furnace, the nitrogen evolved collects in the nitrometer. The volume of the nitrogen collected is recorded after adjusting the levels of potassium hydroxide solution in the two limbs are equal. Room temperature and atmosphere pressure are recorded.
Calculation:
Mass of organic compound = mg
Volume of nitrogen in nitrometer = V cm3
Room temperature = t°C = (273 + t) K
Atmosphere pressure = P1 mm
Aqueous tension at room temperature = P’ mm
Pressure of dry nitrogen gas formed = P = (P – P’) mm
Volume of nitrogen at STP (V0) = \(\frac{PV\times 273}{760\times (273+t)}\)cm3
22,400 cm3 of nitrogen of STP = 28 g of nitrogen
Mass of V0 cm3 of nitrogen = \(\frac{28\times V}{22,400}g\)
Percentage of nitrogen = \(\frac{28\times V_0 \times 100}{22,400\times m}\)
