EFFECT OF CHLORINE ON THE ELECTRO-MOTIVE FORCE OF A VOLTAIC COUPLE.[6]
Reading edition
EFFECT OF CHLORINE ON THE ELECTRO-MOTIVE FORCE OF A VOLTAIC COUPLE.[6]
By D. G. Gore, F.R.S.
If the electro-motive force of a small voltaic couple of unamalgamated magnesium and platinum and distilled water is balanced through the coil of a moderately sensitive galvanometer of about 100 ohms resistance, by means of that of a small Daniells cell, plus that of a sufficient number of couples of iron and German silver of a suitable thermo-electric pile (see Proc. Birm. Phil. Soc., vol. iv., p. 130), the degree of potential being noted, and sufficiently minute quantities of very dilute chlorine water are then added in succession to the distilled water, the degree of electro-motive force of the couple is not affected until a certain definite proportion of chlorine has been added; the potential then suddenly commences to increase, and continues to do so with each further addition within a certain limit. Instead of making the experiment by adding chlorine water, it may be made by gradually diluting a very weak aqueous solution of chlorine.
The minimum proportion of chlorine necessary to cause this sudden change of electro-motive force is extremely small; in my experiments it has been one part in 17,000 million parts of water;[7] or less than 1⁄7000 part of that required to yield a barely perceptible opacity in ten times the bulk of a solution of sal-ammoniac by means of nitrate of silver. The quantity of liquid required for acting upon the couple is small, and it would be easy to detect the effect of the above proportion or of less than one ten-thousand millionth part of a grain of chlorine in one tenth of a cubic centimeter of distilled water by this process. The same kind of action occurs with other electrolytes, but requires larger proportions of dissolved substance.
As the degree of sensitiveness of the method appears extreme, I add the following remarks: The original solution of washed chlorine in distilled water was prepared in a dark place by the usual method from hydrochloric acid and manganic oxide, and was kept in an opaque, well-stoppered bottle in the dark. The strength of this liquid was found by means of volumetric analysis with a standard solution of argentic nitrate in the usual manner. The accuracy of the silver solution being proved by means of a known weight of pure chloride of sodium. The chlorine liquid contained 2.3 milligrammes or 0.03565 grain of chlorine per cubic centimeter, and was just about three-fourths saturated.
One tenth of a cubic centimeter of this solution ("No. 1") or 0.003565 grain of chlorine was added to 9.9 c. c. of distilled water and mixed. One cubic centimeter of this second liquid ("No. 2"), or 0.0003565 grain of chlorine, was added to 99 c. c. of water and mixed; the resulting liquid ("No. 3") contained 0.000003565 grain of chlorine per cubic centim. To make the solutions ("No. 4") for exciting voltaic couple, successive portions of 1⁄10 or 1⁄20 c. c. of "No. 3" liquid were added to 900 cubic centimeters of distilled water and mixed.
I have employed the foregoing method for examining the states and degrees of combination of dissolved substances in electrolytes, and am also investigating its various relations.