Conjugate (acid-base theory) in the context of Brønsted acid

An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO₂OH, are relatively stronger acids. Alcohols, with –OH, can act as acids but they are usually very weak. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: the thiol group –SH, the enol group, and the phenol group. In biological systems, organic compounds containing these groups are generally referred to as organic acids.

The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory, developed independently in 1923 by physical chemists Johannes Nicolaus Brønsted (in Denmark) and Thomas Martin Lowry (in the United Kingdom). The basic concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (the hydrogen cation, or H). This theory generalises the Arrhenius theory.

In chemistry and biochemistry, the pH of weakly acidic chemical solutions can be estimated using the Henderson-Hasselbalch Equation: $${\displaystyle {\ce {pH}}={\ce {p}}K_{{\ce {a}}}+\log _{10}\left({\frac {[{\ce {Base}}]}{[{\ce {Acid}}]}}\right)}$$

The equation relates the pH of the weak acid to the numerical value of the acid dissociation constant, K_a, of the acid, and the ratio of the concentrations of the acid and its conjugate base.