acidconjugate

Acid conjugate, in the context of acid–base chemistry, refers to the concept of conjugate acid–base pairs. In Brønsted–Lowry terms, an acid HA donates a proton to form its conjugate base A−, while a base B accepts a proton to form its conjugate acid BH+. The two species in each pair are interconvertible by proton transfer and are used to describe how acids and bases behave in solution.

Common examples include HA = HCl and its conjugate base Cl−, or CH3COOH and its conjugate base CH3COO−. Similarly, NH3 acts as a base to form the conjugate acid NH4+, and HSO4− is the conjugate base of H2SO4. These pairs help explain how reactions proceed: an acid donates a proton to become its conjugate base, and a base accepts a proton to become its conjugate acid.

The relative strengths of conjugate pairs are tied to pKa values. The stronger an acid HA, the more stable its conjugate base A−, and thus the lower the pKa of HA. In buffers, a weak acid and its conjugate base maintain pH by shifting between HA and A−; the Henderson–Hasselbalch equation, pH = pKa + log([A−]/[HA]), quantifies this balance. In non-aqueous systems, the same idea applies with solvent-specific pKa values.

In biochemistry, many functional groups exist as conjugate acids or bases depending on pH, influencing protein charge, enzyme activity, and metabolite solubility. While the term acidconjugate is not standard, it is often used to reference the broader concept of conjugate acids and bases formed by proton transfer.