pKa1

pKa1 denotes the first acid dissociation constant of a polyprotic acid. It is the negative logarithm of Ka1, the equilibrium constant for the first deprotonation of a species HnA. For a diprotic acid H2A, the process H2A ⇌ HA− + H+ has Ka1 = [H+][HA−]/[H2A], and pKa1 = -log10(Ka1). At pH equal to pKa1, the concentrations of H2A and HA− are equal, so the first proton is half dissociated. Among the successive dissociations of a polyprotic acid, pKa1 is the lowest and governs behavior in the lower pH region.

Measurement and interpretation of pKa1 commonly rely on potentiometric titration with a strong base, supplemented by spectroscopic or electrochemical methods. Near the first dissociation, the Henderson–Hasselbalch equation relates pH to the ratio [HA−]/[H2A], providing a practical way to estimate pKa1. In solution, pKa1 can be influenced by factors such as ionic strength, temperature, and solvent.

Examples of pKa1 values include phosphoric acid (H3PO4) with pKa1 ≈ 2.15, carbonic acid (H2CO3) with pKa1 ≈ 6.35, and oxalic acid (H2C2O4) with pKa1 ≈ 1.25. In the context of amino acids, pKa1 often refers to the carboxyl group with a value around 2, while other pKa values correspond to the amino group or side-chain groups.

Applications of pKa1 span buffering design, prediction of speciation and solubility, enzyme chemistry, and the analysis of acid-base titrations in chemistry and biochemistry.