Estimation of Nitrogen

There are two methods for estimating nitrogen in an organic compound:

1. Dumas method — converts all nitrogen to $\ce{N2}$ gas and measures its volume
2. Kjeldahl's method — converts nitrogen to $\ce{NH3}$ and measures it by back-titration

Each has its own conditions and limitations.

Principle: The nitrogen-containing compound is heated strongly with copper oxide ( $\ce{CuO}$ ) in an atmosphere of $\ce{CO2}$. This converts all nitrogen in the compound to free nitrogen gas:

$\ce{C_xH_yN_z + \left(2x + \frac{y}{2}\right)CuO -> xCO2 + \frac{y}{2}H2O + \frac{z}{2}N2 + \left(2x + \frac{y}{2}\right)Cu}$

Any nitrogen oxides ( $\ce{NO_x}$ ) that form are reduced back to $\ce{N2}$ by passing over heated copper gauze.

The gaseous mixture ( $\ce{CO2}$ + $\ce{N2}$ ) is collected over aqueous KOH solution. KOH absorbs the $\ce{CO2}$, and the remaining gas is pure $\ce{N2}$ collected in a graduated tube.

Calculating percentage of nitrogen:

Let mass of compound = $m$ g, volume of $\ce{N2}$ collected = $V_1$ mL at temperature $T_1$ K and pressure $p_1$ mm Hg.

First convert to STP:
$V_{\text{STP}} = \frac{p_1 V_1}{T_1} \times \frac{273}{760}$

Note: $p_1 = \text{atmospheric pressure} - \text{aqueous tension}$ (since gas is collected over water)

Since 22400 mL of $\ce{N2}$ at STP weighs 28 g:
$\% \text{N} = \frac{28 \times V_{\text{STP}}}{22400 \times m} \times 100$

Principle: The compound is heated with concentrated sulphuric acid ( $\ce{H2SO4}$ ). All nitrogen in the compound converts to ammonium sulphate:
$\ce{\text{Organic N} + H2SO4 -> (NH4)2SO4}$

The mixture is then made alkaline by adding excess NaOH, which liberates ammonia:
$\ce{(NH4)2SO4 + 2NaOH -> Na2SO4 + 2NH3 ^ + 2H2O}$

The $\ce{NH3}$ gas is distilled over and absorbed in a known excess volume of standard sulphuric acid ( $V$ mL of molarity $M$ ):
$\ce{2NH3 + H2SO4 -> (NH4)2SO4}$

The unreacted (excess) acid is back-titrated with standard NaOH ( $V_1$ mL of molarity $M$ ).

Formula:
$\% \text{N} = \frac{1.4 \times M \times (2V - V_1)}{m}$

where $m$ = mass of compound in grams, $V$ = mL of $\ce{H2SO4}$ taken, $V_1$ = mL of NaOH used in back-titration.

Limitation: Kjeldahl's method cannot be used for compounds where nitrogen is in a nitro group ( $\ce{-NO2}$ ), azo group ( $\ce{-N=N-}$ ), or in a ring (like pyridine). In these cases, the nitrogen does not convert to ammonium sulphate under the reaction conditions.