pKa Calculator for Acid Dissociation and Buffers

The acid dissociation constant Ka describes the equilibrium for an acid donating a proton in water. Larger Ka values indicate that dissociation is more favorable and the acid is stronger. Because equilibrium constants can span many orders of magnitude, chemists commonly report pKa, defined as the negative base-10 logarithm of Ka. A lower pKa therefore corresponds to a stronger acid, while each one-unit change in pKa represents a tenfold change in Ka. This calculator converts in both directions. To find pKa from Ka, it evaluates the negative logarithm. To recover Ka from pKa, it raises 10 to the negative pKa. Ka must be positive because logarithms of zero or negative values are not defined in this chemical context. pKa itself may be negative for very strong acids, so the conversion mode accepts any finite pKa value. The buffer modes use the Henderson-Hasselbalch equation: pH equals pKa plus the base-10 logarithm of the conjugate-base concentration divided by the weak-acid concentration. The ratio is dimensionless when both concentrations use the same units. When base and acid concentrations are equal, the ratio is one, its logarithm is zero, and pH equals pKa. A tenfold excess of conjugate base raises pH by one unit; a tenfold excess of acid lowers it by one unit. Henderson-Hasselbalch is most useful for mixtures containing appreciable amounts of a weak acid and its conjugate base. It assumes activities can be approximated by concentrations and that the equilibrium model is appropriate. Accuracy can decrease in very dilute solutions, at high ionic strength, near complete neutralization, or when additional equilibria are important. Polyprotic acids have multiple dissociation steps and pKa values, so use the pair relevant to the buffer region being studied. A calculated pH is not a substitute for measurement when temperature, ionic strength, activity coefficients, or complex sample matrices matter. Ka and pKa values depend on solvent and temperature, and tabulated values should match the experimental conditions. Buffer capacity also depends on total acid and base concentration, not only their ratio. Two buffers with the same calculated pH can resist added acid or base very differently. Use this tool for transparent equilibrium arithmetic, then consider concentration, capacity, and experimental calibration when designing a real buffer.