Buffer Region

In chemistry, a buffer zone is a region where the pH of a solution remains constant. When a weak acid is titrated against a strong base, the pH of the solution rises, levels off through the buffer zone, and then rises quickly to reach the equivalence point.

There are also buffer zones with weak bases and strong acids. The buffer zone is essential for biological processes that use enzymes because it provides the correct pH for the reaction to occur and protects the enzymes. Enzymes exposed to pH levels outside of their normal range can lose their ability to act as catalysts for their intended biological function. Manufacturers use buffers in the production of alcohol and the colouring of fabric.

Table of Contents

• What is a Buffer Region?
• Relationship between Titration and Buffer Region
• How do Buffers work?
• Examples of Buffers
• Frequently Asked Questions – FAQs

What is a Buffer Region?

A weak acid and its conjugate base are typically present in a buffer solution. When H⁺ is added to a buffer, the conjugate base of the weak acid accepts a proton (H⁺), “absorbing” the H⁺ before the pH of the solution drops significantly. When OH^– is added, the weak acid donates a proton (H⁺) to its conjugate base, resisting any increase in pH before shifting to a new equilibrium point. To maintain an optimal pH, buffers in biological systems prevent pH fluctuations caused by processes that produce acid or base by-products.

Each conjugate acid-base pair has a distinct pH range in which it functions as an effective buffer. The buffering region is approximately 1 pH unit on either side of the conjugate acid’s pKa. The buffering region’s midpoint occurs when one-half of the acid reacts to dissociation, and the proton donor (acid) concentration equals that of the proton acceptor (base).

In other words, the pH of an equimolar acid solution (e.g., when the concentration ratio of acid to the conjugate base is 1:1) equals the pKa. This represents the halfway point in the titration to the equivalence point. This region is the most effective at resisting large pH changes when either acid or base is added.

Relationship Between Titration and Buffer Region

A titration curve graphically depicts buffer capacity. The middle of the curve is flat because the addition of base or acid has little effect on the pH of the solution. This is known as the buffer zone. When a small amount of acid or base is added to the buffer system, the curve will increase dramatically once it exits the buffer region. If too much acid is added to the buffer, or if the concentration is too high, extra protons remain free, and the pH drops dramatically. This effect demonstrates the solution’s buffer capacity.

The acid-base properties of the solution are dominated by the equilibrium for dissociation of the weak acid, corresponding to Ka, in the region of the titration curve at the lower left, before the midpoint. The acid-base properties of the solution are dominated by the equilibrium for the reaction of the conjugate base of the weak acid with water, corresponding to Kb, in the region of the titration curve at the upper right, after the midpoint. However, because Ka and Kb are related by Kw, we can calculate one from the other.

Read More: Acid-Base Titration

How do Buffers Work?

A buffer works by substituting a weak acid or base for strong acid or base.

Consider the action of a buffer made up of the weak base ammonia, NH₃, and its conjugate acid, NH₄⁺.

• When HCl is added to the buffer, the NH₃ “soaks up” the proton from the acid to form NH₄⁺. Since that proton is locked up in the ammonium ion, it does not significantly affect the solution’s pH.
• When NaOH is added to the same buffer, the ammonium ion donates a proton to the base, resulting in the formation of ammonia and water. The buffer also serves to neutralise the base in this case.

Examples of Buffers

• Blood – contains a bicarbonate buffer system
• TRIS buffer
• Phosphate buffer