Determination of thermodynamic parameters from Langmuir isotherm constant-revisited

Abstract

An analytical approach for estimation of the thermodynamic parameters from the Langmuir isotherm constant has been introduced in the present paper. The concept of the thermodynamic equilibrium constant for the Langmuir isotherm based adsorption process was critically analysed. The study was attributed to the fact that the use of the numerical value of the Langmuir isotherm constant (b) in the van't Hoff equation is supported strictly for the solute concentration presented in mol/L in the non-ionic solution or the dilute solution of ionic species. Otherwise, necessary modifications of b must be conducted to adopt b as the thermodynamic equilibrium constant. A critical review of some related studies on thermodynamics of adsorption was performed in this paper, which unequivocally demonstrated the improper estimations of the thermodynamic parameters are in practice. The nature of the reaction, spontaneity, etc. is incorrectly appraised in many literature. The present study attempted to represent correct estimations of the thermodynamic parameters appraised by many published literature by the proposed approach and exhibited significant deviation from the original works. The influence of activity on thermodynamic equilibrium constant for few ionic solutions has also been estimated. It may be ascribed to the fact that the activity of the solute on the thermodynamic equilibrium constant has more influence at the higher concentrations than at the lower concentrations.