Element

Method and its principle

Formula

Carbon and Hydrogen

Liebig's combustion method : In this process, a given weight of organic compound is heated with pure and dry cupric oxide in a steam of pure and dry oxygen, when carbon is oxidised to carbon dioxide while hydrogen is oxidised to water. From the weight of CO₂ and H₂O, the percentage of C and H can be calculated.

Nitrogen

(i) Duma's method : Elemental nitrogen is converted into molecular nitrogen by a suitable chemical method and its voiume is changed to STP data.

(ii) Kjeldahl's method : Nitrogen in organic compound is converted into NH₃ by suitable chemical method which, in turn, is absorbed by V₁ mL of N₁H₂SO₄.

% of N = 

Where, V=  volume of N₂ in nitrometer (in ml) at NTP,

W= Weight of substance taken;

% of N = (1.4 * N * V)/ W

Note : This method is, however, not applicable to compounds containing nitrogen in the ring (e.g. Pyridine, quinoline etc) and compounds containing nitro and azo (- N = N -) groups since nitrogen in these compounds is not completely converted into (NH₄)₂SO₄ during digestion.

Halogens

(i) Carius method :  The method is based on the fact that when an organic compound containing halogen (Cl, Br, or I) is heated in a sealed tube with fuming nitric acid in presence of silver nitrate, silver halide is formed. From the mass of silver halide formed, the percentage of the halogen can be calculated.

(ii) Schiff's and Piria method : In this method the accurately weighed organic compound (0.15 - 0.25 g) is taken in a small platinum crucible with a mixture of lime and sodium carbonate, (CaO + Na₂CO₃). It is now heated strongly and then cooled and dissolved in dilute nitric acid in a beaker. The solution is then filtered and the halide is precipitated with silver nitrate solution. Halogen is now calculated as in Carius method.

Sulphur

Carius method : When an organic compound containing sulphur is heated with fuming nitric acid, sulphur is oxidised to sulphuric acid. This is taken as barium sulphate by adding barium chloride solution. From the amount of barium sulphate, percentage of sulphur can be calculated.

phosphorous

Carius method : The organic compound containing phosphorus is heated with fuming nitric acid. Phosphorus is oxidised to phosphoric acid. It is precipitated as magnesium ammonium phosphate, MgNH₄PO₄, by the adding up of magnesia mixture

(MgSO₄ + NH₄OH + NH₄Cl). The magnesium ammonium phosphate is washed, dried and ignited when it is converted to magnesium pyrophosphate (Mg₂P₂O₇).

From the mass of magnesium pyro-phosphate, the percentage of phosphorus in the compound can be calculated.

Oxygen

(i) The usual method of determining the percentage of oxygen in an organic compound is by the method of difference. All the elements except oxygen present in the organic compound are estimated and the total of their percentages subtracted from 100 to get the percentage of oxygen.

(ii) Aluise's method  :. Organic compound containing oxygen is heated with graphite and CO formed is quantitatively converted into CO₂ on reaction with I₂O₅.

Percentage of oxygen = 100 - (Sum of the percentages of all other elements)