( 132 )

CHAPTER XVI.

CHLORINE, CHLORIDES, AND CHLORIMETRY.

HLORINE possesses the valuable property of destroying organic pigments,and consequently is of use to the dyer as a bleaching agent. Under thepressure of the atmosphere and at ordinary temperatures chlorine is a gas ofsp. gr. = i‘33. Its colour is a yellowish green. Its irritating odour is well known.One volume of water absorbs 2'5 volumes of the gas, the result being knownas chlorine-water. The theory of its bleaching properties is based upon itsaffinity for hydrogen, forming, with the hydrogen of the organic matter,hydrochloric acid. Simultaneously, by the decomposition of the water con-tained in the organic body, ozone is formed, and the latter oxidises thecolouring matter. Thus, in the case of bleaching flax and raw cotton, onlythe colouring matter and not the cellulose is destroyed.

For purposes of transport it is usual to employ a combination of chlorinewith oxygen and a base, as a hypochlorite, and of these combinations weshall speak presently.

The methods of preparing chlorine are very various. They may be dividedinto two classes—those employing manganese and those which do not. Thelatter it will be more convenient to describe first. At Mr. Tennant’s works,at Glasgow, Dunlop’s process is employed, consisting in a citing with sulphuricacid upon a mixture of 3 molecules of common salt with 1 molecule of nitrateof soda. Chlorine and hyponitric acid are evolved, the latter being absorbedby passing the mixed gases through concentrated sulphuric acid.

Peligot’s method consists in heating 3 parts of bichromate of potash and4 parts of concentrated hydrochloric acid, to obtain a solution yielding, uponcooling, crystals of bichromate of chloride of potassium, KCI,Cr0 3 ; and thissalt heated to no° yields chlorine.

Messrs. MacDougal, Rawson, and Shanks decompose chromate of lime byhydrochloric acid, with the formation of chloride of chromium, chloride ofcalcium, and the evolution of free chlorine, according to the formula—2CaCr0 4 + 16HCI = Cr 2 Cl6 + 2CaCl 2 +3H 2 0 + 6C1.

The chloride of chromium may be precipitated with carbonate of lime, andconverted into chromate of lime by ignition.

M. Vogel decomposes chloride of copper by heat, the chloride being mixedwith sand, and heated in earthenware retorts to 250°. Chlorine is evolved,and protochloride of copper remains, convertible into perchloride by treatmentwith hydrochloric acid. 100 kilos, of cupric chloride yield 6 to 7 cubic metresof chlorine.