pH Buffer Calculator
Calculate the pH of a weak acid–conjugate base buffer solution using the Henderson-Hasselbalch equation, with a buffer-specific temperature coefficient (dpKa/dT) and a Davies ionic-strength activity correction. Essential for biochemists, molecular biologists, and analytical chemists preparing precise buffer systems.
Last updated: September 2026
Formula below · 2 sources (pubchem.ncbi.nlm.nih.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Buffer pH is governed by the Henderson-Hasselbalch equation: pH = pKa + log₁₀([A⁻] / [HA]), where [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration. This calculator adds two standard corrections. Temperature: each buffer's pKa drifts by its own coefficient dpKa/dT, so pKa(T) = pKa(25 °C) + dpKa/dT × (T − 25). The coefficient is buffer-specific — about −0.028 per °C for Tris, −0.014 for HEPES, −0.0028 for phosphate and essentially 0 for acetate — so you enter it (leave it blank for 0). Ionic strength: pH meters respond to activity, and the conjugate base A⁻ carries charge −1, so its activity coefficient from the Davies equation, log γ = −0.51 × (√I/(1 + √I) − 0.3·I), lowers the pH. The full formula is: pH = pKa + dpKa/dT·(T − 25) + log₁₀(base / acid) − 0.51 × (√I/(1 + √I) − 0.3·I). The Davies equation is reliable up to about I = 0.5 M. It assumes a neutral acid and an anionic base (acetate, MES, HEPES, phosphate H₂PO₄⁻/HPO₄²⁻ approximately); for a cationic acid such as Tris-H⁺ or NH₄⁺ the ionic-strength shift has the opposite sign. Buffer capacity is maximized when [A⁻] = [HA], i.e., pH = pKa, and useful within pKa ± 1.
How to use
Example 1 — Acetate at 37 °C. pKa (25 °C) = 4.76, 0.1 M acetic acid, 0.1 M sodium acetate, temperature 37 °C, dpKa/dT = 0 (acetate barely changes), ionic strength = Medium (0.1 M). Step 1: log₁₀(0.1 / 0.1) = 0. Step 2: temperature term = 0 × (37 − 25) = 0. Step 3: ionic term = 0.51 × (0.3162/1.3162 − 0.03) = 0.51 × 0.2103 = 0.107. Step 4: pH = 4.76 + 0 + 0 − 0.107 = 4.65. Example 2 — HEPES at 37 °C. pKa (25 °C) = 7.48, equal acid and base at 0.05 M, dpKa/dT = −0.014, temperature 37 °C, ionic strength Low (0.01 M). Temperature term = −0.014 × 12 = −0.168; ionic term = 0.51 × (0.1/1.1 − 0.003) = 0.045. pH = 7.48 − 0.168 − 0.045 = 7.27.
Frequently asked questions
What is the Henderson-Hasselbalch equation and when should I use it?
The Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), is a logarithmic rearrangement of the acid dissociation equilibrium expression. It is valid for weak acid–conjugate base buffer systems where the concentrations of acid and base are both significant and not overwhelmed by autoprotolysis of water. It is the standard tool for designing biological buffers (e.g., phosphate, HEPES, TRIS) and for predicting how pH shifts when small amounts of strong acid or base are added. The equation loses accuracy when concentrations fall below about 1 mM, when pH is extreme (below 3 or above 11), or when significant ionic strength effects are ignored.
How does ionic strength affect the pH of a buffer solution?
Ionic strength (I = ½ Σ cᵢzᵢ²) measures the total concentration of charged ions in solution. High ionic strength lowers the activity of ions relative to their molar concentration. For an acid/anion buffer the conjugate base's activity coefficient falls, so the measured pH is lower than the ideal Henderson-Hasselbalch value. With the Davies equation used here the shift is about −0.045 at I = 0.01 M, −0.107 at 0.1 M and about −0.13 at physiological 0.15 M — far from negligible for precision work in cell-culture media or seawater.
Why does buffer pH change with temperature and how can I correct for it?
The pKa of a weak acid depends on temperature because the dissociation equilibrium has a non-zero enthalpy change (ΔH). The size of the effect is buffer-specific: Tris drops about 0.028 pH units per °C (a buffer made at 25 °C is about 0.34 units lower at 37 °C), HEPES about 0.014, phosphate about 0.0028, while carboxylic acids like acetate hardly change. That is why this calculator asks for dpKa/dT instead of applying one generic correction; look up the coefficient for your buffer from tabulated data (for example the Good's buffer tables).