Buffer Solutions & Henderson-Hasselbalch Cheat Sheet

Key Facts

• A buffer made from a weak acid and its conjugate base follows $\text{pH} = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)$.
• A buffer made from a weak base and its conjugate acid can be analyzed using $\text{pOH} = pK_b + \log\left(\frac{[BH^+]}{[B]}\right)$, then $\text{pH} = 14.00 - \text{pOH}$ at $25^\circ\text{C}$.
• The acid dissociation constant is related to acid strength by $pK_a = -\log(K_a)$.
• When $[A^-] = [HA]$, the ratio is $1$, so $\log(1) = 0$ and $\text{pH} = pK_a$.
• If $[A^-] > [HA]$, then $\text{pH} > pK_a$ because the buffer has more conjugate base than acid.
• If $[A^-] < [HA]$, then $\text{pH} < pK_a$ because the buffer has more weak acid than conjugate base.
• A useful buffer range is approximately $pK_a - 1 < \text{pH} < pK_a + 1$.
• Buffer capacity increases when the total concentration $[HA] + [A^-]$ increases, even if the ratio $\frac{[A^-]}{[HA]}$ stays the same.

Vocabulary

Buffer

A solution that resists major pH changes when small amounts of acid or base are added.

Weak acid

An acid that only partially ionizes in water and has an equilibrium described by $K_a$.

Conjugate base

The particle formed when an acid loses a proton, such as $A^-$ from $HA$.

Henderson-Hasselbalch equation

An equation that calculates buffer pH using $\text{pH} = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)$.

Buffer capacity

The amount of acid or base a buffer can neutralize before its pH changes significantly.

Equivalence point

The point in a titration where stoichiometrically equal amounts of acid and base have reacted.