## Diverse ion effect

aeacfm - 5-6-2011 at 03:11

simply i want to understand the diverse ion effect ? some sites said that it increases the solubility (salting in effect ), others said that it decreases the solubility (salting out effect)
i want to know when salting in happen and when salting out happen ?

Sobrero - 5-6-2011 at 15:21

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<sup>+</sup>(aq)+B<sup>-</sup>(aq), but it can be easily generalized):

The solubility equilibrium constant, K, equals (a<sub>A<sup>+</sup></sub>.a<sub>B<sup>-</sup></sub> / a<sub>AB</sub>,
with a<sub>i</sub> 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<sub>A<sup>+</sup></sub> = γ<sub>A<sup>+</sup></sub>.c<sub>A<sup>+</sup></sub>.

From the Debye-HÃ¼ckel 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).

aeacfm - 5-6-2011 at 23:20