140

DYEING AND CALICO PRINTING .

and 2 equivalents of hydrochloric acid. Supposing, then, the equivalent ofarsenious acid to be represented by 99, while that of chlorine is 35-5, 99 grainsof arsenious acid will correspond to 71 (2 equivalents) of chlorine. It is clear,then, for practical purposes there may be taken 139*44 grains of arsenious acidas corresponding to 100 grains of chlorine. Therefore, if 139*44 grains ofpure arsenious acid be dissolved in 3 ozs. of pure hydrochloric acid, and thesolution diluted to 10,000 grains measure, each 1000 grains measure willcontain 13*944 grains of arsenious acid, equivalent to 10 grains weight ofchlorine. This is the arsenious acid test solution. To this solution is to beadded a few drops of sulphate of indigo, in order to afford an indication of thepoint at which the bleaching effedt of the chloride of lime solution predomi-nates, The process of titration is proceeded with as in alkalimetry, and theresult obtained by a proportional equation.

With the description of another chlorimetric process we shall bring thisdivision to a close. For this method it is necessary to prepare some iodide ofpotassium-starch papers, by taking a clear solution of starch and dissolvingtherein some iodide of potassium; filter-paper may then be dipped into thissolution, and then cut into small strips. This paper, when prepared in thismanner, becomes coloured blue by contadt with chlorine and hypochlorites.

The normal arsenious acid solution is prepared from finely-pulverisedarsenious acid dried in a desiccator. From this dried arsenious acid 4*95 grms.are weighed out, and dissolved, in the presence of 10 grms. of carbonate ofsoda, in distilled water, and diluted to 1000 c.c. A c.c. of this solution con-tains therefore 6*00495 grm. of arsenious acid.

Iodine is very little soluble in water, and is best employed for this purposein the form of solution in iodide of potassium. There are to be weighed out12*7 grms. of crystals of iodine, and these are to be rubbed down with18 grms. of iodide of potassium, in a porcelain mortar, with the addition ofa little water. This solution should be diluted to 1000 c.c., and 10 c.c. ofthis diluted solution should be capable of oxidising 1 c.c. of normal arseniousacid into arsenic acid.

Estimation of the Chlorine in Chlorine-Water.

Two equivalents of chlorine are capable of oxidising 1 equivalent of arse-nious acid to arsenic acid, so that 1 c.c. of the normal arsenious acid solutioncorresponds to 0*00355 S rm * chlorine: 50 c.c. of the sample to be testedare to be titrated with the arsenious test-solution until the iodide test-paper isno longer coloured blue. The test may then be discontinued, or, for greateraccuracy, the solution may be titrated back with the normal iodine solution,the number of divisions of the iodine solution employed being divided by 10.An example will make the process clear. We have 50 c.c. of the chlorinewater to be tested, and we will suppose 34 c.c. of the normal arsenious acidsolution to have been employed in the titration, and 18*8 c.c. of the iodinesolution in the re-titration, this representing the excess of normal arsenioussolution employed. Dividing this number by 10, and deducting the resulti*88 from 34, we have 32*12 c.c. of normal arsenious acid solution really em-ployed. This number multiplied by 0*00355 gives 0*114026 grm. chlorine.The 50 c.c. of chlorine water contain, therefore, 0*114026 grm. of chlorine, andfrom the equation—

50 : 0*114026 = 100 : xwe obtain * = 0*228 per cent of chlorine.