A system of physical chemistry - Index of

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A system of physical chemistry - Index of

6 A SYSTEM OF PHYSICAL CHEMISTRY

quantity is obviously a constant for a particular gas at constant tempera-

ture, and hence is really included in the proportionality factor or velocity

constant k.

Another illustration of the application of probability considerations

is afforded by the phenomenon of coagulation of colloids by electrolytes

{cf. Whetham, Phil. Mag., [v.], 48, 474 (1899). It is supposed that

the coagulation is due solely to the neutralisation of the electrical

charge of the colloid, and that one trivalent ion is exactly equivalent to

I '5 divalent ions, or to three univalent ions as far as coagulating efficiency

is concerned. This is, of course, too simple an assumption, as we

have already seen in the criticism of Schultze's rule (Vol. II., Chap. VIII. ).

Adopting this simple view of the effect, however, Whetham has shown

how the numerical values obtained experimentally may be accounted

for approximately. The coagulating power of an ion is defined as the

reciprocal of the number of gram-equivalents of the ion which are just

capable of bringing about coagulation of a given quantity of colloid.

Linder and Picton found that negatively charged colloidal arsenic

sulphide could be coagulated by tri-, di-, or uni-valent cations, the

relative coagulating powers being in the ratios—

1023 : 35

Let us suppose that in order to produce coagulation a certain

minimum electric charge has to be brought into contact with the

colloidal particle, and that such conjunctions must occur with a certain

minimum frequency throughout the solution. We shall get equal

charges by the conjunction of 2« trivalent ions, or 3^ divalent ions,

or 6« univalent ions, where n is any whole number. In a solution

where ions are moving freely the probability that an ion is at any instant

at a certain position is represented by a fraction which is proportional

to the ratio between the volume occupied by the sphere of influence of

the ion and the total volume of the solution. The probability is therefore

proportional to the concentration of the ion, and may be written

as (A

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