How this instrument works
pKa is defined as the negative base-10 logarithm of Ka, the acid dissociation constant: pKa = -log10(Ka). Ka itself measures how far a weak acid's dissociation equilibrium, HA <=> H+ + A-, sits toward the dissociated products — a larger Ka means a stronger acid that dissociates more completely. Because Ka values for weak acids typically range from about 10^-2 down to 10^-14, comparing them directly means comparing awkward exponents; pKa compresses that same range into ordinary numbers usually between roughly 2 and 14.
The logarithm inverts the usual sense of 'bigger means more': since pKa is a negative log, a smaller Ka produces a larger pKa, and a larger Ka produces a smaller pKa. This means stronger acids have lower pKa values and weaker acids have higher pKa values — a detail worth remembering, since it runs opposite to how Ka itself scales with acid strength.
pKa is also the single number the Henderson-Hasselbalch equation needs to describe a buffer: it's the pH at which a weak acid is exactly half-dissociated, meaning the concentrations of the acid (HA) and its conjugate base (A-) are equal. Chemists select a buffering acid by matching its known, tabulated pKa to the pH they need their buffer to hold, which is why pKa tables for common acids are a standard reference in any general or analytical chemistry course.
- Enter the acid's dissociation constant into Acid dissociation constant Ka — a small positive number, usually looked up from a reference table or a lab measurement.
- Read pKa directly beneath the field — the instrument computes the negative base-10 logarithm of your Ka value automatically.
- Enter Ka in plain decimal or scientific notation (e.g. 0.0000178 or 1.78e-5) — both represent the same value.
- Ka must be greater than zero; the instrument requires a positive value since zero and negative numbers have no defined logarithm.
- Once you have pKa, this site's Henderson-Hasselbalch calculator uses it directly, alongside conjugate base and acid concentrations, to compute a buffer's pH.
Worked example — acetic acid, Ka = 1.78 x 10^-5
Enter 1.78e-5 (or 0.0000178) into Acid dissociation constant Ka — acetic acid's standard textbook dissociation constant, the acid that gives vinegar its sourness. pKa reads 4.74957999769.
That value matches acetic acid's well-known textbook pKa of about 4.75, confirming the conversion. A pKa near 4.75 means acetic acid is roughly half-dissociated at pH 4.75 in solution — a figure that also makes acetic acid/acetate a common choice for laboratory buffers targeting that pH range, since the Henderson-Hasselbalch equation centers a buffer's most stable behavior right around its acid's pKa.
Questions
Why does a smaller pKa mean a stronger acid?
Because pKa is a negative logarithm of Ka, and stronger acids have larger Ka values — so taking -log10 of a larger number produces a smaller (or more negative) result. A strong acid with Ka around 10^2 has a pKa near -2, while a very weak acid with Ka around 10^-10 has a pKa near 10; the logarithm inverts the ordering, which is the detail most likely to trip someone up when first learning pKa.
What does pKa actually represent physically?
It's the pH at which the acid is exactly half-dissociated — where the concentration of the undissociated acid (HA) equals the concentration of its conjugate base (A-). This falls directly out of the Henderson-Hasselbalch equation, pH = pKa + log10([A-]/[HA]): when the two concentrations are equal, their ratio is 1, log10(1) is 0, and pH reduces to exactly pKa.
How is pKa different from pH?
pKa is a fixed, tabulated property of a specific acid — it doesn't change with concentration or how much of the acid has dissociated. pH describes the acidity of a particular solution at a particular moment, and it does change with concentration and with how much acid or base has been added. An acid's pKa stays constant across every solution containing it; the pH of those solutions varies.
Why do chemists pick a buffer acid whose pKa matches the target pH?
Because a buffer resists pH change best when it holds roughly equal reserves of acid (HA) and conjugate base (A-) — and per the Henderson-Hasselbalch equation, that balance occurs exactly when pH equals pKa. Choosing an acid whose pKa sits close to the desired working pH gives the buffer maximum capacity to absorb small additions of acid or base in either direction.
Can pKa be negative?
Yes — very strong acids, with Ka values greater than 1, produce a negative pKa. Hydrochloric acid, for instance, has an estimated pKa around -6 to -7 because it dissociates almost completely in water, making Ka enormous. The formula pKa = -log10(Ka) has no built-in floor, so a sufficiently large Ka simply pushes pKa below zero.
Does this calculator work for polyprotic acids like phosphoric acid?
Yes, one step at a time. A polyprotic acid such as H3PO4 has multiple, distinct Ka values, one for each proton it can lose (Ka1, Ka2, Ka3), and each converts to its own pKa independently using this same formula. Enter each Ka value separately to get that dissociation step's pKa rather than expecting one input to represent the whole acid.