For detecting the presence of elements in a compound, these have to be converted into ionic forms. This is done by fusing the compound with sodium metal. The elements present in the compound are converted into soluble sodium salts.
Na + X → NaX
where X is Cl, Br or I
Halogens are present as halides in sodium extract.
(i) Silver Nitrate Test
Chlorides give white precipitate of silver chloride with silver nitrate. The precipitate is soluble in ammonium hydroxide.
NaCl + AgNO3 → AgCl ↓ + NaNO3
AgCl + 2NH4OH → [Ag(NH3)2] Cl + 2H2O
Bromides yield yellow precipitate, sparingly soluble in ammonium hydroxide. Iodides yield dark yellow precipitate, insoluble in ammonium hydroxide.
(ii) When a solution containing bromide or iodide is treated with chlorine water, the bromide and iodide are oxidized to the corresponding halogens. Being covalent, these halogens get dissolved in carbon tetrachloride CCl4. Bromine imparts yellow to brown colour and iodine imparts purple colour in CCl4 layer.
2NaBr + Cl2 → 2NaCl + Br2
2Nal + Cl2 → 2NaCl + l2
Chlorine being more electronegative, displaces bromides and iodides from solution.
The experiment consists of two stages:
Preparation of Sodium Fusion Extract
Take out a small piece of sodium metal from a petri dish with the help of pair of forceps. Dry the sodium piece between the folds of filter papers. Put a small piece of sodium metal into a clean and dry ignition tube. Hold the tube with a pair of tongs and heat it gently over the flame so that sodium melts and makes a stirring ball. Remove the ignition tube away from the flame and immediately add a small amount of solid organic compound into it.
Heat the tube to red hot. Again add a little of the compound into the tube and heat it again to red hot. Plunge the red hot ignition tube in about 10 mL of distilled water taken in a porcelain dish. Immediately cover the porcelain dish with a wire gauze to avoid bumping of any unreacted sodium piece. Repeat the process of sodium fusion using two more ignition tubes. Break the tubes into small pieces with a clean glass rod. Boil the mixture for about 2-3 minutes and filter.
Test for Halogens
(i) Take about 1 mL of sodium fusion extract in a test tube and acidify it with dilute nitric acid. Add to it about 2 mL of silver nitrate solution. Observe the appearance of any precipitate and its colour.
Add 2-3 mL of ammonium hydroxide to the precipitate and shake the contents. Observe the solubility of precipitate.
(ii) If chloride is absent, perform the following test for bromide and iodide.
Take 2 mL of sodium fusion extract in a test tube and acidify it with dilute hydrochloric acid. Add 1 mL of carbon tetrachloride. Being denser, CCl4 forms the lower layer.
Add excess of chlorine water and shake the mixture vigorously. Observe the appearance of any colour in CCl4 layer.