Use the Henderson-Hasselbalch Calculator to solve for pH, pKa, or the [A⁻]/[HA] ratio in any buffer system — just pick What to Calculate, enter your pKa and concentrations (or grab a preset from Select Common Acid), and get your calculated result alongside the buffer range and the full Henderson-Hasselbalch equation.
Calculated Result
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[A⁻]/[HA] Ratio
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Buffer Range (pH ± 1)
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Equation
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First select what you want to calculate (pH, pKa, or ratio). Then input the known values - for pH calculation, enter pKa and concentrations of acid and conjugate base. The calculator will determine the missing value using the Henderson-Hasselbalch equation.
The Henderson-Hasselbalch equation calculates the pH of buffer solutions. It relates pH to the pKa of the weak acid and the ratio of conjugate base to acid concentrations. It's essential in biochemistry, analytical chemistry, and blood gas analysis.
The equation is pH = pKa + log([A⁻]/[HA]). Input the acid dissociation constant (pKa), conjugate base concentration [A⁻], and weak acid concentration [HA]. The calculator computes the logarithm and gives you the buffer pH.
The conjugate base [A⁻] is formed when the weak acid [HA] loses a proton. If you know the pH, pKa, and acid concentration, you can calculate the conjugate base concentration using the rearranged Henderson-Hasselbalch equation.
The Henderson-Hasselbalch equation is most accurate when the [A⁻]/[HA] ratio is between 0.1 and 10, corresponding to a pH range of pKa ± 1. Outside this range, the equation becomes less reliable due to activity coefficient effects.
This calculator focuses on acid buffers. For base buffers, you would use pOH = pKb + log([BH⁺]/[B]), then convert to pH using pH = 14 - pOH at 25°C. The same principles apply but with base dissociation constants.
Buffer solutions require both the weak acid and its conjugate base to resist pH changes. The ratio between these concentrations determines the buffer's pH according to the Henderson-Hasselbalch equation. Equal concentrations give pH = pKa.
When the [A⁻]/[HA] ratio is outside 0.1-10, the solution loses its buffering capacity and the Henderson-Hasselbalch equation becomes less accurate. The pH will be dominated by either the acid (low ratio) or base (high ratio) component.