A conjugate acid is the species formed when a Brønsted base accepts a proton (H⁺). It differs from the original base by one extra proton and can itself donate that proton in the reverse reaction. For example, when ammonia (NH₃) accepts a proton, its conjugate acid is the ammonium ion (NH₄⁺).

共轭酸是布朗斯特碱接受一个质子(H⁺)后形成的物种。它比原来的碱多一个质子,并能在逆反应中给出该质子。例如,氨(NH₃)接受一个质子后,其共轭酸是铵离子(NH₄⁺)。