Pharmaceutical Chemistry
pKa Suite Calculator
Convert between Kₐ and pKₐ, find how ionized an acid is at a pH, and estimate the pH of a weak acid solution.
Calculation mode
pKₐ
4.74
Weak acid
Inputs
Enter as decimal or scientific notation (1.8e-5).
Example acids (loads Kₐ → pKₐ)
Working
Your values substituted into the formula.
pKₐ = −log₁₀(Kₐ)
−log₁₀(0.000018) = 4.7447 ≈ 4.74
Acid strength guide
pKₐ = −log₁₀(Kₐ)
Where Kₐ is the acid dissociation constant.
Kₐ = 10^(−pKₐ)
Where pKₐ is the negative log of Kₐ.
pH = pKₐ + log([A⁻]/[HA]) → % ionized = ratio / (1 + ratio) × 100
Henderson–Hasselbalch equation for weak acids; [A⁻] is the ionized form, [HA] the unionized form.
pH ≈ ½(pKₐ − log₁₀C)
Approximation for weak acid solution pH, where C is the molar concentration. It assumes only a small fraction of the acid dissociates.
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
Use e-notation: 1.8 × 10⁻⁵ is typed 1.8e-5. A plain decimal (0.000018) works too.
Kₐ → pKₐ fills the pKₐ used by the other modes (rounded to 2 decimals), pKₐ → Kₐ fills the Kₐ, and the weak-acid pH fills the pH for the ionization mode — so you can chain the calculations.
For a weak acid it is the A⁻ fraction at that pH. The unionized HA form crosses lipid membranes, so a weak acid such as aspirin is better absorbed in the acidic stomach, where it is mostly unionized.
Use the pKₐ of the base's conjugate acid (BH⁺), and remember the ionized form of a base is the protonated one. The calculator's A⁻ percentage is then the unionized free base.
The ½(pKₐ − log C) shortcut ignores the water's own H⁺. At very low concentrations it can even predict a pH above 7 for an acid, which is impossible.
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