Pharmaceutical Chemistry
pKa & pH Calculator
Solve the Henderson–Hasselbalch equation for pH, pKa or the base : acid ratio of a buffer.
What do you want to calculate?
pH
Enter a pKa and a base : acid ratio greater than 0.
Inputs
The acid's pKa — tap a common buffer below to fill it in.
Base form ÷ acid form. 1 means equal amounts.
Common ratios [A⁻] : [HA]
Common biological buffers
Tap a buffer to use its pKa and solve for pH.
| Buffer | pKa | pH range |
|---|---|---|
| 4.76 | 3.8-5.8 | |
| 7.20 | 6.2-8.2 | |
| 7.55 | 6.8-8.2 | |
| 8.06 | 7.0-9.0 | |
| 9.24 | 8.2-10.2 |
pH = pKa + log₁₀([A⁻] / [HA])
[A⁻] is the conjugate base (ionised) form and [HA] the acid (un-ionised) form. Rearranged, pKa = pH − log₁₀(ratio) and [A⁻]/[HA] = 10^(pH − pKa).
% HA = 1 / (1 + ratio) × 100 · % A⁻ = ratio / (1 + ratio) × 100
Relative buffer capacity = 2.303 × α(HA) × α(A⁻) × 100
α is the fraction of each form. The product α(HA)·α(A⁻) is largest when the two forms are equal (pH = pKa), which is why a buffer works best within about one pH unit of its pKa.
Keep learning
References
- Sinko PJ, editor. Martin's Physical Pharmacy and Pharmaceutical Sciences. 8th ed. Philadelphia: Wolters Kluwer; 2023.
- Harris DC, Lucy CA. Quantitative Chemical Analysis. 10th ed. New York: W. H. Freeman (Macmillan Learning); 2019.
Common questions
Base over acid: [A⁻]/[HA]. A ratio above 1 means more of the base form, so the pH is above the pKa; below 1 means more acid form and a pH below the pKa.
No, as long as [HA] and [A⁻] are in the same unit — only their ratio enters the equation. The unit selector changes both fields together.
log₁₀(0) is undefined (it heads to minus infinity). A solution with no base form at all is not a buffer, so the equation does not apply.
It is 2.303 × α(HA) × α(A⁻) × 100, which peaks at 57.6% when pH = pKa. Use it to compare compositions, not as an absolute buffer capacity — that also depends on the total buffer concentration.
For a weak acid drug, % A⁻ is the ionised fraction at that pH. The un-ionised fraction (% HA) is the one that crosses lipid membranes, so it predicts absorption and renal trapping.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy