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 solution-solid adsorption data and estimating adsorption equilibrium constants. The applicability of the Langmuir equation to estimation of adsorption equilibrium constants at solid-aqueous solution interfaces was theoretically examined in terms of (1) comparisons of solid-solution adsorption with solid-gas adsorption, (2) deductions of the Langmuir mathematical expression for the nonelectrolyte/solvent exchange, the phase exchange, the ion exchange, and the simple association reactions, and (3) the fitting procedure. The restrictive assumptions underlying the Langmuir equation for solid-gas adsorption 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 adsorption data to the Langmuir mathematical expression cannot be ascertained solely 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 solution. Therefore, the parameters of fitting to the Langmuir mathematical expression may not provide an unambiguous chemical meaning. In addition, we attempt to bring attention to the importance of the standard states of surface phase and solution phase. Copyright 2000 Academic Press.
Zuyi
T
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, People's Republic of China
Taiwei
C