Explain the uses of the common ion effect.

(A) The common ion effect is widely used in various chemical processes:
$1$. Purification of $NaCl$: To remove impurities like $Na_{2}SO_{4}$ and $MgSO_{4}$ from a saturated $NaCl$ solution, $HCl$ gas is passed through it. The increase in $[Cl^{-}]$ concentration causes the precipitation of $NaCl$ due to the common ion effect.
$2$. Salting out of soap: Soap is a sodium salt of a fatty acid $(RCOONa)$. When $NaCl$ is added to the soap solution, $Na^{+}$ acts as a common ion. The equilibrium $RCOONa_{(aq)} \rightleftharpoons RCOO^{-}_{(aq)} + Na^{+}_{(aq)}$ shifts to the left, causing the soap to precipitate as a solid.
$3$. Quantitative analysis: To ensure complete precipitation of an ion, an excess of a reagent containing a common ion is added. For example, excess $H_{2}SO_{4}$ is added for $BaSO_{4}$ precipitation, and excess $NaCl$ is used for $AgCl$ precipitation.
$4$. Qualitative analysis:
$(i)$ In Group-$II$ analysis, $HCl$ is added to $H_{2}S$ to suppress the dissociation of $H_{2}S$ via the common ion effect of $H^{+}$, ensuring only $CuS$ and $CdS$ precipitate.
$(ii)$ In Group-$III$ $A$ analysis, $NH_{4}Cl$ is added before $NH_{4}OH$ to suppress the dissociation of $NH_{4}OH$ via the common ion effect of $NH_{4}^{+}$, limiting $[OH^{-}]$ to precipitate only $Fe^{3+}$, $Al^{3+}$, and $Cr^{3+}$.
$5$. Buffer solutions: The common ion effect is essential in maintaining the $pH$ of buffer solutions used in drugs, cosmetics, and fertilizers.