pKa Calculator

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Determine acid strength by calculating the pKa from the dissociation constant (Ka).

Dissociation Parameters

Acidity Constant (Ka)

Speciation Curve (Fraction vs pH)

At pH = pKa (4.74), both forms are present at exactly 50%.

Acidity Results

pKa Value4.74

Strength Analysis:Weak Acid

About this calculator

Overview

The pKa value is the negative base-10 logarithm of the acid dissociation constant (Ka). It is the standard way to express acid strength: a lower pKa indicates a stronger acid that dissociates more readily in water. Crucially, when pH equals pKa, the acid exists in a 50:50 equilibrium between its protonated (HA) and deprotonated (A⁻) forms.

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Pro Tips

Negative pKa: The acid is 'strong' (e.g., HCl is ~ -6). These dissociate almost 100% in water.
pKa between 2 and 7: Weak organic acids, common in biochemistry (like Acetic acid, 4.76).
pKa > 7: Very weak acids, often basic in their salt form.
Buffer Region: The most effective buffering occurs when pH is within ±1 unit of the pKa.
Ka values are often very small, so scientists invented the pKa scale to make numbers manageable.

Fun Facts

"The strongest known superacid, Fluoroantimonic acid, has a pKa estimated at -25! It is trillions of times stronger than 100% sulfuric acid."
"Aspirin has a pKa of 3.5. This allows it to be absorbed in the acidic environment of the stomach (pH 1-2) where it remains unionized."
"The term 'p' in pH and pKa stands for 'potenz' (power in German) or 'puissance' (power in French)."
"Human blood is buffered by the bicarbonate system with a pKa of 6.1—kept at pH 7.4 through complex physiological regulation."
"Glycine, the simplest amino acid, has two pKa values (2.3 and 9.6), making it a 'zwitterion' at neutral pH."

Formula: pKa = -log₁₀(Ka)