1. Principle:
A known weight of organic substance is burnt in excess of oxygen and the carbon and hydrogen present in it are oxidised to CO2 and H2O respectively.
The weight of carbon dioxide and water thus formed are determined and the amount of carbon and hydrogen in the organic substance are calculated.
2. Description of the apparatus:
(a) The oxygen supply (b) combustion tube (c) Absorption tube
Oxygen supply:
To remove the moisture from oxygen. it is allowed to bubble through sulphuric acid and then passed through a U-tube containing sodalime to remove CO2. The oxygen gas free from moisture and CO2 enters the combustion tube.
Combustion tube:
A hard glass tube open at both ends used for the combustion. It contains:
(i) an oxidized copper gauze to prevent the backward diffusion of the products of combustion
(ii) a porcelain boat containing a known weight of the organic substance
(iii) coarse copper oxide on either side and
(iv) an oxidized copper gauze placed towards the end of the combustion tube. The combustion tube is heated by a gas burner.
Absorption apparatus:
The combustion products containing moisture and CO2 are then passed through the absorption apparatus which consists of :
(i) a weighed U-tube packed with pumice soaked in conc. H2SO4 to absorb water
(ii) a set of bulbs containing a Strong solution of KOH to absorb CO2 and finally
(iii) a guard tube filled with anhydrous CaCl2 to prevent the entry of moisture from atmosphere.
3. Procedure:
The combustion tube is heated strongly to dry its content. It is then cooled and connected to absorption apparatus. The other end of the combustion tube is open for a while and the boat containing weighed organic substance is introduced. The tube is again heated strongly till all the substance in the boat is burnt away. This takes about 2 hours. Finally a strong current of oxygen is passed. Then the U-tube and potash bulbs are then detached and increase in weight of each of them is determined.
4. Calculation:
Weight of organic substance = W g
Increase in weight of H2O = x g
Increase in weight of CO2 = y g
18g of H2O contains 2 g of hydrogen
\(\therefore\) x g of H2O contain \(\frac{1}{2}\times\)x g of hydrogen.
\(\therefore\) Percentage of hydrogen = \(\big(\frac{2}{18}\times\frac{x}{w}\times100\big)\)
44g of CO2 contains 12 g of carbon
\(\therefore\) y g of CO2 contain \(\frac{12}{44}\times\) y g of carbon
\(\therefore\) Percentage of carbon = \(\big(\frac{12}{44}\times\frac{12}{44}\times100)\%\)
