Potentiometric titration experiments of poly(propylene imine) dendrimers (up to the fifth generation) were carried out at salt concentrations of 0.1, 0.5 and 1.0 M KCl and NaCl. The experiments were performed at two different locations on different instruments and were converted to titration curves using two different methods, resulting in a consistent experimental data set for the dendrimers measured. The titration curves feature two distinct steps around pH 6 and 10 with an intermediate plateau at 2/3 of the total ionizable groups. This protonation pattern reflects short-ranged repulsive interactions between ionizable sites and can be modeled using an Ising model with nearest-neighbour pair interactions. The intermediate plateau results from the stability of an onion-like structure where all odd shells of the dendrimer are protonated, while the even ones remain deprotonated. The Ising model permits a quantitative analysis of the titration curves. For larger dendrimers, this Ising approach is shown to be superior to the classical analysis in terms of successive protonation equilibria.