pKa & Speciation
Enter the pKa values of a mono- or polyprotic acid and a pH. The tool computes the fraction (α) of every protonation state and flags the dominant species.
How to use this tool
See which form of an acid actually exists at a given pH, how much is protonated versus deprotonated. Enter the acid's pKa value(s) and a pH, and the tool gives the fraction of every species and flags the one that dominates.
What to enter
- pKa values: one value for a monoprotic acid, or a comma-separated ascending list for a polyprotic acid (e.g. phosphoric 2.15, 7.20, 12.35). The example chips load common acids.
- pH: the solution pH (0–14) you want the distribution at.
Reading the result
The headline names the dominant species at that pH and its percentage. Below it, a table lists every protonation state with its fraction α (0–1) drawn as a bar, α is simply the proportion of molecules in that form, and the columns sum to 1.
Worked example
A single pKa of 4.76 (acetic acid) at pH 4.76: the protonated (HA) and deprotonated (A⁻) forms are each 50%, the classic pH = pKa crossover.
Acid & pH
Speciation
α is the fraction of molecules in each protonation state (0–1) and the column sums to 1; the dominant species is simply the largest α at your pH. Each pKa marks a 50/50 crossover between neighbouring forms, for a monoprotic acid, pH = pKa splits HA and A⁻ evenly.
Methodology
For an n-protic acid, the fraction of each protonation state is the standard alpha distribution: αk = (termk) ÷ Σ terms, where term0 = 1 for the fully protonated form and termk = (Ka1·…·Kak) ÷ [H⁺]k. Each Ka = 10−pKa and [H⁺] = 10−pH.
At pH = pKa for a monoprotic acid the protonated and deprotonated forms are equal (α = 0.5 each), the expected Henderson-Hasselbalch crossover.
Known limits
- Activity coefficients are ignored (ideal-dilute assumption); values are thermodynamic pKa at 25 °C.
- pKa values must be entered in ascending order for the labels to read correctly.