Basicity is an important parameter with impact on biological systems and technological problems. The HOMO-LUMO and FERMO theoretical approaches can describe the acid-base behavior of compounds as amines, carboxylic acids and alcohols. In this work, a method was developed using the localization degree Gamma(FERMO) parameter based on projection operators to quantify the localization of molecular orbitals. This new method was employed for the analysis of the protonation reaction of 30 organic compounds. The quantitative data from our findings were able to reproduce experimental data, pointing out that the FERMO approach could better describe the acid-base behavior of the investigated compounds.