pKb Calculator
Convert between Kb and pKb and relate it to the conjugate acid.
About pKb Calculator
A base is half protonated at pH equal to the pK_a of its conjugate acid. Converting pK_b to that pK_a is usually the quickest route to predicting behaviour at a given pH. Amine drugs are largely protonated at stomach pH and largely free base in blood, which is exactly this relationship in action.
Formula
pKb
pK_b = −log₁₀K_b; pK_a(conjugate) = 14 − pK_b
- protonated fraction
- Share of the base carrying a proton at the given pH
Worked example
With the default values below, pKb Calculator returns pKb of 5.7447. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Base dissociation constantminimum 0
- 0.000002
- Solution pHaccepts -2–16
- 9.5
Results
- pKb
- 5.7447
- pKa of the Conjugate Acid
- 8.2553
- Ratio of Protonated to Free Base at That pH
- 0.05692
- Percentage Protonated at That pH
- 5.386%
- Base Strength
- Weak base
Step by step
- 1Values used
- 2pKb
- 3pKb= 5.7447
- 4pKa of the Conjugate Acid= 8.2553
- 5Ratio of Protonated to Free Base at That pH= 0.05692
- 6Percentage Protonated at That pH= 5.386
- 7Base Strength= Weak base
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is pKb calculated?
pK_b = −log₁₀K_b; pK_a(conjugate) = 14 − pK_b A base is half protonated at pH equal to the pK_a of its conjugate acid. Converting pK_b to that pK_a is usually the quickest route to predicting behaviour at a given pH.
Why does pKb matter?
Amine drugs are largely protonated at stomach pH and largely free base in blood, which is exactly this relationship in action.
What values do I need to enter?
This calculator takes 2 inputs: Base dissociation constant, Solution pH. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.