Buffer pH Calculator (from Ratio)
Find the pH of a buffer directly from its base-to-acid ratio and the acid’s pKa using the Henderson–Hasselbalch equation.
How it works
pH = pKa + log₁₀( [A⁻] / [HA] )
The Henderson–Hasselbalch equation relates a buffer’s pH to the pKa of its weak acid and the ratio of conjugate base to acid. When the ratio is 1 the pH equals the pKa exactly — this is the point of maximum buffering. Because the term is a base-10 logarithm, a tenfold change in the ratio shifts the pH by only one unit, which is why buffers resist pH change so effectively.
Worked example
An acetate buffer (pKa 4.76) with equal concentrations of acetate and acetic acid has a ratio of 1: pH = 4.76 + log₁₀(1) = 4.76 + 0 = 4.76. Doubling the acetate so the ratio is 2 gives pH = 4.76 + log₁₀(2) = 4.76 + 0.30 = 5.06.
Frequently asked questions
What ratio gives pH equal to the pKa?
A ratio of exactly 1 (equal moles of conjugate base and weak acid). At that point log₁₀(1) = 0, so pH = pKa. This is the centre of the buffer’s useful range.
What is the useful buffering range?
Roughly pKa ± 1, which corresponds to base-to-acid ratios between 1:10 and 10:1. Outside that window one component is nearly exhausted and the buffer capacity drops sharply.
Can I use molar amounts instead of concentrations?
Yes. Because the acid and base share the same solution volume, the volume cancels and you can use the ratio of moles just as well as the ratio of concentrations.
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This calculator is for educational purposes. Double-check important results before acting on them.