Question

Can a buffer resist \(\mathrm{pH}\) changes for any added amount of strong acid or base? Explain.

Short answer

A buffer can resist pH changes when small amounts of strong acid or base are added, due to the equilibrium between the weak acid/base and its conjugate species. However, there is a limit to the buffer's capacity to maintain pH stability. When a large amount of strong acid or base is added, the buffer loses its ability to resist further pH changes. Therefore, a buffer cannot resist pH changes for any added amount of strong acid or base.

Step-by-step solution

Understanding Buffers

A buffer is a mixture of weak acid and its conjugate base or a weak base and its conjugate acid. It resists changes in pH when small amounts of strong acid or base are added. This ability comes from the equilibrium between the weak acid (or base) and its conjugate species. For example, let's consider an acidic buffer composed of a weak acid, HA, and its conjugate base, A-: \[HA \rightleftharpoons H^+ + A^-\] When a strong acid (HX) is added to the solution, it donates protons (H+) to the conjugate base (A-): \[HX + A^- → HA + X^-\] Since A- acts as a proton acceptor, the increase in H+ concentration, and thus pH change, is minimized. Similarly, when a strong base (MOH) is added to the solution, the weak acid (HA) donates protons (H+) to the hydroxide ions (OH-), forming water (H2O) and the conjugate base (A-): \[MOH + HA → MA + H_2O\] In this case, the weak acid acts as a proton donor, decreasing the effect of added OH- on pH change.

Buffer Capacity

Buffer capacity refers to the amount of strong acid or base that can be added to a buffer solution without causing a significant pH change. Buffers have maximum capacity when the concentrations of their buffer components (weak acid or base and respective conjugate species) are high and equimolar. However, there is a limit to the buffer's resistance to pH changes. If a very large amount of strong acid or base is added, the equilibrium will be significantly disrupted, and the buffer will lose its ability to resist further pH changes. The exact amount of strong acid or base that can be successfully neutralized by a buffer depends on the ratio of the weak acid/base to its conjugate and the magnitude of their concentrations.

Conclusion

A buffer can resist pH changes when small amounts of strong acid or base are added, thanks to the equilibrium between the weak acid/base and its conjugate species. However, there is a limit to the buffer's capacity to maintain pH stability. When a large amount of strong acid or base is added, the buffer components become overwhelmed, and the buffer loses its ability to resist further pH changes. Therefore, it cannot resist pH changes for any added amount of strong acid or base.

Key concepts

Buffer Capacity

Buffer capacity is the measure of how well a buffer solution can resist changes in its pH upon the addition of acids or bases. Think of it as the buffer's ability to "hold its ground" against pH changes. Buffers are made of either a weak acid and its conjugate base or a weak base and its conjugate acid.
They work to maintain a stable pH environment, but their effectiveness, or capacity, depends on the concentration and the balance of the components involved.
Key points to remember about buffer capacity include:

• High concentration of both components results in a more effective buffer, offering higher capacity.
• An equimolar ratio of the acid and conjugate base, or base and conjugate acid, optimizes buffer capacity.
• If too much acid or base is added, the buffer’s components can become overwhelmed, causing the buffer capacity to be exceeded, and allowing significant pH change.

For example, increasing the concentration of a weak acid and its conjugate base means more protons can be accepted or donated, effectively resisting more pH shifts.

Acid-Base Equilibrium

Acid-base equilibrium in a buffer solution is all about the balance between the weak acid/base and their conjugate partners. This balance is why buffers can resist changes in pH.
In the case of an acidic buffer, which typically includes a weak acid (HA) and its conjugate base (A⁻), an equilibrium is established:\[ HA \rightleftharpoons H^+ + A^- \] When you add a strong acid, extra H⁺ ions flood the system. However, the base component (A⁻) helps absorb these protons, re-forming HA and minimizing pH change.
Similarly, when a strong base is introduced, it provides OH⁻ ions. The weak acid, HA, donates protons to form water, ensuring that the excess OH⁻ doesn't drastically increase the pH.
Here’s what you should know about equilibria in buffers:

• The equilibrium shifts in response to the added acids or bases, but not enough to cause a large pH change.
• This equilibrium shift is reversible, returning to balance after the reaction.
• Maintaining equilibrium is crucial for the buffer’s pH stabilization role.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs are crucial players in buffer solutions. Each pair consists of an acid and a base that differ only by the presence or absence of a proton (H⁺). In buffer systems, these pairs work together to maintain a stable pH.
For example, in the buffer solution made of acetic acid (CH₃COOH) and acetate (CH₃COO⁻), the pair works as follows:

• The acetic acid can donate a proton, acting as an acid.
• The acetate can accept a proton, acting as a base.

This interplay is vital because:

• The acid form can "mop up" any strong base added by donating its proton.
• The base form can "soak up" any strong acid by accepting a proton.
• Together, they create a dynamic system capable of adjusting to small changes in pH, crucial in biological systems and industrial processes.

Thus, understanding how conjugate acid-base pairs interact helps explain why buffers can effectively stabilize pH within specific limits.