pKaHB

pKaHB is a term used in chemistry to denote the acidity of a species denoted HB, where HB is a Brønsted acid that donates a proton to form the conjugate base B−. The pKaHB is the negative base-10 logarithm of the acid dissociation constant Ka for the equilibrium HB ⇌ H+ + B−, so pKaHB = -log10 Ka. A smaller pKaHB indicates a stronger acid.

The value of pKaHB depends strongly on the solvent, temperature, and ionic strength. In aqueous solution at room temperature, pKaHB values can span a wide range, with strong acids having negative pKaHB values and weaker acids having pKaHB values well above 10. Many organic acids have pKaHB values between roughly 0 and 25 in water, but when measured in different solvents the numbers shift markedly. Therefore, pKaHB values are always reported with the solvent and temperature.

Measurement of pKaHB is typically achieved by titration or spectroscopic methods. In a titration, the pH at the half-equivalence point equals pKaHB, because at that point [HB] equals [B−]. Other methods include UV-visible, NMR, or infrared techniques that monitor the deprotonation state as a function of pH.

Applications of pKaHB include designing buffer systems, predicting the protonation state and solubility of compounds, and understanding reaction mechanisms in catalysis and biochemistry. Because pKaHB is solvent- and condition-dependent, it serves as a comparative parameter rather than an absolute constant across different environments.