7.12 Common-Ion Effect

Common-Ion Effect

The common-ion effect occurs when a salt is dissolving in a solution that already contains some of the ions that the salt would dissolve into. For example, say you are dissolving NaCl into a 0.1 M solution of Cl-. When considering how many sodium and chlorine ions there will be, you can now no longer consider the ions that are produced when the NaCl dissolves, but also the existing chloride ions. So now, we must reformat the equation to be ${[Na^+]=s}$ and ${[Cl^-]=s+0.1}$, meaning that ${K_{sp}=s(s+0.1)}$. This means that in order to reach the solubility constant, less salt will be dissolved since there are already ions in the solution.