Organic Chemistry – Some Basic Principles and Techniques - Qualitative and Quantitative Analysis

Qualitative Analysis: Detection of Carbon and Hydrogen is done by heating the compound with $CuO$, converting Carbon to $CO_2$ (turns lime water milky) and Hydrogen to $H_2O$ (turns anhydrous $CuSO_4$ blue).

Lassaigne's Test: Used for detection of Nitrogen, Sulphur, and Halogens. The organic compound is fused with metallic Sodium to convert covalent bonds into ionic bonds ($NaCN$, $Na_2S$, $NaX$).

Detection of Nitrogen: The sodium fusion extract is boiled with $FeSO_4$ and acidified with concentrated $H_2SO_4$. Formation of Prussian blue color due to $Fe_4[Fe(CN)_6]_3 \cdot xH_2O$ confirms Nitrogen.

Detection of Sulphur: Sodium fusion extract treated with Sodium Nitroprusside gives a violet color ($[Fe(CN)_5NOS]^{4-}$).

Quantitative Estimation of Carbon and Hydrogen (Liebig's Method): A known mass of compound is burnt in the presence of excess Oxygen and $CuO$. Carbon is oxidized to $CO_2$ (absorbed in $KOH$ solution) and Hydrogen to $H_2O$ (absorbed in anhydrous $CaCl_2$).

Dumas Method for Nitrogen: The compound is heated with $CuO$ in an atmosphere of $CO_2$, yielding $N_2$ gas. The volume of $N_2$ is measured at $STP$.

Kjeldahl's Method for Nitrogen: The compound is digested with concentrated $H_2SO_4$ to form $(NH_4)_2SO_4$. On heating with $NaOH$, $NH_3$ is evolved and absorbed in a known volume of standard acid. Note: This method is not applicable to compounds containing Nitrogen in nitro/azo groups or in the ring (e.g., Pyridine).

Carius Method for Halogens: The compound is heated with fuming $HNO_3$ and $AgNO_3$. Halogen is converted to $AgX$ precipitate, which is filtered, washed, dried, and weighed.