Buffer Definition in Chemistry and Biology

What Buffers Are and How They Work

Saline drips contain buffers to help maintain blood pH.
Saline drips contain buffers to help maintain blood pH. Glow Wellness / Getty Images

Buffer Definition

A buffer is a solution containing either a weak acid and its salt or a weak base and its salt, which is resistant to changes in pH. In other words, a buffer is an aqueous solution of either a weak acid and its conjugate base or a weak base and its conjugate acid.

Buffers are used to maintain a stable pH in a solution, as they can neutralize small quantities of additional acid of base.

For a given buffer solution, there is a working pH range and a set amount of acid or base that can be neutralized before the pH will change. The amount of acid or base that can be added to a buffer before changing its pH is called its buffer capacity. 

The Henderson-Hasselbalch equation may be used to gauge the approximate pH of a buffer. In order to use the equation, the initial concentration or stoichiometric concentration is entered instead of the equilibrium concentration.

The general form of a buffer chemical reaction is:

HA ⇌ H+ + A

Also Known As: Buffers are also called hydrogen ion buffers or pH buffers.

Examples of Buffers

As stated, buffers are useful over specific pH ranges. For example, here is the pH range of common buffering agents:

BufferpKapH range
citric acid3.13., 4.76, 6.402.1 to 7.4
acetic acid4.83.8 to 5.8
KH2PO47.26.2 to 8.2
borate9.248.25 to 10.25
CHES9.38.3 to 10.3

When a buffer solution is prepared, the pH of the solution is adjusted to get it within the correct effective range. Typically a strong acid, such as hydrochloric acid (HCl) is added to lower the pH of acidic buffers. A strong base, such as sodium hydroxide solution (NaOH), is added to raise the pH of alkaline buffers.

How Buffers Work

In order to understand how a buffer works, consider the example of a buffer solution made by dissolving sodium acetate into acetic acid. Acetic acid is (as you can tell from the name) an acid: CH3COOH, while the sodium acetate dissociates in solution to yield the conjugate base, acetate ions of CH3COO-. The equation for the reaction is:

CH3COOH(aq) + OH-(aq) ⇆ CH3COO-(aq) + H2O(aq)

If a strong acid is added to this solution, the acetate ion neutralizes it:

CH3COO-(aq) + H+(aq) ⇆ CH3COOH(aq)

This shifts the equilibrium of the initial buffer reaction, keeping the pH stable. A strong base, on the other hand, would react with the acetic acid.

Universal Buffers

Most buffers work over a relative narrow pH range. An exception is citric acid because it has three pKa values. When a compound has multiple pKa values, a larger pH range becomes available for a buffer. It's also possible to combine buffers, providing their pKa values are close (differing by 2 or less), and adjusting the pH with strong base or acid to reach the required range. For example, McIvaine's buffer is prepared by combining mixtures of Na2PO4 and citric acid. Depending on the ratio between the compounds, the buffer may be effective from pH 3.0 to 8.0.

A mixture of citric acid, boric acid, monopotassium phosphate, and diethyl barbituic acid can cover the pH range from 2.6 to 12!