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Estimation of carbon and hydrogen

Liebig’s combustion method: A known mass of an organic compound is heated in a current of dry oxygen (free from CO2) in the presence of cupric oxide till all the carbon is oxidized to carbon dioxide and all the hydrogen is oxidized to water.
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∴ Percentage of carbon in organic compound
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∴ Percentage of hydrogen in organic compound
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Estimation of nitrogen

Duma’s method: Known mass of an organic compound is heated with cupric oxide in an atmosphere of carbon dioxide. The carbon and hydrogen get oxidized to carbon dioxide and water, while the nitrogen is set free.
C + 2CuO CO2 + 2Cu; 2H + CuO H2O + Cu
2N + CuO N2 + Oxides of nitrogen
Oxides of nitrogen + Cu CuO + N2
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where x is the volume of N2 (in liter) at NTP and w is the mass of the organic compound.
Kjeldahl’s method
  • Organic compounds containing nitrogen in the ring such as pyridine and quinoline.
  • Orgainc compounds containing nitro (—NO2) and diazo (—N=N—) groups.
Principle: A known weight of the organic compound is heated with concentrated H2SO4 so that nitrogen is quantitatively converted into ammonium sulphate. The solution is then heated with excess of sodium hydroxide. The ammonia gas evolved is passed into a known but excess volume of standard acid (HCl or H2SO4). The acid left unused is estimated by titrating the solution with standard alkali. From the amount of acid left unused, the amount of acid used for neutralization of ammonia can be calculated. From this, the percentage of nitrogen can be calculated. The chemical reactions involved are as follows:
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Percentage of nitrogen Description: 51861.png
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where N1 and N2 are the milliequivalents of standard acid and alkali, respectively.

Estimation of halogens

Carius method: In this method, a known mass of the organic substance is heated with fuming nitric acid in the presence of silver nitrate in a special sealed tube known as Carius tube.
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Percentage of halogen = Description: 51914.png

Estimation of sulphur

The organic compound containing sulphur is heated with fuming nitric acid. The sulphur in the compound is oxidized to sulphuric acid, which is then precipitated as barium sulphate by adding excess of barium chloride solution.
Percentage of sulphur
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