Scientia Sinica

62 SCIENTIA SINICA Vol. V

‘THEORETICAL

Debye and McAulay'! proposed an electrostatic theory to explain the salt effect. When a nonelectrolyte is added to water, its dielectric constant is changed. Their calculation shows that if the nonelectrolyte lowers the dielectric constant of water, the addition of salt will increase the activity coeficient of the nonelectrolyte and therefore decrease its solubility, since S=S /f. Conversely, if the nonelectrolyte raises the dielectric constant of water, the addition of salt will lower the value of f and salting-in results. The equation of Debye and McAulay is

2D bhT? — @) where @ and #4 are defined by the following equations,

DI WDe (ean)

In the above equations, #’ is the number of molecules of salt per cc of solution, v= 2 »; 1s the number of ions each molecule of salt gives upon dissociation,

m is the number of molecules of nonelectrolyte per cc of solution, Dy and D are the dielectric constants of water and solution of nonelectrolyte respectively, 6; and 2; are respectively the radius and valence of ion of ith kind, a is a constant, € is the elementary charge, k is the Boltzmann constant and T is the absolute temperature. For a given salt solution saturated by a nonelectrolyte at a given temperature, all the terms on the right side of Eq. (3) are constant except 2’, and log f is proportional to n’. Thus, Eq. (3) gives the theoretical derivation of Eq. (2). The nature of salt effect is determined by the sign of a If a>0, f>1, ie, salting-out; if e<0, f<1, ie, salting-in.

Later, on the basis of the same electrostatic model, Debye! deduced a more exact formula,

Gal-c6, (4)

where o is a parameter. If the nonelectrolyte lowers the dielectric constant of water, o> 0, f >1, 1¢., salting-out; if o<0, f<1, ie, salting-in.

Attempts to improve Debye’s theories were made by Scatchard"!, Butler!>), Gross"!, Belton'!, Kirkwood!®! and IJaxnapasop!!, but these authors built their theories essentially on the same mechanism as that of Debye, namely, electrostatic interaction.