792 CHLOROMETRY 



of the mixture. The acids and water were mixed in a wooden tub, the water being 

 put in first, and then about half the acid ; after cooling, the other half was added. 

 The proportions of water and acid were 9 measures of the former to 10 of the latter. 

 See Watts's ' Dictionary of Chemistry.' 



Many of the compounds of chlorine are of great value in the arts and manufactures. 

 Its hydrogen acid is fully described under HYDROCHLORIC Aero, while its compounds 

 with the metals, forming important substances called chlorides, are noticed under the 

 names of their respective metals. With oxygen, chlorine forms some unimportant 

 anhydrides ; but with oxygen and hydrogen it gives rise to a series of ox-acids, 

 known respectively as hypochlorous, chlorous, chloric, and perchloric acids. Some of 

 the salts of hypochlorous acid are important as constituents of the bleaching compounds 

 popularly called ' chlorides,' as chloride of lime ; and some of the salts of chlorous acid, 

 as chlorate of potash, are also of great value in the arts. See CHLORATE OF POTASH ; 

 CHLORIDE OF LIME. 



CHLORITE. A term applied to a family of minerals of a dark green colour and 

 lamellar structure. They are hydrated silicates of alumina, magnesia, protoxide of 

 iron, &c. The chlorite occurring in the Cornish tin-veins is known as Peach. 



CHLOROMETRY. The name given to the process or processes by which the 

 amount of available chlorine is estimated in substances containing it, which are em- 

 ployed in bleaching, or as disinfectants. The chlorides (hypochlorites) of lime, of potash, 

 or of soda are the most important of these compounds. All these decolourising chlorides 

 are decomposed by the action of acids, even by that of the carbonic acid of the atmo- 

 sphere, chlorine being set free. The amount of chlorine capable of being thus evolved 

 is required to be known for two reasons ; to ascertain its commercial value, and to learn 

 whether the strength of the solution employed is within the limits which enable it to 

 be effectively and yet safely used in bleaching. A number of processes have been 

 devised for this purpose. The method proposed by Gay-Lussac has been extensively 

 used in manufactories ; it depends upon the fact that arsenious acid in solution is 

 oxidised by free chlorine and converted into arsenic acid with the simultaneous forma- 

 tion of hydrochloric acid. A solution of arsenious acid in hydrochloric acid was first 

 prepared, of a known strength, and also a solution of the chloride of lime to be 

 tested, by triturating a weighed quantity with water and making the mixture up to 

 a certain bulk. A measured quantity of the arsenious acid liquor was placed in a 

 beaker and coloured with a solution of sulphate of indigo, and the chloride of lime 

 solution run into it from a graduated burette till the blue colour was destroyed. This 

 process, though capable of giving accurate results when due care was taken, was liable 

 to some objections. It was difficult to hit the exact point when the whole of the 

 arsenious acid was converted into arsenic acid, and a little chlorine was apt to be 

 evolved if the chloride solution was run in too quickly ; moreover, it necessitated the 

 measurement of a turbid liquid which required frequent agitation to keep the 

 suspended matter equally diffused through it, and consequently of uniform strength 

 throughout. To obviate these defects, the following modification of this process has 

 been proposed by Penot. It is based upon the same reaction as Gay-Lussac's, but the 

 change is effected in an alkaline solution, and a test-paper moistened with starch paste 

 containing iodide of potassium is used to show when the reaction is completed. 

 Whatever process may be employed, the mode of preparing the solution of the chloride 

 of lime is the same. Weigh out 100 grains of the chloride to be examined, after it 

 has been well mixed so as to obtain a fair average sample, put them into a mortar 

 with a little water and triturate together so as to break up all lumps and produce a 

 smooth paste, continue to rub whilst gradually adding more water, allow the heavier 

 particles to subside, pour off the supernatant liquid into a measuring flask or cylinder 

 capable of holding 10,000 grains, treat the residue in the mortar as before, and finally 

 rinse the mortar out into the flask and make up the solution to exactly 10,000 grains 

 measure and mix thoroughly. 



Preparation of the Solution of Arsenite of Soda. 139 - 44 grains of the purest 

 arsenious acid prepared either by crystallisation from hot dilute hydrochloric acid or 

 by re-subliming a quantity of the arsenious acid of commerce free from sulphide, are 

 dissolved in a flask in a few ounces of water with the addition of about 700 grains of 

 crystallised carbonate of soda free from sulphide or hyposulphite. To facilitate the 

 solution, the arsenious acid should be in fine powder and the mixture should be kept 

 near the boiling point and frequently shaken. The contents of the flask with the 

 water required to rinse it out are now transferred to a 10,000 grains measure, and when 

 quite cold filled up to the mark and the whole then carefully mixed. The starch test 

 may be prepared by boiling 3 parts of starch with 500 of water, and afterwards adding 

 1 part of iodide of potassium. The starch should be first mixed smoothly with a 

 portion of cold water, then the remainder of the water may be added boiling, and the 

 whole boiled for a short time. Slips of white unsized paper (filtering paper) are 



