pKa Calculator

The pKa Calculator uses the Henderson-Hasselbalch equation to find the acid dissociation constant (pKa) — a measure of how readily an acid releases a proton in solution, widely used in chemistry, biochemistry, and pharmacology. Select your Calculation Type (pKa from pH, pKa from Ka, or pH from pKa), then enter your pH value, conjugate base concentration [A⁻], acid concentration [HA], or Ka value as needed. Results include the pKa value, Ka value, pH, and acid strength classification.

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Results

pKa Value

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Ka Value

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pH Value

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Acid Strength

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Frequently Asked Questions

What is pKa?

pKa is the negative logarithm of the acid dissociation constant (Ka). It measures how strongly an acid holds onto its proton (H+). Lower pKa values indicate stronger acids that readily donate protons.

How do I calculate pKa from pH?

Use the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). Rearrange to get pKa = pH - log([A⁻]/[HA]), where [A⁻] is the conjugate base concentration and [HA] is the acid concentration.

How do I calculate pKa from Ka?

The relationship is simple: pKa = -log₁₀(Ka). Since pKa is the negative logarithm of the acid dissociation constant, you just take the negative base-10 logarithm of your Ka value.

What is the difference between pKa and Ka?

Ka is the acid dissociation constant showing the equilibrium concentration ratio, while pKa is the negative logarithm of Ka. pKa values are easier to work with since they're typically small positive numbers instead of very small decimal fractions.

Are pH and pKa the same?

No, pH measures the acidity of a solution at a given moment, while pKa is an intrinsic property of the acid molecule itself. pH can change with concentration and conditions, but pKa remains constant for a specific acid.

What does a low pKa value mean?

A low pKa value (typically < 2) indicates a strong acid that readily donates protons. The lower the pKa, the more acidic the compound and the more it will be deprotonated at physiological pH.