CHLOROMETRY 793 



dipped into the mixture and used whilst still damp, in which state they are far more 

 sensitive than when dried as Penot proposed. To perform the operation, 1,000 grains 

 of the solution of chloride of lime are measured out with a pipette immediately after 

 thorough mixing, and placed in a beaker or other suitable vessel ; the graduated burette 

 having been filled up to the proper height with the arsenite-of-soda solution, its con- 

 tents are gradually run into the chloride of lime till a drop of the latter taken out 

 with a glass rod ceases to produce any colouration upon a piece of the starch test- 

 paper above described. The operation is then completed. If the burette used 

 contains 1,000 grains divided into 100 parts, each division corresponds to 1 per cent, 

 of chlorine, when the solutions used are of the strength just described. The reason of 

 this will be evident from the following considerations : one molecule of arsenious acid 

 requires four atoms of chlorine with two atoms of water to form one molecule of 

 arsenic acid and four molecules of hydrochloric acid. Taking the molecule of 

 arsenious acid as 198 and the four molecules of chlorine as 132, that is 35'5 x 4, a 

 simple calculation will show that 139'436 grains of arsenious acid are equivalent to 

 100 grains of chlorine, and the 1,000 grains measure of the arsenious test-liquor in the 

 burette contain one-tenth of this quantity, and are consequently equal to 10 grains of 

 chlorine. Hence the number of divisions read off upon the burette indicates at once 

 the percentage of chlorine in the sample examined. As it may sometimes be preferable 

 to operate upon a larger quantity than 10 grains of chloride of lime, 2,000 grains 

 of its solution may be taken, or it may be made of double strength ; of course, in 

 this case the number of divisions observed must be divided by two to obtain the 

 correct result. It is as well to mention that the solution of arsenite of soda is liable 

 to change slightly by the action of the air converting a portion into arsenate of soda, 

 and it is recommended to keep it in small stoppered bottles entirely filled. When the 

 store bottle is provided with the means of drawing off the solution from the bottom, a 

 layer of petroleum oil floating upon the top of the liquid -will defend it from the 

 action of the air. 



Bunsen's Method. When free chlorine is brought in contact with an excess of solu- 

 tion of iodide of potassium, iodine is set free and hydrochloric acid is formed. Each 

 atom of chlorine sets free one atom of iodine, which by means of the exceedingly 

 delicate reaction with starch may be estimated with the greatest accuracy. 100 or 

 200 grains of the solution of chloride of lime prepared as before described are 

 measured, placed in a beaker or mixing jar, and about 60 or 120 grains measure of 

 solution of iodide of potassium containing 1 part iodide in 10 parts of water added, 

 diluted with about 1,000 grains of water, acidulated with hydrochloric acid, and a 

 standard solution of arsenite or hyposulphite of soda rim in from a burette, till only 

 a yellow tint remains. A little starch-paste is now added and the arsenite again 

 added cautiously drop by drop till the blue colour just disappears. When an 

 occasional test only is required, the method subjoined may be followed with advantage, 

 as it obviates the necessity of preparing a standard solution. 



This process is founded upon the following reaction : One atom of chlorine, in pre- 

 sence of free sulphuric acid and water, converts two atoms of protosulphate of iron into 

 one atom of persulphate of iron, with the production of one atom of hydrochloric acid : 



2FeO.SO s + SO 3 + HO + Cl=Fe 2 3 .3SO s + HCl. 

 2FeSO 4 + H-SO 1 + Cl- =Fe 2 (SO') 3 +2HC1. 



For the experiment either pure sulphate of iron, free from peroxide, or the double 

 sulphate of iron and ammonia, or finally a weighed quantity of thin annealed iron 

 wire may be used, dissolved in hydrochloric or preferably in dilute sulphuric acid. 

 Of the crystallised sulphate of iron, as 278 parts correspond to 35'5 parts of chlorine, 

 it is easily calculated that 7'831 grains will be peroxidised by one grain of chlorine. 

 31 '324 grains (7'831 x 4) of sulphate of iron are, therefore, weighed out, dissolved in 

 water, with the addition of a few drops of sulphuric acid, and the volume made up to 

 2,000 grains measure. One fourth of this, or 500 grains, is taken out with a pipette, 

 diluted with 1,500 to 2,000 grains of water, acidulated with hydrochloric acid, and 

 the chloride-of-lime solution, madeas previously described, is run into it from a 1,000- 

 grain burette, with constant agitation, till the whole of the iron is brought into the 

 state of peroxide. To ascertain -when the reaction is finished, a dilute solution of 

 ferricyanide of potassium is used. A drop being placed upon a white plate, a stirring 

 rod, dipped into the mixture of the iron salt and chloride of lime, is brought into con- 

 tact with it. As long as a blue colour is produced it shows that the whole of the 

 protosalt of iron is not yet oxidised, and the addition of the chloride-of-lime solution 

 is, therefore, cautiously continued, until, on mixing the drops, no further shade of 

 blue is perceptible. The reading of the burette is then taken ; and, as the volume 

 used contains one grain of chlorine, the percentage of that element is easily obtained 

 by dividing 10,000 (the number of grains of solution in which 100 graina of chloride 



