Calculadora de Henderson-Hasselbalch
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
pH = pKa + log([A⁻]/[HA])
About this calculator
The Henderson-Hasselbalch Calculator is an online tool that calculates the pH of a buffer solution using the Henderson-Hasselbalch equation. This equation relates pH to the pKa of a weak acid and the ratio of the concentrations of its conjugate base and the weak acid. It is widely used in chemistry and biochemistry to predict the pH of buffered systems, such as biological fluids and laboratory solutions.
How it works: you enter the pKa of the weak acid (or the acid dissociation constant Ka, which is automatically converted) and the concentrations of the conjugate base ([A⁻]) and the weak acid ([HA]). The calculator then applies the formula pH = pKa + log₁₀([A⁻]/[HA]) and displays the resulting pH. The tool can also calculate the [A⁻]/[HA] ratio from a desired pH, helping in the preparation of buffers with a specific pH.
When to use? This calculator is ideal for students and professionals who need to determine the pH of buffers in experiments, adjust the pH of culture media, or understand the acid-base behavior of biological systems. For example, in human blood, the bicarbonate/carbonic acid buffer system maintains pH around 7.4. With the calculator, you can simulate how changes in concentrations affect pH.
Cautions: The Henderson-Hasselbalch equation is valid only for buffer solutions where the concentrations of the acid and conjugate base are significant and dilution is moderate. It assumes that ion activities are approximately equal to concentrations, which may not hold in very concentrated solutions or those with high ionic strength. Also, the equation does not apply to strong acids or bases, nor to systems with multiple equilibria.
Frequently asked questions
What is pKa and how do I obtain it?
pKa is the negative logarithm of the acid dissociation constant (Ka). pKa values for many weak acids are found in tables or literature. If you have Ka, you can calculate pKa = -log(Ka).
Can I use the calculator for strong acids?
No. The Henderson-Hasselbalch equation is valid only for weak acids and their conjugate bases. For strong acids, pH is calculated directly from the acid concentration.
How do I prepare a buffer with a specific pH using this calculator?
Enter the pKa of the acid and the desired pH. The calculator will provide the required [A⁻]/[HA] ratio. Then you can mix appropriate volumes of solutions of the acid and conjugate base to achieve that ratio.
Does the calculator account for temperature?
Not directly. pKa varies with temperature, so make sure to use the correct pKa for your solution's temperature. The calculator assumes constant temperature.
What does it mean if the [A⁻]/[HA] ratio is very high or very low?
If the ratio is very high (above 10), the pH will be much higher than the pKa, and the buffer will have low capacity for acids. If very low (below 0.1), the pH will be much lower than the pKa, with low capacity for bases. Effective buffers have a ratio between 0.1 and 10.