log10Ka

Log10Ka is the base-10 logarithm of the acid dissociation constant Ka, a measure of acid strength in aqueous solution. For an acid HA that dissociates as HA ⇌ H+ + A−, Ka = a(H+) a(A−) / a(HA). Taking the common logarithm (base 10) of Ka yields log10 Ka, which expresses acidity on a logarithmic scale and highlights order-of-magnitude differences between acids. A larger log10 Ka corresponds to a stronger acid because Ka is larger.

Log10 Ka is closely related to pKa, since pKa is defined as −log10 Ka. Therefore log10 Ka = −pKa and, in buffer calculations, pH ≈ pKa + log10([A−]/[HA]) (the Henderson–Hasselbalch equation). This relation means acids with small pKa values have large (less negative) log10 Ka values, while weak acids with larger pKa values have more negative log10 Ka values. For example, an acid with pKa ≈ 4.8 has log10 Ka ≈ −4.8 and Ka ≈ 1.6×10−5.

Calculation and usage: log10 Ka is commonly tabulated for acids in water at a given temperature and is useful for comparing acid strength and for back-calculating Ka from pKa data. It also aids qualitative assessments of proton-transfer equilibria in reaction networks.

Limitations: Ka and log10 Ka depend on temperature, solvent, and activity coefficients; they are meaningful primarily for weak to moderately strong acids in water. For very strong acids (Ka ≫ 1) or non-aqueous systems, pKa and log10 Ka may be less applicable or defined differently.