Chlorine : Chlorine, Preparation

Expt. 1. - A stream of air is passed through concentrated sulphuric acid in a Woulfe's bottle, and concentrated hydrochloric acid allowed to drop slowly into the latter. The mixture of air and hydrochloric acid gas is passed through a hard glass tube packed with pieces of pumice which have been soaked in a solution of copper sulphate and dried, and the tube is heated in a furnace

Fig: Chlorine formation Oxidation of hydrochloric acid gas by atmospheric oxygen with formation of chlorine. to a dull red heat. The gas is then passed through litmus solution, which is rapidly bleached by the chlorine evolved. The copper sulphate is first converted into chloride: CuSO4 + 2HCl = CuCl2 + H2O + SO3.The oxidation by means of manganese dioxide also occurs with gaseous hydrogen chloride, and takes place very slowly in the cold but rapidly on heating. In this case no free oxygen is required, as the oxygen needed is provided by the manganese dioxide: MnO2 + 4HCl = MnCl2 + 2H2O + Cl2 Expt. 2. - Place powdered manganese dioxide in one bulb of a hard glass tube, leaving the other bulb empty. Pass a slow current of hydrogen chloride, obtained by dropping concentrated hydrochloric acid into concentrated sulphuric acid in the apparatus of Expt. 1, over the dioxide, and allow the gas to pass into a bottle containing litmus solution and a piece of moist red flannel. Fig: Oxidation of hydrochloric acid gas by heated manganese dioxide Heat the manganese dioxide. Moisture collects in the second bulb, and chlorine passes on to the bottle and bleaches the litmus and the red flannel. The usual laboratory method of preparing chlorine is to decompose aqueous hydrochloric acid with manganese dioxide: the mineral form, called pyrolusite, in small pieces, is most convenient. Expt. 3. - About 100 gm. of pyrolusite in small pieces are placed in a two-litre flask fitted with a long thistle funnel and delivery tube through a black rubber stopper. About 300 c.c. of concentrated hydrochloric acid are poured into the flask, care being taken thoroughly to moisten the solid, and the mixture is heated gently over wire gauze. [fig:fig_099.jpg text:Preparation of chlorine from hydrochloric acid and manganese dioxide.

The chlorine evolved is washed with a little water and collected in jars by downward displacement (it is 2½ times as heavy as air). It may also be collected over saturated brine, which dissolves 0.36 its volume of chlorine. If required dry, the gas is passed through concentrated sulphuric acid in a second wash-bottle. The preparation is carried out in a good draught cupboard, as the gas has a powerful corrosive action on the mucous membranes. The inhalation of alcohol vapour, and diluted ammonia gas, relieves the bad effects produced by breathing air containing chlorine.

Expt. 4. - The red crystals of potassium dichromate are heated in a flask with concentrated hydrochloric acid; a green solution of chromic chloride, CrCl₃, is formed, and practically pure chlorine is evolved.

Expt. 5. - Drop concentrated hydrochloric acid slowly on crystals of potassium permanganate in a flask. Chlorine is evolved in the cold and may be washed with water and concentrated sulphuric acid. When the evolution of gas ceases, a further supply is obtained on warming. By passing this gas into a glass bulb cooled in solid carbon dioxide and ether, liquid chlorine is formed, by the evaporation of which almost perfectly pure chlorine may be obtained.