pKa Calculator - Acid Dissociation Constant (Ka)

pKa is a number chemists use to measure how strong an acid is in water. It equals the negative base-10 logarithm of the acid dissociation constant, Ka, and shows how easily a molecule gives up a hydrogen ion (H⁺). Lower pKa values mean stronger acids; higher values mean weaker ones. This calculator looks up the pKa value for a chemical formula, such as CH3COOH or HCl, in a reference table of common acids and bases.

What is pKa?

An acid is a substance that can give up a hydrogen ion, H⁺, when dissolved in water. Some acids give up that ion easily. Others hold onto it tightly. pKa is the number that tells the difference.

A low pKa, including negative numbers, means the acid gives up its proton easily and is a strong acid. A high pKa means the acid holds onto its proton and is a weak acid.

pKa formula

When an acid (HA) dissolves in water, it reacts in a reversible reaction:

HA + H₂O ⇌ A⁻ + H₃O⁺

HA is the acid. A⁻ is its conjugate base, the part left over after the acid loses a proton. H₃O⁺ is the hydronium ion, a water molecule that has picked up the released proton.

At equilibrium, the concentrations of these three parts set the acid dissociation constant, Ka:

Ka = [A⁻][H₃O⁺] / [HA]

pKa is the negative base-10 logarithm of Ka:

pKa = −log₁₀(Ka)

Chemists use pKa instead of Ka because Ka values can be extremely large or extremely small, spanning many powers of ten. Taking the logarithm turns those numbers into a compact scale that is easier to read and compare, the same reason the pH scale is used instead of raw hydrogen-ion concentration. A larger Ka means more of the acid splits apart into ions, so a larger Ka always corresponds to a smaller pKa. That is why a lower pKa means a stronger acid.

How to calculate pKa

The database behind this calculator stores pKa values measured directly in experiments. But pKa can also be worked out from a solution's pH if the concentrations of the acid and its conjugate base are known, by rearranging the Henderson-Hasselbalch equation:

pKa = pH − log₁₀([A⁻] / [HA])

This method is most accurate when the solution's pH is close to the pKa itself, since that is where small measurement errors have the least effect on the result.

Worked example

Acetic acid (CH3COOH), the acid in vinegar, has a pKa of 4.76.

To find its Ka, reverse the formula:

Ka = 10^(−pKa) = 10^(−4.76) ≈ 1.7 × 10⁻⁵

To find the pH of a solution where the acetate ion (A⁻) is 1.7 times as concentrated as the undissociated acid (HA), use the Henderson-Hasselbalch equation the other way:

pH = pKa + log₁₀([A⁻] / [HA]) = 4.76 + log₁₀(1.7) ≈ 5.0

pKa and acid strength

pKa values usually range from about −10, for the strongest acids, to 50, for the weakest. Most acids handled in a school or research lab fall between −10 and 15. The table below lists example values.

A useful rule: an acid's pKa equals the pH at which exactly half of its molecules have released their proton. That point matters for buffer solutions, described below.

Why pKa matters

A buffer solution resists changes in pH when small amounts of acid or base are added. A buffer works best when its pKa sits within about one pH unit of the target pH, since that is where the mix of acid and conjugate base can soak up the most added acid or base. A phosphate buffer near pH 7 and an acetate buffer near pH 4.76 both rely on this rule.

In the body, pKa affects how drugs cross cell membranes. Only the non-ionized form of a molecule, the form without an electric charge, passes easily through the fatty membrane of a cell. Aspirin, a weak acid, stays mostly non-ionized in the acidic stomach (pH 1 to 2) and is absorbed there. Once it reaches the near-neutral bloodstream, more of it turns into its ionized form.

pKa also shapes how proteins fold and how enzymes work. Amino acid side chains gain or lose protons depending on the surrounding pH, which changes their electric charge and behavior.

Some acids have more than one pKa, because they can lose more than one proton. Phosphoric acid (H3PO4) can lose three, one at a time. Each successive pKa is higher than the last, because it takes more energy to pull a proton off a molecule that is already negatively charged. This calculator reports the first, lowest pKa for such acids.

Frequently asked questions

What is the difference between pKa and pH?

pKa is a fixed property of a specific acid. pH describes the acidity of an actual solution and changes whenever acid or base is added to it. Acetic acid always has a pKa of 4.76, but a solution containing acetic acid can have almost any pH, depending on how much is dissolved and what else is mixed in.

Can pKa be negative or higher than 14?

Yes. The pH scale in water runs from roughly 0 to 14, but pKa has no such limit. Strong acids such as hydrochloric acid (pKa −6.3) have negative values, and very weak acids such as ethane (pKa 50) have very high values.

Why do some acids have more than one pKa value?

An acid with more than one hydrogen ion to give up loses them one at a time, and each step has its own equilibrium and its own pKa. Phosphoric acid has three pKa values because it can lose three protons in sequence, each one harder to remove than the last.

How does temperature affect pKa?

Most pKa values shift slightly with temperature, because acid dissociation is usually an endothermic reaction, meaning it absorbs heat. Warming a solution favors more dissociation, which raises Ka and lowers pKa a small amount. Reference tables, including the one behind this calculator, normally quote a single pKa value for standard conditions.

Why isn't a compound showing up in the calculator?

The calculator matches formulas against a fixed list of common acids and bases. It expects plain text notation, such as H2SO4 rather than H₂SO₄, with the elements in the standard order. If a formula is typed correctly but still returns no result, the compound is likely outside this reference list. A chemistry database such as PubChem may list a value for it instead.

How is pKa used to pick a buffer?

A buffer works best within about one pH unit of its acid's pKa. To hold a solution near pH 7, for example, a chemist would choose an acid with a pKa close to 7, since that is where the acid and its conjugate base are present in similar amounts and can neutralize the most added acid or base.