On the applicability of the Langmuir equation to estimation of adsorption equilibrium constants on a powdered solid from aqueous solution

The Langmuir adsorption equation has become popular for summarizing solutio n-solid adsorption data and estimating adsorption equilibrium constants. Th e applicability of the Langmuir equation to estimation of adsorption equili brium constants at solid-aqueous solution interfaces was theoretically exam ined in terms of (1) comparisons of solid-solution adsorption with solid-ga s adsorption, (2) deductions of the Langmuir mathematical expression for th e nonelectrolyte/solvent exchange, the phase exchange, the ion exchange, an d the simple association reactions, and (3) the fitting procedure, The rest rictive assumptions underlying the Langmuir equation for solid-gas adsorpti on may not be valid for adsorption of solute of small molecules or ions by a powdered solid surface from the aqueous solution phase. The fit of adsorp tion data to the Langmuir mathematical expression cannot be ascertained sol ely by the linearity of the commonly used equilibrium concentration/amount adsorbed versus the equilibrium concentration plot and does not provide the actual mechanism of the loss of small molecules or ions from aqueous solut ion. Therefore, the parameters of fitting to the Langmuir mathematical expr ession may not provide an unambiguous chemical meaning. In addition, we att empt to bring attention to the importance of the standard states of surface phase and solution phase, (C) 2000 Academic Press.