When the pH is specified in studying equilibria in a system of biochemical reactions, the criterion for spontaneous change and equilibrium is provided by a transformed Gibbs energy G' that is the thermodynamic potential corresponding to a semigrand ensemble. The semigrand partition function for a system at a specified pH provides a global view of the thermodynamics of the reaction system because all the other thermodynamic properties can be calculated from this function. These operations are illustrated by a system consisting of two species with different numbers of hydrogen atoms, by a system involving two isomeric monoprotic acids, and by a series of metabolic reactions that reduce to a pseudoisomer group when the steady-state concentrations of coenzymes are specified.