pH Buffer Calculator
Calculate the pH of a weak acid–conjugate base buffer solution using the Henderson-Hasselbalch equation, with corrections for temperature and ionic strength. Essential for biochemists, molecular biologists, and analytical chemists preparing precise buffer systems.
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 extends the classic equation with two empirical corrections. First, pKa values shift with temperature — for every degree above 25 °C, pH decreases by approximately 0.02 units (for most biological buffers), captured by the term −0.02 × (T − 25). Second, high ionic strength compresses activity coefficients, slightly lowering effective pH; this is approximated by −0.1 × I, where I is the ionic strength in mol/L. The full formula is: pH = pKa + log₁₀(base / acid) − 0.02 × (T − 25) − 0.1 × I. Buffer capacity is maximized when [A⁻] = [HA], i.e., pH = pKa ± 1 unit.
How to use
Prepare an acetate buffer (pKa = 4.76) with 0.1 M acetic acid and 0.1 M sodium acetate at 37 °C and ionic strength 0.15 M. Step 1: log₁₀(0.1 / 0.1) = log₁₀(1) = 0. Step 2: temperature correction = −0.02 × (37 − 25) = −0.24. Step 3: ionic strength correction = −0.1 × 0.15 = −0.015. Step 4: pH = 4.76 + 0 − 0.24 − 0.015 = 4.505. Enter pKa = 4.76, acid = 0.1, base = 0.1, temperature = 37, ionic strength = 0.15 to confirm this result.
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 decreases the thermodynamic activity of ions relative to their molar concentration, effectively shifting equilibria. For buffers, this means the apparent pKa increases and the measured pH is lower than predicted by the ideal Henderson-Hasselbalch equation. Physiological saline (0.15 M NaCl) contributes an ionic strength of 0.15 M, which causes a pH depression of roughly 0.015 units per the correction used here. In high-salt environments such as cell culture media or seawater, this correction becomes non-negligible for precision work.
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). For most biological buffers, pKa decreases as temperature rises, meaning pH decreases at higher temperatures. TRIS buffer is particularly sensitive, dropping nearly 0.03 pH units per °C — a buffer made at room temperature may be 0.5 units off at 37 °C. This calculator applies a general correction of −0.02 per °C above 25 °C, which is a reasonable average for common biological buffers. For critical applications, always verify the temperature coefficient (dpKa/dT) for your specific buffer from tabulated data.