Henderson-Hasselbalch Calculator

Determine the exact pH of a buffer solution by utilizing the pKa and the ratio of conjugate base to weak acid concentrations.

pKa of Weak Acid

Concentration of Conjugate Base [A⁻]

Concentration of Weak Acid [HA]

Buffer pH

4.7600

[A⁻]/[HA] Ratio1.0000

Acid Dissociation50.00

Buffer EfficiencyMaximum Buffering Capacity

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The Most Important Equation in Biochemistry

The Henderson-Hasselbalch equation is the absolute foundation of buffer chemistry. It is universally used in laboratories, medical diagnostics, and pharmacology to calculate the pH of a buffer solution based on the ratio of the weak acid to its conjugate base.

A buffer is a special solution that violently resists changes to its pH. Your blood is a highly regulated buffer system (using carbonic acid and bicarbonate) that must strictly remain between a pH of 7.35 and 7.45. If it strays outside this tiny window, proteins denature and death occurs rapidly.

How the Equation Works

The equation beautifully connects the physical properties of the molecule (the pKa) directly to the surrounding environment (the pH).

The Formula

pH = pKa + log₁₀([A⁻] / [HA])

Where:

pH=

Potential of Hydrogen

pKa=

Logarithmic Acid Constant

[A⁻]=

Concentration of Conjugate Base

[HA]=

Concentration of Weak Acid

The Ratio is Everything

The brilliance of the Henderson-Hasselbalch equation lies in the logarithm of the ratio.

If [A⁻] = [HA]: The ratio is 1. The log of 1 is zero. Therefore, pH = pKa. This is the point of maximum buffering capacity.
If [A⁻] > [HA]: You have more base than acid. The log is positive, so the pH > pKa.
If [A⁻] < [HA]: You have more acid than base. The log is negative, so the pH < pKa.

By simply tweaking the amounts of acid and base you mix together, you can dial in a buffer to any exact pH you desire, as long as it is within ±1 unit of the pKa.

Frequently Asked Questions

The equation is an approximation. It assumes that the initial concentrations of the acid and base don't change significantly when they reach equilibrium. It fails spectacularly for relatively strong acids (pKa < 2), incredibly weak acids (pKa > 12), or highly dilute solutions (< 1 mM).

Yes! Because the acid and base are mixed in the exact same physical volume of liquid, the volume units cancel out in the division ratio. You can plug raw moles directly into the equation and get the exact same answer.

You must choose a weak acid whose pKa is within ±1.0 of your target pH. If you need to run a biological assay at pH 4.8, you should use Acetic Acid (pKa 4.76). You would never use Phosphate (pKa 7.2) because it has zero buffering capacity at pH 4.8.

The strong acid will instantly react with the conjugate base [A-], converting it into weak acid [HA]. You must subtract the moles of strong acid from the [A-] value, and add it to the [HA] value, then run the Henderson-Hasselbalch equation again.

If you add pure water to a buffer, you dilute both the acid [HA] and the base [A-] by the exact same amount. Because they are divided against each other in a ratio, the dilution factor cancels out completely. The ratio remains identical, so the pH does not move.

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