⚗️ Buffer pH Calculator

Calculate buffer solution pH using Ka/pKa and component concentrations.

Enter pKa or Ka

pKa

— or Ka

[HA] Weak Acid (M)

[A⁻] Conjugate Base (M)

—pH

pKa used—

[A⁻]/[HA] ratio—

Buffer range—

How to Use This Calculator

This calculator finds the pH of a buffer solution using the Henderson-Hasselbalch equation. You need to know the pKa (or Ka) of the weak acid and the molar concentrations of both the weak acid and its conjugate base in the buffer. The tool accepts Ka directly if you do not have pKa handy.

Enter the pKa of your weak acid. For acetic acid enter 4.74; for carbonic acid enter 6.35; for dihydrogen phosphate enter 7.21. If you only have Ka, type that value into the Ka field instead.

Enter [HA], the molar concentration of the weak acid in your buffer. For example, if you dissolved 0.1 mol of acetic acid in 1 L, enter 0.1.

Enter [A⁻], the molar concentration of the conjugate base. This is typically the salt you added, such as sodium acetate.

Click Calculate pH. The result shows pH, the [A⁻]/[HA] ratio, and the effective buffer range for your acid.

The Henderson-Hasselbalch Equation

pH = pKa + log₁₀([A⁻] / [HA]) Effective buffer range: pKa ± 1 Maximum capacity when [A⁻] = [HA], giving pH = pKa

pKa is the negative log of the acid dissociation constant. [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration, both in mol/L. When the two concentrations are equal, the log term equals zero and pH equals pKa exactly.

Worked Examples

Acetate buffer: 0.1 M acetic acid, 0.1 M sodium acetate (pKa 4.74)pH = 4.74 + log(1.0) = 4.74

Acetate buffer: 0.05 M acetic acid, 0.15 M sodium acetatepH = 4.74 + log(3.0) = 5.22

Phosphate buffer: 0.1 M H₂PO₄⁻, 0.1 M HPO₄²⁻ (pKa 7.21)pH = 7.21 + log(1.0) = 7.21

Carbonate buffer: 0.2 M H₂CO₃, 0.6 M HCO₃⁻ (pKa 6.35)pH = 6.35 + log(3.0) = 6.83

Where This Calculation Comes Up

Buffer pH calculations appear in virtually every wet biochemistry or analytical chemistry course. In a physiology context, blood is a bicarbonate buffer with pKa 6.1. The blood's normal pH of 7.4 is maintained because the [HCO₃⁻]/[H₂CO₃] ratio is approximately 20:1. A pH drop below 7.35 is acidosis; above 7.45 is alkalosis. Both can be fatal. When you work out the H-H equation for blood, you immediately see why the lungs (which control CO₂ and therefore carbonic acid) play such a central role in acid-base balance.

In the lab, you will prepare buffers to run enzyme assays, run HPLC mobile phases, or keep a cell culture at a specific pH. Picking the right acid for the job means choosing one where your target pH is within 1 unit of the pKa. If you need pH 7.0, phosphate buffer (pKa 7.21) is a better choice than acetate buffer (pKa 4.74), which would have almost no buffering capacity that far from its pKa.

Frequently Asked Questions

What makes a solution a buffer?

A buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid) in comparable concentrations, resisting pH change when small amounts of strong acid or base are added.

What is the ideal ratio for maximum buffer capacity?

When [A⁻]/[HA] = 1 (equal concentrations), pH = pKa and buffer capacity is maximised. Effective range is pKa ± 1.

Can I use Ka or pKa?

Yes! Enter Ka directly (e.g. 1.8e-5) or pKa (e.g. 4.74). The calculator accepts both formats.

How do I prepare a pH 5 acetate buffer?

Acetic acid Ka=1.8×10⁻⁵ (pKa=4.74). Using H-H: 5=4.74+log(r) → r=1.82. Mix sodium acetate and acetic acid in ratio 1.82:1.

What is buffer capacity?

Buffer capacity (β) = the moles of strong acid/base required to change pH by 1 unit. Maximum at pH=pKa.