Diverse ion effect

The "salting out effect" is that the solubility of an non-ionic compound (typically an organic molecule) in water decreases when the concentration of salt in the water increases. I don't know the exact thermodynamical reason for this, but I picture this as "more water molecules being used for hydrating the salt ions, so less free water molecules are available for interaction with the non-ionic solute".

The "salting in effect" is the increase of the solubility of an ionic compound with increasing (indifferent*) salt concentration. Thermodynamically, the story goes like this (for the simple case AB(s)-->A⁺(aq)+B^-(aq), but it can be easily generalized):

The solubility equilibrium constant, K, equals (a_A⁺.a_B^- / a_AB,
with a_i the activity of the species i.
The activity of a pure solid AB is 1, per definition.
The activity of the dissolved ions can be written as their activity coefficient γ times their concentration: a_A⁺ = γ_A⁺.c_A⁺.

From the Debye-Hückel theory (which, within its limits, agrees well with reality) follows that the activity coefficient of a dissolved ion decreases with increasing ionic strength of the solution. Increasing the concentration of salt in the solution increases it's ionic strength. Since at a given temperature, the solubility equilibrium constant is, well, constant, a decrease in activity coefficients means an increase in concentration of the dissolved ions (hence, increased solubility of the ionic compound).

So, for example, silver chloride is more soluble in an aqueous solution of ammonium nitrate than it is in pure water.


*Indifferent, in the sense that it doesn't react with the compound's ions, nor shares common ions with it (the well known common ion effect).


There you go. In retrospect, a "like dissolves like"-ish answer would've been simpler (though less correct).

Quote: Originally posted by Sobrero

The "salting out effect" is that the solubility of an non-ionic compound (typically an organic molecule) in water decreases when the concentration of salt in the water increases. I don't know the exact thermodynamical reason for this, but I picture this as "more water molecules being used for hydrating the salt ions, so less free water molecules are available for interaction with the non-ionic solute".

Hi Sobrero, also from Belgium .
Yes indeed it is the water activity of the salt vs the water activity of the compound that explains that fenomenon.
Water solvate the salt in priority and thus is less prone to correctly solvate the compound...this later goes where it is best accepted and where it suffers no competition with the salt...in the organic layer.