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The solubility of a salt at a particular temperature can vary with different pH. If the salt contains a specie which has a conjugate weak acid or base, or can react with additional `H^(+)` or `OH^(-)` , the solubility can be affected. Let me show you an example:
`AgCN <=> Ag^(+) (aq) + CN^(-) (aq)`
If you add some acid, the pH of the solution will decrease and proton, `H^(+)` concentration will increase. Since `CN^(-)` is a conjugate of a weak acid `HCN` , the solubility will vary.
`CN^(-) (aq) + H^(+) (aq) <=> HCN (aq)`
Cyanide ion, `CN^(-)` will react with the added `H^(+)` to form `HCN` . Consequently, cyanide ions will be depleted. Now as the cyanide ion is being consumed, `AgCN` will be forced to shift the equilibrium to the right thus making `AgCN` more soluble.
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