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When does pH affect the solubility of a slightly soluble solute, and when does it not? Give some examples.
The solubility of an ionic salt will be affected by pH if either the cation or the anion can undergo significant hydrolysis. This includes most salts.
FePO4(s)Fe3+(aq) + PO43(aq)
Both ions can undergo hydrolysis:
|Fe3+(aq) + 2 H2O(l)FeOH2+(aq) + H3O+(aq)||(1) Ka = 6.3×103|
|PO43(aq) + H2O(l)HPO42(aq) + OH(aq)||(2) Kb = 1.0×1014/4.2×1013 = 2.4×102|
The base hydrolysis is slightly more important under neutral conditions.
At higher pH, where there is an excess of hydroxide ions, reaction (2) will shift towards reactants, thereby tending to reduce solubility. However, an acid/base reaction can occur:
OH(aq) + H3O+(aq) 2 H2O(l)
This will drive reaction (1) towards products, which will help increase solubility. The net change in solubility depends upon the pH and the balance of the various equilibria.
Likewise, at low pH, where there is an excess of hydronium ions, reaction (1) will shift towards reactants, with a tendency for reducing solubility. Again, an acid/base reaction can occur between hydroxide ions and hydronium ions which, at low pH, will drive reaction (2) towards products and increase solubility.