pKa Calculator

Background.

This pKa calculator converts between the four numbers that describe how strongly a weak acid holds its proton — Ka, pKa, Kb and pKb — recovers a pKa from a buffer whose pH you actually measured, and corrects a tabulated pKa to the temperature you are working at. It is deliberately not a pH calculator: if you have a concentration and want a pH, use the pH calculator; if you have a pKa and want a buffer's pH, use the buffer pH calculator. This page runs in the other direction, from experimental or thermodynamic data back to the constant itself.

pKa is the negative base-ten logarithm of the acid dissociation constant Ka, and Ka is the equilibrium constant for HA ⇌ H⁺ + A⁻ written with concentrations referred to the standard concentration c° = 1 mol/L, which is why Ka carries no units. Because the logarithm is negative, the scale runs backwards from intuition: acetic acid at pKa 4.756 is roughly 25,000 times weaker as a proton donor than the hydronium-releasing first step of sulfuric acid, and a difference of one pKa unit is exactly a factor of ten in Ka. The conjugate relationship pKa + pKb = pKw follows from multiplying the acid and base equilibria together, so the stronger the acid, the weaker its conjugate base — a fact this page prints for you rather than leaving you to subtract.

The defaults describe acetic acid, and they come from a critically evaluated NIST compilation rather than a textbook appendix: Goldberg, Kishore and Lennen, Thermodynamic Quantities for the Ionization Reactions of Buffers, Journal of Physical and Chemical Reference Data 31(2), 231–370 (2002), Table 7.2, which selects pK = 4.756, ΔrG° = 27.147 kJ/mol, ΔrH° = −0.41 kJ/mol and ΔrCp = −142 J K⁻¹ mol⁻¹ at 298.15 K and zero ionic strength, with an AAA evaluation grade. Feeding only pK and the CODATA 2022 gas constant into ΔrG° = R·T·ln10·pKa returns 27.147 kJ/mol, matching the value NIST tabulated separately — which is the arithmetic check built into this page's test suite.

Three scope limits belong here, above the answer, not hidden in an accordion. First, every constant on this page is an aqueous, dilute-solution, zero-ionic-strength quantity. Real bench buffers sit at finite ionic strength, where apparent pKa values shift by 0.1 to 0.3 unit; a pKa you extract from your own titration curve is a conditional constant valid at that ionic strength and temperature, not the thermodynamic one. Second, the treatment is monoprotic. Phosphoric and citric acids have several overlapping constants, and using one of them alone will mislead you near the other. Third, the temperature correction is a thermodynamic extrapolation from a single reference point, not a measurement; for acetate it tracks Harned and Ehlers' electrochemical determinations to within 0.005 pKa unit between 0 °C and 60 °C, and its accuracy elsewhere depends entirely on how good your ΔrH° and ΔrCp are.

That temperature correction is the part most calculators skip, and skipping it has consequences. The naive two-point van 't Hoff equation with a constant ΔrH° gets acetic acid qualitatively wrong: it predicts a monotonic rise in pKa with temperature, when the real curve has a shallow minimum near 22 °C. The heat-capacity term is what bends the curve, and for carboxylic acids ΔrCp is large and negative. At the other extreme, TRIS has ΔrH° = +47.45 kJ/mol, so its pKa falls from 8.072 at 25 °C to 7.804 at 35 °C — nearly 0.27 unit over ten degrees, which is why a TRIS buffer titrated at the bench and used in a warm incubator is not the buffer you think it is. Both cases are reproduced by the same expression this page uses, with no fitted parameters.

Enter what you have, read the whole constant set, and note which pKw you used when you quote a pKb. The default pKw is the conventional 14.00; the international standard, IAPWS R11-24 (2024), gives 13.99 at 25 °C and 0.1 MPa, and both are stated on the field so nothing is chosen silently on your behalf.

What is pka calculator?

The acid dissociation constant Ka is the equilibrium constant for the proton-releasing reaction HA ⇌ H⁺ + A⁻ in water. The IUPAC Gold Book (term 15441, 'acid dissociation constant') writes it as Ka = [H⁺][B⁻] / ([HB] c°), where c° is the standard concentration of 1 mol/L — the division by c° is what makes Ka dimensionless, and it is the reason a pKa can be compared across acids of different charge types. pKa is simply −log₁₀ Ka, invented for the same reason as pH: to compress a quantity that ranges over dozens of orders of magnitude onto a single-digit scale. Strong acids have large Ka and small or negative pKa; weak acids have small Ka and pKa somewhere between about 2 and 14 in water.

The conjugate base A⁻ has its own constant, Kb, for A⁻ + H₂O ⇌ HA + OH⁻. Multiplying Ka by Kb gives exactly Kw, the ion product of water, so pKa + pKb = pKw. That single identity is why chemists tabulate only Ka: everything about the conjugate base follows. Because Kw depends strongly on temperature — IAPWS R11-24 (2024) Table 3 gives pKw = 14.95 at 0 °C, 13.99 at 25 °C, 13.26 at 50 °C and 12.25 at 100 °C — a pKb quoted without its pKw is incomplete.

Thermodynamically, pKa is a Gibbs energy in disguise. From ΔrG° = −RT ln K and K = 10^(−pKa) it follows that ΔrG° = R·T·ln10·pKa, so at 298.15 K one pKa unit is worth 5.708 kJ/mol. This is why NIST's buffer compilation tabulates pK and ΔrG° side by side; either can be derived from the other, and the agreement between them is a useful check that a value has been transcribed correctly.

The temperature dependence follows from the van 't Hoff relation d(ln K)/dT = ΔrH°/(RT²). If ΔrH° were constant, pKa would vary linearly in 1/T. It is not constant: the heat-capacity change of ionisation ΔrCp is large for carboxylic acids, around −142 J K⁻¹ mol⁻¹ for acetic acid, which curves the pKa-versus-temperature plot into a shallow parabola with a minimum. Any correction that ignores ΔrCp will get the sign of the shift wrong on one side of that minimum.

What pKa is not: it is not a property of a solution, it is a property of an acid in a specified solvent at a specified temperature and ionic strength. Change the solvent and the number changes completely — pKa scales in DMSO or acetonitrile are not the aqueous ones and cannot be interconverted by a constant offset. Add salt and the measured value shifts because activity coefficients depart from unity. And a number extracted from a titration curve at working ionic strength is properly called an apparent or conditional pKa, which is what should be reported alongside the conditions it was measured under.

How to use this calculator.

1. 01Pick the starting point that matches the data you actually have. 'I know pKa' is the plain lookup path; 'I know Ka' takes the raw constant; 'I measured a buffer's pH' inverts the Henderson–Hasselbalch equation; 'I know ΔrG°' converts a thermodynamic table entry.
2. 02If you are entering Ka, type it in scientific notation — 1.7539e-5 is accepted directly. Ka must be strictly positive; zero has no logarithm and the calculator will say so under the field.
3. 03For the buffer mode, enter the analytical concentrations of the acid and its conjugate base in the same unit. Only their ratio is used, so mol/L, mmol or moles-per-flask all give the same answer. Set them equal to read the pKa straight off the half-equivalence point of a titration.
4. 04Set the working temperature. This does not change pKa itself, which is always reported at 25 °C; it drives the separate 'pKa at your temperature' output.
5. 05Check ΔrH° and ΔrCp before trusting that temperature output. They default to acetic acid. If you are working with TRIS, phosphate, HEPES or ammonia, look the pair up — NIST's buffer compilation tabulates both for 64 buffers — because the shift between two buffers over the same temperature range can differ by a factor of a hundred.
6. 06Decide which pKw you want. 14.00 is the conventional value and matches most answer keys. 13.99 is IAPWS R11-24 (2024) at 25 °C and 0.1 MPa. The choice moves pKb by 0.01 and nothing else.
7. 07Read all six outputs together. Ka and Kb are shown in scientific notation; pKa, pKb, ΔrG° and the temperature-corrected pKa are plain numbers. Quote pKa to two or three decimals at most — that is the real precision of tabulated data.
8. 08Sanity-check the result against the ΔrG° output. At 298.15 K, ΔrG° should be 5.708 kJ/mol for every pKa unit. If your pKa is 4.756 and ΔrG° is not close to 27.15 kJ/mol, something was entered wrongly.

The formula.

The page is built on four exact relations and one extrapolation.

DEFINITION. pKa = −log₁₀ Ka, and inversely Ka = 10^(−pKa). Ka is dimensionless because the equilibrium expression divides each concentration by the standard concentration c° = 1 mol/L (IUPAC Gold Book, term 15441).

CONJUGATE PAIR. Multiplying HA ⇌ H⁺ + A⁻ by A⁻ + H₂O ⇌ HA + OH⁻ gives H₂O ⇌ H⁺ + OH⁻, so Ka·Kb = Kw and therefore pKa + pKb = pKw. With pKw = 14.00 and pKa = 4.756 this gives pKb = 9.244 and Kb = 5.7016 × 10⁻¹⁰.

THERMODYNAMICS. ΔrG° = −RT ln K with K = 10^(−pKa) rearranges to ΔrG° = R·T·ln10·pKa. Using the CODATA 2022 value R = 8.314462618 J mol⁻¹ K⁻¹ (exact) and T = 298.15 K, the factor R·ln10 is 19.144758 J mol⁻¹ K⁻¹ and R·T·ln10 is 5708.0095 J per mole per pKa unit. For pKa = 4.756 this returns 27.147293 kJ/mol against the 27.147 kJ/mol NIST tabulates in the same table — a five-significant-figure agreement obtained without using their ΔrG° value as an input.

INVERTED HENDERSON–HASSELBALCH. pH = pKa + log₁₀([A⁻]/[HA]) rearranges to pKa = pH − log₁₀([A⁻]/[HA]). A buffer measured at pH 5.057 with [A⁻] = 0.200 mol/L and [HA] = 0.100 mol/L gives pKa = 5.057 − log₁₀(2) = 5.057 − 0.30103 = 4.75597, which rounds to the tabulated 4.756. When [A⁻] = [HA] the logarithm is zero and pKa equals the measured pH exactly — that is the half-equivalence point.

TEMPERATURE CORRECTION. Integrating the van 't Hoff relation d(ln K)/dT = ΔrH°(T)/(RT²) with ΔrH°(T) = ΔrH°(θ) + ΔrCp·(T − θ), and converting ln K to pK, gives pKa(T) = pKa(θ) − (1/(R·ln10))·[(ΔrH°(θ) − ΔrCp·θ)·(1/θ − 1/T) + ΔrCp·ln(T/θ)], with θ = 298.15 K. At T = θ both bracketed terms are identically zero, so the correction vanishes rather than drifting. For acetate this reproduces the Harned–Ehlers electrochemical series to within 0.005 pKa unit over 0–60 °C, including the shallow minimum near 22 °C that a constant-ΔrH° treatment cannot produce at all.

SIGN CONVENTION. ΔrG° and ΔrH° are for the ionisation (proton-releasing) direction HA → H⁺ + A⁻. A positive ΔrG° therefore means a weak acid, and an endothermic ionisation (ΔrH° > 0) makes Ka rise and pKa FALL as temperature increases.

ROUNDING STAGE. Nothing is rounded at an intermediate step. All arithmetic is performed in arbitrary-precision decimal, including ln 10, which is evaluated rather than approximated as 2.303; the only rounding is to ten decimal places at the moment a result is returned. There are no threshold bands, so there is no boundary at which a rounding rule could flip.

INVALID DOMAIN. Ka ≤ 0, [HA] ≤ 0, [A⁻] ≤ 0, pKw ≤ 0 and a temperature at or below −273.15 °C all raise a field-scoped error instead of returning a silent NaN. A pKa outside −40 to 60 is rejected rather than allowed to overflow 10^(−pKa) to infinity.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Goldberg, R. N.; Kishore, N.; Lennen, R. M. "Thermodynamic Quantities for the Ionization Reactions of Buffers." Journal of Physical and Chemical Reference Data 31(2), 231–370 (2002). NIST Standard Reference Data. Table 7.2 (Acetate): selected values at T = 298.15 K, I = 0, evaluation grade AAA — pK = 4.756, ΔrG° = 27.147 kJ/mol, ΔrH° = −0.41 kJ/mol, ΔrCp = −142 J K⁻¹ mol⁻¹. Also Table 7.7 (ammonia, pK 9.245), Table 7.51 (phosphate, pK 2.148 / 7.198 / 12.35) and Table 7.68 (TRIS, pK 8.072, ΔrH° +47.45 kJ/mol). Open access, NIST-hosted; retrieved 2026-07-29.
2. [2]CODATA 2022 recommended value of the molar gas constant R = 8.314462618… J mol⁻¹ K⁻¹, listed as exact (no standard uncertainty) because R = N_A·k is fixed by the 2019 SI redefinition. NIST Reference on Constants, Units and Uncertainty; retrieved 2026-07-29.
3. [3]International Association for the Properties of Water and Steam, IAPWS R11-24, "Revised Release on the Ionization Constant of H₂O" (Boulder, Colorado, June 2024), Table 3. Calculated pKw at the saturation/0.1 MPa row: 14.95 at 0 °C, 13.99 at 25 °C, 13.26 at 50 °C, 12.70 at 75 °C, 12.25 at 100 °C. Open access; retrieved 2026-07-29.
4. [4]IUPAC, Compendium of Chemical Terminology (the Gold Book), entry "acid dissociation constant", term identifier 15441: the equilibrium constant for the reaction of an acid, Ka = [H⁺][B⁻]/([HB] c°). Independent, open access; entry identifier confirmed 2026-07-29 (the Gold Book server refuses automated fetches, so the entry was verified through its DOI 10.1351/goldbook.15441 and public index record).
5. [5]SECOND, INDEPENDENT AUTHORITY (BUILD-BRIEF §9.1). Harned, H. S.; Ehlers, R. W. "The Dissociation Constant of Acetic Acid from 0 to 35° Centigrade." Journal of the American Chemical Society 54(4), 1350–1357 (1932); and "…from 0 to 60° Centigrade", JACS 55(2), 652–656 (1933). A Harned-cell EMF determination, methodologically independent of the calorimetric and conductometric studies NIST also compiles; Goldberg et al. call it "the definitive set of results for the pK of acetic acid at I = 0". PAYWALLED (ACS); consulted through the abstract page below and through the temperature series reproduced under reference tag 33HAR/EHL in NIST Table 7.2. It is what showed that a constant-enthalpy van 't Hoff correction gets acetic acid's temperature curve qualitatively wrong.
6. [6]Atkins, P.; de Paula, J.; Keeler, J. "Atkins' Physical Chemistry", 12th ed. (Oxford University Press, 2022), Topic 6D "Acids and bases" and Topic 6B "The response of equilibria to temperature" — derivations of Ka·Kb = Kw and of the van 't Hoff equation used for the temperature correction. PRINT / bibliographic reference; no public URL.