Skip to main content

Buffer pH Calculator

Calculate the pH of a buffer solution using the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]).

Common pKa Values Reference

Buffer pH and the Henderson-Hasselbalch equation

The pH of a buffer is set by two things: the pKa of the weak acid, and the ratio of conjugate base to acid in solution. Henderson-Hasselbalch packages both into one line:

pH = pKa + log([A⁻]/[HA])

This is just the Ka expression — Ka = [H⁺][A⁻]/[HA] — solved for pH after taking the negative log of both sides. The calculator takes pKa, [HA], and [A⁻] as inputs and returns pH along with the substituted log term.

The structure of the equation tells you everything about how a buffer behaves. When [A⁻] = [HA], the log is zero and pH equals pKa exactly — this is the center of the buffer’s range and where capacity is highest. As the ratio shifts away from 1, pH drifts above or below pKa, but only logarithmically: a tenfold excess of conjugate base raises pH by exactly one unit, and a hundredfold excess raises it by two. That slow drift is what makes a buffer resist pH change.

The formula assumes the equilibrium concentrations of HA and A⁻ are close to the analytical concentrations you mixed. That holds when both species are present at comparable concentrations and the acid is genuinely weak. Once the ratio gets past 10:1 in either direction, you’re at the edge of the useful range; past 100:1 the approximation has broken down and you need the full equilibrium expression.

Worked examples

Acetate buffer: 0.10 M acetic acid (pKa 4.76) and 0.15 M acetate. pH = 4.76 + log(0.15/0.10) = 4.76 + 0.176 = 4.94

Phosphate at physiological pH: 0.050 M H₂PO₄⁻ (pKa 7.20) and 0.050 M HPO₄²⁻. pH = 7.20 + log(1) = 7.20 — the equimolar case where pH equals pKa.

Ammonia/ammonium: 0.20 M NH₄⁺ (pKa 9.25) and 0.30 M NH₃. pH = 9.25 + log(0.30/0.20) = 9.25 + 0.176 = 9.43

Frequently Asked Questions

What is the Henderson-Hasselbalch equation?
Henderson-Hasselbalch is pH = pKa + log([A-]/[HA]). It comes from rearranging the Ka expression and taking the negative log of both sides. The pKa fixes the center of the buffer's effective range, and the log of the conjugate-base-to-acid ratio shifts pH up or down from that center. When [A-] equals [HA], the log term is zero and pH equals pKa exactly.
What is a buffer solution?
A buffer is a weak acid and its conjugate base (or a weak base and its conjugate acid) sitting in solution at comparable concentrations. Add a bit of strong acid and the conjugate base soaks up the H+; add a bit of strong base and the weak acid donates an H+ to neutralize it. The pH barely moves until one component runs out. Blood at 7.4, intracellular cytosol, and most enzyme assays rely on this kind of equilibrium.
When is the Henderson-Hasselbalch equation most accurate?
The equation assumes the equilibrium concentrations of HA and A- are well-approximated by the analytical (initial) concentrations you put in. That holds when the [A-]/[HA] ratio is between roughly 0.1 and 10 (so pH is within 1 unit of pKa), the buffer is not too dilute, and the acid is genuinely weak. Outside that window, autoionization of water and the actual extent of dissociation start to matter and the simple log expression deviates.
How do you choose a buffer for a target pH?
Pick a weak acid whose pKa is within about 1 unit of the pH you want. Inside that window the [A-]/[HA] ratio sits between 0.1 and 10, which is where the buffer has real capacity in both directions. Acetic acid (pKa 4.76) is the standard choice near pH 5; dihydrogen phosphate (pKa 7.20) covers the physiological range; tris (pKa 8.07) is common for biochemistry just above neutral.
What is buffer capacity?
Buffer capacity is how many moles of strong acid or base the buffer can absorb per unit pH change. It scales with the total concentration of HA plus A-, and it peaks when [HA] equals [A-]. A 1.0 M acetate buffer at pH 4.76 holds the line through much larger additions than a 0.01 M one at the same pH; both have the same starting pH, but the dilute version exhausts almost immediately.