Estimation of Nitrogen

Two methods for estimation of nitrogen: Dumas method and Kjeldahl’s method.

Dumas method

1. Heating of organic compound (weight of sample mass = m (g) ) gas containing nitrogen with copper oxide in the environment of Carbon dioxide,
2. Free nitrogen in addition to carbon dioxide and water are produced $$ce{C_xH_yN_z + CuO ->[CO2] CO2 + H2O + N2 + Cu}$$
3. During this oxidation, traces of nitrogen oxides are also formed which are reduced to nitrogen by passing over a heated copper gauze
4. The mixture of gases so produced are collected over an aqueous solution of potassium hydroxide, KOH, which absorbs Carbon dioxide.
5. Nitrogen is collected in the graduated tube, as shown in the figure:Courtesy: NCERT
6. Let the volume collected be (V_n), its temperature be (T_n) and its partial pressure be (P_n) where (P_n) = atmospheric pressure – aqueous tension, as only nitrogen and water vapour are present over the KOH solution.
7. (P_1.V_1 over T_1) = (P_2.V_2 over T_2)
8. For STP conditions (temperature, (T_{STP}) = 273 K, pressure, (P_{STP}) = 760 mm of Hg) volume of Nitrogen at STP, (V_{STP}) can be related with experimenting conditions as $$ {P_{STP}. V_{STP} over T_{STP}} = {P_n.V_n over T_n}$$
9. Calculate (V_{STP}) for (N_2) gas
10. Calculate mass of (N_2) gas using the concept that 22,400 ml of (N_2) gas at STP weighs 28 g. Let it be (m_n )g
11. Percentage of nitrogen present = ({m_n over m} times 100) %

Kjeldahl’s method

1. The organic compound (weight of sample mass = m (g) ) containing nitrogen is heated with concentrated sulphuric acid. It converts nitrogen in the compound to ammonium sulphate. $$ Compound + ce{H2SO4 -> (NH4)2SO4} $$ Courtesy: NCERT
2. The acid mixture thus obtained is heated with excess of sodium hydroxide. It liberates ammonia gas. $$ ce{(NH4)2SO4 ->[2NaOH] Na2SO4 + 2NH3 + 2H2O} $$
3. The liberated ammonia gas is absorbed in an excess but measured volume ( (V_{a}) ) of standard solution of sulphuric acid of known molarity, ( (M_{a}) ). $$ ce{2NH3 + H2SO4 -> (NH4)2SO4}$$
4. This sulphuric acid solution is now titrated with standard alkali (NaOH) solution of known molarity, (M_{b}). $$ ce{H2SO4 + 2NaOH -> Na2SO4 + 2H2O} $$
5. The titration tells how much of the acid remains after reaction with ammonia gas. If ( (V_{b}) ) of alkali NaOH is used to titrate with remaining acid, number of moles of NaOH consumed is (M_{b} times V_{b}). By the titration formula, $$acidity times text {number of moles of base} = basicity times text{number of moles of acid} $$
   $$acidity times M_{b} times V_{b} = basicity times text{number of moles of acid} $$
   $$ 1 times M_{b} times V_{b} = 2 times text {number of moles of acid remained in the solution that titrated with NaOH} $$
   number of moles of acid remained in the solution that titrated with NaOH is $$ M_{b} times V_{b} over 2 $$
6. Number of moles of (ce{H2SO4}) consumed = $$ M_{a} times V_{a} – {M_b times V_b over 2} $$
7. One mole of (ce{H2SO4}) reacts with two moles of (ce{NH3}). Thus, number of moles of ammonia that consumed the sulphuric acid is $$ 2 times left( {M_{a} times V_{a} – {M_b times V_b over 2}}right) $$
8. The mass of nitrogen (let it be (m_1) ) is
   $$ 2 times left({M_{a} times V_a – {M_b times V_b over 2}}right) times 14 g$$
9. Mass % of nitrogen is ({m_1 over m}times 100) %

Note: Kjeldahl method is not applicable to compounds containing nitrogen in nitro and azo groups and nitrogen present in the ring (e.g. pyridine) as nitrogen of these compounds does not change to ammonium sulphate under the conditions used in the method.