pKa / pH / Buffer Capacity Explorer

The Henderson-Hasselbalch Equation

This equation relates the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base [A⁻] to weak acid [HA]. When the ratio is 1:1, pH = pKa.

What is pKa?

pKa = -log₁₀(Ka) where Ka is the acid dissociation constant. It measures the strength of a weak acid:

• • Lower pKa = stronger acid (gives up H⁺ more easily)
• • Higher pKa = weaker acid (holds onto H⁺ more tightly)
• • At pH = pKa, exactly half the acid is dissociated

Buffer Capacity (β)

Buffer capacity measures how well a buffer resists pH changes. It's defined as the amount of acid or base needed to change the pH by one unit.

• • Maximum when pH = pKa (equal [A⁻] and [HA])
• • Increases with total buffer concentration
• • Drops rapidly outside the effective range

Effective Buffer Range

Buffers work effectively within one pH unit of the pKa. This corresponds to:

• • [A⁻]/[HA] ratio between 0.1 and 10
• • At least 10% of both components present
• • Outside this range, one component is depleted

Effect of Adding Strong Acid or Base

Common Buffer Systems