Calculate pKa from Ka or buffer information
The pKa calculator offers two clearly separated questions. Direct mode converts a positive acid dissociation constant with pKa = −log10(Ka). Henderson–Hasselbalch mode rearranges pH = pKa + log10(base/acid) to find pKa from a stated pH and a positive conjugate-base to acid ratio. Choose the mode that matches the quantities actually supplied.
A smaller Ka produces a larger pKa, so pKa is a convenient logarithmic way to compare acid dissociation over many orders of magnitude. In buffer mode, equal compatible amounts of base and acid give a ratio of one and log10(1) = 0, so pKa equals pH. The result shows this substitution rather than hiding the logarithm.
Direct conversion between Ka and pKa
Enter Ka as a positive decimal or scientific-notation value such as 1.8e-5. Zero and negative values are invalid because a real logarithm of those values is undefined. The calculator keeps very small positive values in scientific notation and rejects values that fall outside the supported floating-point range instead of displaying a false zero or infinity.
The reverse check is Ka = 10^(−pKa). Apply it to the displayed pKa and confirm that it reproduces the entered Ka within rounding. A thermodynamic equilibrium constant is dimensionless relative to a standard state. Many classroom problems write a concentration-based numerical Ka; retain the convention and conditions stated by the source when comparing values.
Find pKa with Henderson–Hasselbalch
Buffer mode requires pH and two positive values representing compatible amounts or concentrations of conjugate base and weak acid. The units must match so their division is dimensionless. Moles may be used when both species share the same final volume; concentrations may be used when they refer to the same solution. Mixing millimoles with moles without conversion creates a thousand-fold ratio error.
The rearranged equation is pKa = pH − log10(base/acid). A base-rich ratio above one makes pH higher than pKa, while an acid-rich ratio below one makes pH lower than pKa. That direction is a useful check. The page does not infer whether the entered species truly form a conjugate pair or whether a reaction has reached the assumed buffer state.
Know when the approximation is appropriate
Henderson–Hasselbalch is most useful when both members of a weak-acid conjugate pair are present in meaningful amounts and activity effects are modest. It can mislead for extremely dilute solutions, ratios near exhaustion of one buffer species, strong acids or bases, polyprotic systems without a chosen step, or conditions where ionic strength changes activities substantially.
Temperature and solvent can change dissociation constants. A pKa quoted for water at one temperature should not be assumed identical in another medium or condition. For laboratory preparation, use a validated equilibrium model, calibrated measurement, and the pKa source required by the method. This page checks arithmetic and relationships, not experimental suitability.
Scope, privacy, and result checking
The result is a learning and estimation aid. It does not identify an unknown acid, calculate full speciation, solve charge and mass balances, apply activity coefficients, or replace a measured titration curve. Keep the original temperature, solvent, ionic-strength, and standard-state information with any copied pKa value.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.