Buffer pH Calculator
Compute the pH of a buffer solution from its pKa and conjugate acid/base concentrations. Essential for biochemistry labs, pharmaceutical formulation, and any experiment requiring a stable pH.
About this calculator
The Henderson-Hasselbalch equation relates the pH of a buffer to the acid dissociation constant and the ratio of base to acid concentrations: pH = pKa + log₁₀([A⁻] / [HA]), where [A⁻] is the molar concentration of the conjugate base and [HA] is the molar concentration of the weak acid. When [A⁻] equals [HA], log₁₀(1) = 0, so pH = pKa exactly — this is the buffer's midpoint. Moving the ratio above 1 raises pH above pKa; moving it below 1 lowers pH below pKa. The equation is valid when concentrations are well above the Ka and when the buffer components are not too dilute. It is the cornerstone of buffer design in biological, environmental, and industrial chemistry.
How to use
Imagine a acetate buffer with a pKa of 4.76, a sodium acetate (base) concentration of 0.1 M, and acetic acid (acid) concentration of 0.05 M. Apply the formula: pH = 4.76 + log₁₀(0.1 / 0.05) = 4.76 + log₁₀(2) = 4.76 + 0.301 = 5.06. Enter 4.76 for pKa, 0.1 for Base Concentration, and 0.05 for Acid Concentration. The result, pH 5.06, confirms the buffer sits slightly above the pKa due to the 2:1 base-to-acid ratio.
Frequently asked questions
What is the Henderson-Hasselbalch equation and when should I use it?
The Henderson-Hasselbalch equation is pH = pKa + log₁₀([A⁻] / [HA]). It applies to buffer solutions containing a weak acid and its conjugate base in meaningful concentrations. It is most accurate when the ratio [A⁻]/[HA] falls between 0.1 and 10 — roughly one pH unit on either side of the pKa. Outside this range the buffering capacity is low and the approximation becomes less reliable.
How does changing the base-to-acid ratio affect buffer pH?
The pH shifts by log₁₀ of the ratio change. Doubling the base concentration while keeping acid constant raises pH by log₁₀(2) ≈ 0.30 units. Halving it lowers pH by the same amount. This logarithmic relationship means large changes in concentration ratio produce modest pH shifts, which is precisely why buffers resist pH change when small amounts of acid or base are added.
Why is the pKa important when choosing a buffer for an experiment?
A buffer works best within ±1 pH unit of its pKa because buffering capacity is highest at the midpoint where [A⁻] = [HA]. Choosing a buffer whose pKa matches your target pH ensures maximum resistance to pH fluctuations. For example, if you need pH 7.4 for a physiological buffer, phosphate (pKa ≈ 7.2) or HEPES (pKa ≈ 7.5) are ideal candidates, while acetate (pKa 4.76) would be a poor choice.