pKa Calculator

How to use this pKa calculator

What this calculator does

This calculator estimates an acid's pKa in either of two ways. The first method rearranges the Henderson – Hasselbalch equation using a measured pH, the conjugate base concentration, and the weak acid concentration. The second method converts an acid dissociation constant, Ka, with pKa = – log₁₀(Ka). The result is a logarithmic acidity-strength descriptor, not a complete prediction of solution behavior, reaction rate, buffer capacity, or biological effect. For dilute solutions, high ionic strength, mixed solvents, or temperatures far from the conditions used to measure Ka, activity-based equilibrium calculations may be more appropriate. The Henderson – Hasselbalch equation reference explains both the relationship and its approximation limits.

When to use it

Use the calculator to check a buffer-preparation calculation, infer pKa from experimental pH and composition data, convert a published Ka value into the more readable pKa scale, or compare the relative acidity of compounds measured under similar conditions. It is especially useful when the acid and conjugate base coexist in appreciable amounts and their concentrations are expressed on the same basis.

How to calculate

1. The calculator opens with a complete demonstration: pH 4.8, conjugate base 0.01 mol/L, and weak acid 0.10 mol/L. The result is pKa 5.800, and a validated example workbook is immediately available.
2. Choose pKa from pH to use composition data, or choose pKa from Ka when you already know the dissociation constant.
3. Replace the sample values. Results update live after each valid change. Read the primary pKa, then use the secondary values and breakdown table to check the logarithmic step.
4. Select Download Excel to export the current typed inputs, outputs, and calculation steps. Select Reset to clear the demonstration and all calculated content; export remains unavailable until a complete valid state is entered again.

Input guide

Calculation method is required and selects one of the two formulas. pH is required in the pH method, accepts an ordinary decimal from 0 to 14, and is dimensionless; 4.8 is a realistic example. Raising pH while keeping the ratio fixed raises calculated pKa by the same amount. Do not enter a concentration in this field. Conjugate base concentration is required, must be positive, and is entered in mol/L; 0.01 is the startup example. A larger base concentration increases the base-to-acid ratio and lowers the inferred pKa for a fixed pH. Weak acid concentration is also required and positive; 0.10 mol/L is the startup example. Increasing it decreases the ratio and raises inferred pKa. The two concentrations may use another common concentration unit, but they must use the same unit because the ratio must be dimensionless. Acid dissociation constant (Ka) is required only in the Ka method, must be greater than zero, and accepts decimals or scientific notation such as 1.8e-5. Larger Ka values produce smaller pKa values. Do not enter “10^-5” as text; use 1e-5 or a decimal.

Output guide

Calculated pKa is the primary logarithmic result. Lower values usually correspond to stronger acids, while higher values correspond to weaker acids under comparable conditions. Equivalent Ka is 10 raised to the negative pKa and provides the inverse conversion. Base-to-acid ratio is [A⁻]/[HA]; a value of 1 means equal concentrations. log₁₀ ratio is the logarithmic adjustment used by the Henderson – Hasselbalch equation. Relationship states whether pH is below, equal to, or above pKa. The Calculation breakdown table lists the ratio, logarithm, and final substitution for the current method. These are exact mathematical identities for the supplied values, while their chemical applicability depends on the equilibrium assumptions.

Worked example

With pH = 4.8, [A⁻] = 0.01 mol/L, and [HA] = 0.10 mol/L, the ratio is 0.01/0.10 = 0.100. Its base-10 logarithm is – 1.000. Rearranging pH = pKa + log₁₀([A⁻]/[HA]) gives pKa = 4.8 – ( – 1.000) = 5.800. The equivalent Ka is 10⁻⁵·⁸, or approximately 1.585 × 10⁻⁶. Because the conjugate base concentration is lower than the weak acid concentration, pH is below pKa.