QUANTITATIVE ANALYSIS OF ORGANIC COMPOUNDS

The percentage composition of elements present in an organic compound is determined by the following methods:

1. Estimation of Carbon and Hydrogen

Carbon and hydrogen are estimated by Liebig’s combustion method. In this method, a known mass of an organic compound is burnt in the presence of excess of oxygen and copper(II) oxide. Then carbon is oxidised to CO2 and hydrogen is oxidised to H2O.

CxHy + (x + y/4) O2 → x CO2 + (y/2) H2O

The water so produced is absorbed in a weighed U-tube containing anhydrous calcium chloride and carbon dioxide is absorbed in another U-tube containing concentrated solution of potassium hydroxide. These tubes are connected in series. The increase in masses of calcium chloride and potassium hydroxide gives the amounts of water and carbon dioxide from which the percentages of carbon and hydrogen are calculated.

Calculations:  Let the mass of organic compound be m g, mass of water and carbon dioxide produced be m1 and m2 g respectively.

Percentage of hydrogen = 2 x m1 x 100 / 18 x m

Percentage of carbon = 12 x m2 x 100 % / 44 x m

2. Estimation of Nitrogen

There are two methods for estimation of nitrogen: (i) Dumas method and (ii) Kjeldahl’s method.

(i) Dumas method:

Here the organic compound is heated with copper oxide in an atmosphere of carbon dioxide so that free nitrogen, carbon dioxide and water are produced.

CxHyNz + (2x + y/2) CuO → x CO2 + y/2 H2O + z/2 N2 + (2x + y/2) Cu

This mixture of gases is collected over an aqueous solution of potassium hydroxide which absorbs carbon dioxide. Nitrogen is collected in the upper part of the graduated tube

Calculations:

Let the mass of organic compound = m g Volume of nitrogen collected = V1 mL Room temperature = T1 K

Volume of nitrogen at STP = P1V1 x 273 / 760 × T1 = V mL

Where P1 and V1 are the pressure and volume of nitrogen gas.

P1= Atmospheric pressure – Aqueous tension

We know that 22400 mL N2 at STP weighs 28 g. Therefore, VmL N2 at STP weighs = 28 x V / 22400  g

Percentage of nitrogen = 28 x V x 100 % / 22400 x m

(ii) Kjeldahl’s method:

Here the organic compound containing nitrogen is heated with concentrated sulphuric acid. Nitrogen in the compound gets converted to ammonium sulphate. 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.

Organic compound + H2SO4 → (NH4)2SO4 

2 NaOH Na2SO4 + NH3 + H2O 2NH3 + H2SO4 → (NH4)2SO4

Calculations:

Let the mass of organic compound taken = m g Volume of H2SO4 of molarity M, taken = V ml

Note: Kjeldahl’s 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.

3. Estimation of halogens (Carius method):

Here a known mass of an organic compound is heated with fuming nitric acid in the presence of silver nitrate contained in a hard glass tube known as Carius tube, in a furnace. Carbon and hydrogen present in the compound are oxidised to carbon dioxide and water. The halogen present forms the corresponding silver halide (AgX). It is filtered, washed, dried and weighed.

Calculations:

Let the mass of organic compound taken = m g

Mass of AgX formed = m1 g

1 mol of AgX contains 1 mol of halogen

4. Estimation of Sulphur (Carius method):

A known mass of an organic compound is heated in a Carius tube with sodium peroxide or fuming nitric acid. Sulphur present in the compound is oxidised to sulphuric acid. It is precipitated as barium sulphate by adding excess of barium chloride solution. The precipitate is filtered, washed, dried and weighed. The percentage of sulphur can be calculated from the mass of barium sulphate (BaSO4).

Calculations:

Let the mass of organic compound taken = m g and the mass of barium sulphate formed = m1 g 1 mol of BaSO4 = 233 g BaSO4 = 32 g sulphur

5. Estimation of Phosphorus

A known mass of an organic compound is heated with fuming nitric acid. Phosphorus present in the compound is oxidised to phosphoric acid. It is precipitated as ammonium phosphomolybdate [(NH4)3PO4.12MoO3] by adding ammonia and ammonium molybdate.

Calculations:

Let the mass of organic compound taken = m g and mass of ammonium phosphomolydate = m1g

6. Estimation of Oxygen

The percentage of oxygen in an organic compound is usually found by difference between the total percentage composition (100) and the sum of the percentages of all other elements.

i.e. percentage of oxygen = 100 – sum of the percentage of all the other elements.