KJELDAHL ’ S METHOD : A known mass of organic compound is heated with conc . H2SO4 in presence of K2SO4 and little CuSO4 or Hg in a long necked flask called Kjeldahl ’ s flask when nitrogen present in the organic compound is quantitatively converted into ( NH4 ) 2SO4 . ( NH4 ) 2SO4 thus obtained is boiled with excess of NaOH solution to liberate NH3 gas which is absorbed in a known excess of a standard acid such as H2SO4 or HCl .
The vol of acid unused is found by titration against a standard alkali solution . From the vol of the acid used , the percentage of nitrogen is determined by applying the equation ,
% N = 1.4 x Molarity of the acid x Basicity of the acid x Vol of the acid used
Mass of the substance taken Halogens 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 test tube known as carius tube in a furnace . Carbon and hydrogen present in the compound are oxidized to carbon dioxide and water . The halogen present forms the corresponding silver halide . It is filtered , dried , and weighed .
Let the mass of the organic compound taken = m g Mass of AgX formed = m1 g 1 mol of AgX contains 1 mol of of X Mass of halogen in m1 g of AgX
= at mass of X xm1 g Molecular mass of AgX
% of halogen
= at mass of X xm 1 g x 100 % Molecular mass of AgX x m