Acid-Base Definitions and Conjugate Pairs

The Arrhenius description concerns aqueous solutions: an acid increases hydronium and a base increases hydroxide. Metal hydroxides supply OH⁻ by dissociation, while NH₃ raises [OH⁻] by reacting with water. A narrow historical definition requiring a base to contain OH⁻ misses ammonia, but the operational “increases OH⁻ in water” definition includes it. State which version is being used.

Brønsted–Lowry focuses directly on proton transfer and applies beyond water. In HCl + NH₃ → NH₄⁺ + Cl⁻, HCl donates H⁺ and NH₃ accepts it, even without an aqueous hydroxide source. The Lewis description instead tracks an electron pair used to form a bond, covering reactions such as NH₃ donating its lone pair to BF₃ without transferring a proton.

Because a proton has to go somewhere, an acid and a base always react together. HA + B gives A- + HB⁺. The pair HA/A- differs by exactly one proton, and so does B/HB⁺. Members of a conjugate pair never differ by anything else: not by an oxygen, not by two protons.

Write the before-and-after formula for each candidate. H₂PO₄⁻ losing H⁺ gives HPO₄²⁻, while gaining H⁺ gives H₃PO₄. Removing a positive charge makes the remaining ion one unit more negative. Conjugate pairs are usually found across the arrow; the two starting reactants are the acid and base reacting, not automatically one another's conjugates. A species containing H is not necessarily an appreciable acid in water: the specified proton transfer and equilibrium strength matter.

Strong acids such as HCl, HBr, HI, HNO₃ and HClO₄, plus the first ionization of H₂SO₄, transfer a proton essentially completely in dilute water, so their conjugate bases (Cl⁻, NO₃⁻ and so on) have almost no tendency to take it back and are spectator ions. A weak acid such as CH₃COOH holds its proton well, so its conjugate base CH₃COO⁻ is a meaningful base.

For a conjugate pair HA/A⁻ at one temperature, K_a(HA)K_b(A⁻) = K_w. Increasing K_a forces the partner K_b smaller. A weak acid's conjugate base can therefore be a meaningful weak base; it is not automatically a strong base. For proton transfer HA + B ⇌ A⁻ + BH⁺, K ≈ 10^[pK_a(BH⁺) − pK_a(HA)] in compatible activity conventions. If the product acid has larger pK_a, it is weaker and the transfer tends toward products. This connects the pair labels to a prediction rather than merely a naming exercise.

For H₂SO₄, the first proton transfers essentially completely in dilute water, but its conjugate HSO₄⁻ has a separate weaker second ionization. Do not treat sulfate as its one-proton conjugate or infer that both acid steps are identical.