There are two methods for estimation of nitrogen: (i) Dumas method and (ii) Kjeldahl’s method.
(i) Dumas method: The nitrogen containing organic compound, when heated with copper oxide in an atmosphere of carbon dioxide, yields free nitrogen in addition to carbon dioxide and water.
`color{red}(C_x H_y N_z + (2x+y/2) CuO → xCO_2 + y/2 H_2O + z/2 N_2 + (2x+y/2) Cu)`
Traces of nitrogen oxides formed, if any, are reduced to nitrogen by passing the gaseous mixture over a heated copper gauze. The mixture of gases so produced is collected over an aqueous solution of potassium hydroxide which absorbs carbon dioxide. Nitrogen is collected in the upper part of the graduated tube (Fig.12.15).
Let the mass of organic compound `color{red}(= m g)` Volume of nitrogen collected `color{red}(= V_1 mL)` Room temperature `color{red}(= T_1K)`
Volume of nitrogen at STP ` color{red}(= (p_1 V_1 xx 273)/(760 xx T_1))`
(Let it be `color{red}(V mL)`) Where `color{red}(p_1)` and `color{red}(V_1)` are the pressure and volume of nitrogen, `color{red}(p_1)` is different from the atmospheric pressure at which nitrogen gas is collected. The value of `color{red}(p_1)` is obtained by the relation;
`color{red}(p_1=)` Atmospheric pressure – Aqueous tension `color{red}(22400 mL N_2)` at STP weighs `28 g.`
`color{red}(V m L N_2)` at STP weighs `color{red}( = ( 28 xx V)/(22400) g)`
`color{red}("Percentage of nitrogen " = ( 28xxV xx 100)/(22400 xx m))`
(ii) Kjeldahl’s method: The compound containing nitrogen is heated with concentrated sulphuric acid. Nitrogen in the compound gets converted to ammonium sulphate (Fig. 12.16). The resulting acid mixture is then heated with excess of sodium hydroxide. The liberated ammonia gas is absorbed in an excess of standard solution of sulphuric acid. The amount of ammonia produced is determined by estimating the amount of sulphuric acid consumed in the reaction. It is done by estimating unreacted sulphuric acid left after the absorption of ammonia by titrating it with standard alkali solution. The difference between the initial amount of acid taken and that left after the reaction gives the amount of acid reacted with ammonia.
`color{red}("Organic compound" + H_2SO_4 → (NH_4)_2 SO_4 overset(2NaOH)→ Na_2SO_4 +2NH_3 + 2H_2O)`
`color{red}(2NH_3 + H_2SO_4 → (NH_4)_2 SO_4)`
Let the mass of organic compound taken `color{red}(= m g)`
Volume of `color{red}(H_2SO_4)` of molarity, `color{red}(M),` taken `color{red}(= V mL)`
Volume of `color{red}(NaOH)` of molarity, `color{red}(M),` used for titration of excess of `color{red}(H_2SO_4 = V_1 mL V_1mL)` of `color{red}(NaOH)` of molarity `color{red}(M = V_1 /2 mL)` of `color{red}(H_2SO_4)` of molarity `color{red}(M)`
Volume of `color{red}(H_2SO_4)` of molarity `color{red}(M)` unused `color{red}(= (V - (V_1)/2) mL)`
`color{red}((V- (V_1)/2) mL)` of `color{red}(H_2SO_4)` of molarity `color{red}(M)`
`color{red}(= 2(V-(V_1)/2) mL)` of `color{red}(NH_3)` solution of molarity `color{red}(M)`.
`color{red}(1000 mL)` of `color{red}(1 M NH_3)` solution contains `17g` `color{red}(NH_3)` or `14 g` of `color{red}(N)`
`color{red}((2(V-(V_1)/2) mL)` of `color{red}(NH_3)` solution of molarity `color{red}(M)` contains:
`color{red}((14 xxM xx 2 (V - (V_1)/2))/(1000) g N)`
Percentage of `color{red}(N = (14 xx M xx 2 ( V - (V_1)/2))/(1000) xx 100/m)`
`color{red}( = ( 1.4 xx M xx2 (V - (V_1)/2))/m)`
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 these conditions.
There are two methods for estimation of nitrogen: (i) Dumas method and (ii) Kjeldahl’s method.
(i) Dumas method: The nitrogen containing organic compound, when heated with copper oxide in an atmosphere of carbon dioxide, yields free nitrogen in addition to carbon dioxide and water.
`color{red}(C_x H_y N_z + (2x+y/2) CuO → xCO_2 + y/2 H_2O + z/2 N_2 + (2x+y/2) Cu)`
Traces of nitrogen oxides formed, if any, are reduced to nitrogen by passing the gaseous mixture over a heated copper gauze. The mixture of gases so produced is collected over an aqueous solution of potassium hydroxide which absorbs carbon dioxide. Nitrogen is collected in the upper part of the graduated tube (Fig.12.15).
Let the mass of organic compound `color{red}(= m g)` Volume of nitrogen collected `color{red}(= V_1 mL)` Room temperature `color{red}(= T_1K)`
Volume of nitrogen at STP ` color{red}(= (p_1 V_1 xx 273)/(760 xx T_1))`
(Let it be `color{red}(V mL)`) Where `color{red}(p_1)` and `color{red}(V_1)` are the pressure and volume of nitrogen, `color{red}(p_1)` is different from the atmospheric pressure at which nitrogen gas is collected. The value of `color{red}(p_1)` is obtained by the relation;
`color{red}(p_1=)` Atmospheric pressure – Aqueous tension `color{red}(22400 mL N_2)` at STP weighs `28 g.`
`color{red}(V m L N_2)` at STP weighs `color{red}( = ( 28 xx V)/(22400) g)`
`color{red}("Percentage of nitrogen " = ( 28xxV xx 100)/(22400 xx m))`
(ii) Kjeldahl’s method: The compound containing nitrogen is heated with concentrated sulphuric acid. Nitrogen in the compound gets converted to ammonium sulphate (Fig. 12.16). The resulting acid mixture is then heated with excess of sodium hydroxide. The liberated ammonia gas is absorbed in an excess of standard solution of sulphuric acid. The amount of ammonia produced is determined by estimating the amount of sulphuric acid consumed in the reaction. It is done by estimating unreacted sulphuric acid left after the absorption of ammonia by titrating it with standard alkali solution. The difference between the initial amount of acid taken and that left after the reaction gives the amount of acid reacted with ammonia.
`color{red}("Organic compound" + H_2SO_4 → (NH_4)_2 SO_4 overset(2NaOH)→ Na_2SO_4 +2NH_3 + 2H_2O)`
`color{red}(2NH_3 + H_2SO_4 → (NH_4)_2 SO_4)`
Let the mass of organic compound taken `color{red}(= m g)`
Volume of `color{red}(H_2SO_4)` of molarity, `color{red}(M),` taken `color{red}(= V mL)`
Volume of `color{red}(NaOH)` of molarity, `color{red}(M),` used for titration of excess of `color{red}(H_2SO_4 = V_1 mL V_1mL)` of `color{red}(NaOH)` of molarity `color{red}(M = V_1 /2 mL)` of `color{red}(H_2SO_4)` of molarity `color{red}(M)`
Volume of `color{red}(H_2SO_4)` of molarity `color{red}(M)` unused `color{red}(= (V - (V_1)/2) mL)`
`color{red}((V- (V_1)/2) mL)` of `color{red}(H_2SO_4)` of molarity `color{red}(M)`
`color{red}(= 2(V-(V_1)/2) mL)` of `color{red}(NH_3)` solution of molarity `color{red}(M)`.
`color{red}(1000 mL)` of `color{red}(1 M NH_3)` solution contains `17g` `color{red}(NH_3)` or `14 g` of `color{red}(N)`
`color{red}((2(V-(V_1)/2) mL)` of `color{red}(NH_3)` solution of molarity `color{red}(M)` contains:
`color{red}((14 xxM xx 2 (V - (V_1)/2))/(1000) g N)`
Percentage of `color{red}(N = (14 xx M xx 2 ( V - (V_1)/2))/(1000) xx 100/m)`
`color{red}( = ( 1.4 xx M xx2 (V - (V_1)/2))/m)`
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 these conditions.