Use of the Tóth and Unilan Equations for Chromatographic Testing of the Adsorption Properties of Active Carbon

The adsorption and chromatographic properties of active carbon samples obtained by adsorptive/desorptive modification have been studied by inverse gas chromatography to assess their suitability for adsorption of analytes from gas and liquid phases. This paper may help in the choice of an appropriate method for characterizing the surface of an active carbon with regard to particular analytes. Therefore, the specific, V _{g (T )}, and net, V _{S ( T )}, retention volumes were determined from the retention times of the peak maxima and centres of gravity of the peaks. A mathematical link between retention volume data and the monolayer capacity, a _m , was derived by use of a hyperbolic-type equation which enables the values of the terms of the Tóth and Unilan equations to be found. Adsorption second virial coefficients, B _{2 S} , calculated from the values of the terms of the Tóth and Unilan equations were used as a sensitive test of the accuracy of the calculated data. There were no significant differences between the isosteric enthalpies obtained by use of virial coefficients or retention volumes. It was shown experimentally there is substantial parallelism between the magnitudes of the differential isosteric enthalpy and the differential entropy for propane chromatographed on the adsorption systems tested. The cause of non-linear behaviour of plots of ln V _{g ( T )} against T ^?1 for different dynamic adsorption systems is discussed.