This article discusses the charging behavior of particles in a colloidal suspension allowing for a simple model reaction at the surface of the colloidal particles that regulates the total number Z(f) of free counterions. This number is to be distinguished from an effective number of charges Z* calculated in the Wigner-Seitz cell model to determine the interaction between the colloidal particles. We discuss the problem in terms of the full free energy of the system. The number of free counterions Z(f) as a function of the structural charge of the colloid is derived from the minimum of this free energy. Our results are compared with experimental data.