Some consideration on the Langmuir isotherm equation

The Langmuir isotherm equation has been widely used in adsorption process for decades. This note analyzed application constrain of the Langmuir isotherm equation. In a strict theoretical sense, the concentration of adsorbate used in the Langmuir isotherm equation must be expressed as its molar concentration. However, in the literature of adsorption research, the volumetric concentration of adsorbate has been commonly used in the Langmuir isotherm equation without any theoretical consideration. This would eventually lead to misapplication of the Langmuir isotherm equation in calculation of change in standard free energy (ΔG°) of adsorption. Two examples from the literature were examined, and it was shown that calculation of ΔG° presented in these two references seemed to be incorrect. This note offers deeper insights into the Langmuir isotherm equation, and can help avoid misuse of the Langmuir isotherm equation in adsorption research.