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Quantitative Analysis

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

Carbon and Hydrogen

Both carbon and hydrogen are estimated in one experiment. A know mass of an organic compound is burnt in the presence of excess of oxygen and copper (II) oxide. Carbon and hydrogen in the compound are oxidised to carbon dioxide and water respectively.

The mass of water produced is determined by passing the mixture through a weighed U-tube containing anhydrous calcium chloride. Carbon dioxide is absorbed in another U-tube containing concentrated solution of potassium hydroxide. These tubes are connected in series (fig 12.14). The increase in masse of calcium chloride and potassium hydroxide gives the amounts of water and carbon dioxide from which the percentages of carbon and hydrogen are calculated.
Let the mass of organic compound be m g, mass of water and carbon dioxide produced be g respectively,
Percentage of carbon =
Percentage of hydrogen =


There are two methods for estimation of nitrogen (i) Dumas method and (ii) kjeldahl's method
  1. 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.

Traces of nitrogen oxides formed, if any are reduced to nitrogen by passing the gaseous mixture over 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.

Let the mass of organic compound = mg volume of nitrogen collected
Room temperature

Volume of nitrogen at STP (Let it be V ml) where e the pressure and volume of nitrogen where is different from the atmospheric pressure at which nitrogen gas is collected. The value of is obtained by the relation;

atmospheric pressure - Aqueous tension 22400 mlat STP weigh 28g
VmlL at STP weighs
Percentage of nitrogen
  1. 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.
Organic compound

Let the mass of organic compound taken = mg
volume of H2SO4 of molarity, M taken = Vml
Volume of NaOH of molarity, M used for titration excess of
of NaOH of molarity M
of of molarity M unused
Volume of of molarity M solution of molarity M.
1000mLl of 1M solution contains 17g 14 g of N
solution of molarity M contains:

Percentage of

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 the conditions.


Carius Method: 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 (fig 12.17) 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.
Let the mass of organic compound taken =m g
Mass of AgX formed =
1 mol of AgX contains 1 mol of X
Mass of halogen in of AgX

Percentage of halogen


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 in water. The precipitate is filtered, washed, dried and weighed. The percentage of sulphur can be calculated from the mass of barium sulphate.
Let the mass of organic compound taken = mg
And the mass of barium sulphate formed =
1 mol of sulphur
contains g sulphur
Percentage of sulphur


A known mass of an organic compound is heated with fuming nitric acid whereupon phosphorus present in the compound is oxidised to phosphoric acid. It is precipitated as ammonium phosphomolybdate. by adding ammonia and ammonium molybdate. Alternatively, phosphoric acid may be precipitated as by adding magnesia mixture which on ignition yields .
Let the mass of organic compound taken =m g and

mass of ammonium phosphomolyate =
Molar mass of
Percentage of phosphorus
If phosphorus is estimated as
Percentage of phosphorus where 222 u is the molar mass of m, the mass of organic compound taken , the mass of formed and 62, the mass of two phosphorus atoms present in the compound


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. However, oxygen can also be estimated directly as follows :
A defined mass of an organic compound is decomposed by heating in a stream of nitrogen gas. The mixture of gaseous products containing oxygen is passed over red-hot coke when all the oxygen is converted to carbon, warm iodine pentoxide when carbon monoxide is oxidised to carbon dioxide producing iodine.
Compound + other gaseous products

The percentage of oxygen can be derived from the amount of carbon dioxide or iodine produced
Let the mass of organic = mg compound taken be mg
Mass of carbon dioxide =
44 g carbon dioxide = 32 g oxygen
carbon dioxide contains
Percentage of oxygen
Presently, the estimation of elements in an organic compound is carried out by using micro quantities of substance and automatic experimental techniques. The elements, carbon, hydrogen and nitrogen present in a compound are determined by an apparatus known as CHN elemental analyzer. The analyzer requires only a very small amounts of the substance (1-3mg) and displays the values on a screen within a short time. A detailed discussion of such methods is beyond the scope of this book.

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