log10Ka1

Log10 Ka1 refers to the base-10 logarithm of the first acid dissociation constant (Ka1) for polyprotic acids. Ka1 characterizes the equilibrium of the initial deprotonation step, for example in a triprotic acid H3A, where the reaction H3A ⇌ H+ + H2A− has Ka1 = [H+][H2A−]/[H3A]. The value of log10 Ka1 thus conveys the strength of the first deprotonation: larger Ka1 gives a less negative or even positive log10 Ka1, indicating a stronger first dissociation.

Relationship to pKa: pKa1 is defined as −log10 Ka1, so log10 Ka1 equals −pKa1. Consequently, the common pKa scale is the negative of the log10 Ka1 scale; lower pKa1 (stronger first dissociation) corresponds to less negative log10 Ka1.

interpretation and examples: Many common polyprotic acids have negative log10 Ka1 values, reflecting weaker first dissociations. For example, orthophosphoric acid (H3PO4) has pKa1 around 2.1, which corresponds to log10 Ka1 near −2.1. Stronger first dissociations yield higher log10 Ka1 values, potentially approaching or exceeding zero for very strong acids. Weaker acids (higher pKa1) yield more negative log10 Ka1 values. A simple reference point is acetic acid, with Ka ≈ 1.8×10^−5 and log10 Ka ≈ −4.74.

Measurement and use: Ka1 and log10 Ka1 are determined experimentally from acid–base titration data or spectroscopic measurements, typically under standard conditions (aqueous solution at 25°C, activities approximated by concentrations). These values are reported in chemical data tables to compare acid strengths, especially in systems with multiple acidic groups where the first dissociation dominates the early protonation state.