The way a buffer stabilizes pH during the addition of acid differs from how it stabilizes pH during the addition of base due to the roles of its conjugate acid-base components:

• When acid (H⁺ ions) is added: The buffer's conjugate base component reacts with the added H⁺ ions to neutralize them. This converts the conjugate base into its weak acid form, thereby minimizing the increase in free hydrogen ion concentration and preventing a significant drop in pH. For example, in an acetic acid/acetate buffer, the acetate ion (A⁻) consumes the added H⁺ to form acetic acid (HA)

• When base (OH⁻ ions) is added: The buffer's weak acid component donates protons (H⁺) to react with the added OH⁻ ions, forming water and the conjugate base. This neutralizes the base and prevents a significant rise in pH. Using the same acetic acid/acetate example, acetic acid (HA) donates a proton to OH⁻, producing acetate ion (A⁻) and water

In summary, the buffer stabilizes pH by shifting the equilibrium between its weak acid and conjugate base forms:

Addition of Acid (H⁺)| Addition of Base (OH⁻)
---|---
Conjugate base neutralizes H⁺, forming weak acid| Weak acid donates H⁺ to neutralize OH⁻, forming conjugate base
Prevents large decrease in pH| Prevents large increase in pH

This equilibrium adjustment follows Le Chatelier's principle, allowing the buffer to resist changes in pH by converting one component into the other as needed

. The buffer capacity depends on having sufficient amounts of both components; once one is depleted, the buffer "breaks," and pH changes more drastically with further acid or base addition

. Thus, the key difference lies in which component of the buffer reacts: the conjugate base neutralizes added acid, and the weak acid neutralizes added base, maintaining pH stability through these complementary reactions.