Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

108 LAW OF DISTRIBUTION.

follows: First dissolve a sample of about 0.25 g. in a solution

of potassium iodide and titrate with 0.1-normal thiosulphate;

second, expose about 0.25 g. to the air in a moderately warm

place until it has become colorless, and weigh the residue of

ammonium bromide which is left.

On dissolving the tribromide in water, free bromine separates

in considerable quantity and one molecule of Br 2 can be com-

pletely removed by shaking the solution with carbon bisulphide,

or chloroform.

70. Law of Distribution; Proof of the Existence of Potassium
    Tribromide and Potassium Tri-iodide.
    A. Law of Distribution.
    If to two non-miscible solvents standing in contact with one another, a third
    substance is added which is soluble in both, the ratio of the concentration^1
    of this substance in the two solvents after equilibrium has been established,
    i.e., the distribution coefficient, will be constant for a given temperature pro-
    vided the solute has the same molecular weight in both solutions:

If the dissolved substance exists in one of the two solvents in a disso-

ciated or associated condition, the distribution law holds only for the same

sort of molecular aggregate in both solvents. This fact finds expression in

c n2

the formula in such a way that now the ratio -!— remains constant if nt

and n 2 are the number of simple molecules associating to give the prevalent

form of molecules in the respective solutions.

I. Dissolve about 4 g. of succinic acid in 200 c.c. of water,
shake the solution with 50 c.c. of ether in a 500 c.c. separatory
funnel, allow the two liquids to separate, and titrate 10 c.c. from

both layers with 0.1-normal sodium hydroxide, using phenolphtha-

lein as an indicator. To what is left in the separatory funnel add

about 100 c.c. of water and some ether, shake thoroughly again,
and titrate portions of 20 c.c. each from both layers. Again add

50 c.c. of water and half as much ether, and determine the concen-

trations in 20 c.c. portions. Finally add 100 c.c. of water and

this time titrate 40 c.c. portions. The four distribution coefficients
1
Concentration = the amount of substance contained in a unit of volume,
TO