Estimation of Carbon and Hydrogen by kjeldahl's method

A known mass (0.3 to 0.5 g) of the given organic compound is digested with concentrated H₂SO₄, in presence of a small quantity of potassium sulphate and copper sulphate in a Kjeldahl's flask. Potassium sulphate raises the boiling point of sulphuric acid and copper sulphate catalyzes the digestion. In 3 to 4 hours, the organic compound is completely decomposed to form ammonium sulphate.

Distillation

The digested reaction mixture, on cooling, is transferred to a round bottomed distillation flask, and distilled with a concentrated alkali solution (NaOH). Ammonia produced is absorbed in a known volume of HCl solution of a known strength.

The un-neutralised HCl is then back-titrated against a standard alkali. From the acid consumed, the amount of ammonia produced and hence the mass of nitrogen is calculated.

Fig: Apparatus for the estimation of nitrogen by Kjeldahl's method

Calculation

Let,

Mass of the organic compound = W g

Volume of the standard acid required for complete neutralization of the evolved ammonia = V mL

Normality of the standard solution of acid = N

From the law of equivalence (normality equation),

1000 mL of 1 N acid = 1000 mL of 1 N NH_3 = 17g NH_3 = 14 g nitrogen

Then,

V mL of N acid = V mL of NH₃

NV milli equivalent of acid = NV milli equivalent of ammonia

Therefore,

Then,

Percentage of nitrogen in the sample =