C + N + S → NaSCN Fe + +SCN – → [Fe(SCN)] + Blood red If sodium fusion is carried out with excess of sodium, the thiocyanate decomposes to yield cyanide and sulphide. These ions give their usual tests. NaSCN + 2Na → NaCN+Na S (C) Test for Halogens The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.
X – + Ag + → AgX X represents a halogen – Cl, Br or I. If nitrogen or sulphur is also present in the compound, the sodium fusion extract is first boiled with concentrated nitric acid to decompose cyanide or sulphide of sodium formed during Lassaigne’s test. These ions would otherwise interfere with silver nitrate test for halogens. (D) Test for Phosphorus The compound is heated with an oxidising agent (sodium peroxide).
The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus. Na PO + 3HNO → H PO +3NaNO H PO + (NH ) MoO + 21HNO → Ammonium molybdate (NH ) PO .12MoO + 21NH NO + 12H O Ammonium phosphomolybdate .
Quantitative Analysis Quantitative analysis of compounds is very important in organic chemistry. It helps chemists in the determination of mass per cent of elements present in a compound. You have learnt in Unit- that mass per cent of elements is required for the determination of emperical and molecular formula. The percentage composition of elements present in an organic compound is determined by the following methods: .
. Carbon and Hydrogen Both carbon and hydrogen are estimated in one experiment. A known mass of an organic compound is burnt in the