How a Buffer Works The Common-Ion Effect

Buffers work through a phenomenon known as the common-ion effect. An example of this effect occurs when acetic acid dissociates in water and some sodium acetate is added. As you know, acetic acid dissociates only slightly in water  

From Le Chatelier s principle (Section 17.6), we know that if some CH3COO ion is added (from the soluble sodium acetate), the equilibrium position shifts to the left thus, [H30 ] decreases, in effect lowering the extent of acid dissociation  

Similarly, if we dissolve acetic acid in a sodium acetate solution, acetate ion and H30 ion from the acid enter the solution. The acetate ion already present in the solution acts to suppress as much acid from dissociating, which lowers [H3O ]. Thus, the effect again is to lower the acid dissociation. Acetate ion is called the common ion in this case because it is common to both the acetic acid and sodium acetate solutions that is, acetate ion from the acid enters a solution in which it is already present. The common-ion effect occurs when a given ion is added to an equilibrium mixture that already contains that ion, and the position of equilibrium shifts away from forming more of it. 

The Effect of Added Acetate Ion on the Dissociation of Acetic Acid 

Consider what happens to a solution containing high [CH3COOH] and high [CH3COO ] when we add small amounts of strong acid or base. The expression for HA dissociation at equilibrium is 

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