1. Estimation of C and H (Liebig method). Carbon and hydrogen are always estimated simultaneously. A weighed amount of the compound is heated strongly with dry copper oxide in presence of oxygen or air, its carbon and hydrogen are oxidised to carbon dioxide and water vapours respectively. The resultant CO2 and H2O vapours are absorbed separately in potash and calcium chloride solutions respectively and hence the weight of absorbed gases (CO2 and H2O) is determined.
\(C +2 CuO \longrightarrow CO_2 + 2Cu\)
\(2H+ Cu O \longrightarrow H_2 O + Cu\)
From the weights of CO2 and H2O vapours formed, the % of carbon and hydrogen in the compound can be calculated.
2. Estimation of nitrogen
(i) Duma's method : The organic compound is heated with copper oxide when its nitrogen is converted into nitrogen gas (N2) which is collected over caustic potash which absorbs all other gases except N2. The method is applicable to all nitrogenous organic compounds.
\(N + CuO \longrightarrow N_2 +\) small amount of oxides of nitrogen
Oxides of nitrogen \(+ Cu \longrightarrow CuO + N2 \)
(ii) Kjeldahl's method : The compound is heated with conc. H2SO4 in the presence of a catalyst (e.g. a mixture of CuSO4 and K2SO4)1, when its nitrogen is quantitatively converted into ammonium sulphate. The ammonium sulphate so obtained is decomposed with excess of NaOH solution to give ammonia which is estimated.
This method is simpler and convenient, and can be used in compounds in which nitrogen is directly linked to carbon and hydrogen. Therefore, this method is largely used for the estimation of nitrogen in food, fertilizers and drugs. However, the method fails in compounds con- taining nitrogen joined either to other nitrogen or to oxygen as in nitro, nitroso, azo (- N = N-), azoxy, diazo, hydrazones, oximes, etc. since in these compounds nitrogen is not converted into \((NH_{4}) 2 SO _4 \).
3. Estimation of halogens and sulphur (Carius method) : In case of halogens, the weighed amount of organic compound is heated in a sealed tube with fuming nitric acid in presence of silver nitrate when its halogen is converted into silver halide (AgX) which is weighed. In case sulphur, the compond is heated with conc. HNO3 when its sulphur is oxidised to \(H_2SO_4\) which is collected as barium sulphate (BaSO4) by using barium chloride solution.
4. Estimation of phosphorus: Weighed amount of the organic compound is heated with conc. HNO3 in a Carius tube. Phosphorus present in the compound is converted into phosphoric acid. Contents of the tube are heated with MgCl2 solution in the presence of NH4Cl and NH4OH (magnesia mixture); the MgNH4PO4 so formed is collected, washed, dried and ignited to get magnesium pyrophosphate (Mg2P2O7) which is weighed.
\(
\mathrm{MgCl}_2+\mathrm{NH}_4 \mathrm{Cl}+\mathrm{H}_3 \mathrm{PO}_4 \longrightarrow \mathrm{MgNH}_4 \mathrm{PO}_4+3 \mathrm{HCl} \)
\(2 \mathrm{MgNH}_4 \mathrm{PO}_4 \longrightarrow \mathrm{Mg}_2 \mathrm{P}_2 \mathrm{O}_7 \downarrow+2 \mathrm{NH}_3+\mathrm{H}_2 \mathrm{O}
\)
\(\text { Knowing the weight of } \mathrm{Mg}_2 \mathrm{P}_2 \mathrm{O}_7 \text {, percentage of } \mathrm{P} \text { can be calculated. }\)
\(% \text { of } \mathrm{P}=\frac{66 \times \text { Wt. of } \mathrm{Mg}_2 \mathrm{P}_2 \mathrm{O}_7 \text { formed }}{222 \times \text { Wt. of the compound }} \times 100
\)\(\% \text { of } \mathrm{P}=\frac{66 \times \text { Wt. of } \mathrm{Mg}_2 \mathrm{P}_2 \mathrm{O}_7 \text { formed }}{222 \times \text { Wt. of the compound }} \times 100
\)
5. Estimation of oxygen : The amount of oxygen is calculated by difference. In case, the sum of precentages of all other elements present in the organic compound is less than 100, the remiander gives the precentage of oxygen i.e. % of oxygen 100 Sum of % of all other elements.
