Question

How does ph affects the solubility of a salt???

Answer 1

here are several competing factors that determine the solubility of any substance:

(1) Each solute atom/molecule is competing for water molecules, because the hydration of the various solute species is an important, sometimes the most important, driving force that ultimately determines the solubility of the particular solute. This competition can work both ways -- increasing or decreasing the solubility of a particular solute. This is the mechanism behind the so-called "salting out effect".

(2) If the acid/base has an ion common to the salt -- for example HCl added to NaCl, the common ion effect comes into play so I would guess the addition of aqueous HCl to a saturated solution of NaCl would reduce the concentration of Na(+1) because it will combine with the additional Cl(-1) coming from the acid added.

(3) In cases where there is a "real" chemical reaction. For example, adding Ag(NO3) forming insoluble AgCl, the "apparent" solubility of NaCl increases because the excess solid NaCl will disappear.

(4) For any or all of the above reasons, it depends upon what acid or base is used to change the pH. If the acid or base has greater "affinity" for water than the salt, the acid or base will "steal" water, making the water less "available" to the solute salt; if the acid or base has less "affinity" for water than the salt the reverse will occur. I used the terms above in "quotation marks" because I am not sure of a general, measurable, operationally defined method for measuring those terms.

However, I am pretty sure that there is no general method for predicting the change insolubility of a salt with changes in pH. In fact there is no good quantitative model for the solubility salts, or even most neutral solutes especially in the very complex solvent, water.