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.
[6] Read before the Royal Society, May 3, 1888.
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.
[7] As one part of chlorine in 17,612 million parts of water had no
visible effect, and one in 17,000 million had a distinct effect, the
influence of the difference, or of one part in 500,000 millions, has been
detected.
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.