Ising models, which have received much attention in statistical mechanics, are used to predict acidity constants (pK values) of polyprotic acids or bases. The present model circumvents the calculation of microscopic pK values, which is necessary in the classical approach, and leads directly to all experimentally observable macroscopic pK values. The Ising model allows a transparent parametrization of the problem where the basic parameters are the microscopic pK values of the acid-base groups (given all other groups in the molecule are deprotonated) and interaction energies between pairs and triplets of protonated sites. The classical description is recovered if only pair interactions are considered while the model can be systematically improved by including triplet (and higher order) interactions. As an illustration, the model is applied to linear aliphatic amines and polyphosphoric acids.