The central objective of this work is to revisit the development of the widely used Lewis acidity/basicity scales and seek possibilities for their eventual unification. The primary focus is on hydrogen bonding interactions which are dealt with the estimation of hydrogen-bond-donor (HBD) and hydrogen-bond acceptor (HBA) capacity of the compounds. The developments are made by having in mind their implementation in a thermodynamic equation-of-state framework for the reliable estimation of all basic thermodynamic quantities of compounds over a broad range of external conditions. Gutmann's donor and acceptor numbers, the Hard-Soft Acid and Base principle, Drago's electrostatic and covalent interaction parameters E and C, the LFER/LSER acidity and basicity, alpha and beta (A and B), parameters, the Hansen partial hydrogen - bonding solubility parameters and Klamt's quantum-chemically derived hydrogen bonding cosmoments are all combined with the recent partial solvation parameter (PSP) approach in an effort to develop a unified hydrogen bonding scale. The challenges and the perspectives of this unification are critically discussed. (C) 2017 Elsevier Ltd.