110 LAW OF DISTRIBUTION.
existing in the pure water, is determined solely by the concentration of
bromine in the carbon bisulphide layer.
Finally, [Br~] = A — (B — D), i.e., the equilibrium concentration of
bromine ions is equal to the difference between the original concentration
of the bromine ions and that of the part which forms the complex. By
inserting these values in the mass-action equation, we obtain:
D[A - (B - D)]
B~^D =K-Mix one volume of bromine with four volumes of carbon bisul-
phide and add 5 c.c. of the mixture to each of six 50 c.c. glass-
stoppered bottles; add about 20 c.c. of water to each of two of
the bottles, and to the other four add equal volumes of potassium
bromide solution of the following normal concentrations:A = 1/1; 1/2; 1/4; 1/8.
Establish equilibrium between the layers by shaking vigorously
for a long time; place the bottles in water at room temperature,
and give them an occasional lateral motion in order to make the
drops of carbon bisulphide solution floating on the surface sink.
After about an hour, when the aqueous layer has become entirely
clear, pipette off 10 c.c. from each bottle, and after adding potas-
sium iodide, titrate with 0.1-normal thiosulphate to disappear-
ance of color. In this way the concentrations B and D are
obtained. Substitute these values, as well as the corresponding
values of A, in the mass-law equation and see whether K remains
constant throughout the experiment. If this is the case, it proves
the existence of the compound KBr 3. It is a good plan to make
the assumption, by way of a check, that more than one molecule
of bromine combines with the potassium bromide in the formation
of the complex, to substitute the values in the corresponding
mass-law equation, and to compare the values of K thus obtained.
C. Proof of the Existence of Potassium Tri-iodide in AqueousSolution. The method of proof is the same here as in the above
case. Place a few grams of finely powdered iodine in each of
seven bottles, and add 250 c.c. of water to the first, 250 c.c. of
TJ 5 normal KI to the second, 250 c.c. of ^ normal KI to the
third, 150 c.c. of ^ normal KI to the fourth, 150 c.c. of ^ normal
KI to the fifth, 75 c.c. of J normal KI to the sixth, and 75 c.c. of
J normal KI to the seventh. The concentration of these solutions
must be accurately known, but the amount taken need only be