Comparison Of Different Adsorption Enthalpies Calculated From Chromatographic Data
COMPARISON OF DIFFERENT ADSORPTION ENTHALPIES CALCULATED FROM CHROMATOGRAPHIC DATA
H. Grajek
Military Technical Academy, Institute of Chemistry,
Kaliski str. 2, 00-908 Warsaw, Poland
Inverse gas chromatography is widely used for investigation of adsorbents and catalysers. The enthalpies and entropies of adsorption, the isotherms and isosters of adsorption, and the parameters of adsorbents characterizing the structural and adsorption properties of adsorbents one can calculate. There are only very few papers which describe the enthalpies and entropies of adsorption on the basis of the adsorption second virial coefficient, B01S, reffered to zero coverage of the adsorbent surface with adsorbate:
where: m the amount of adsorbate injected on column.
The B01S values calculated in the proposed manner are linked to the same partition isotherm and the same adsorption isotherm for the chromatographed adsorbates at the adsorbent interfaces.
The Tóth and UNILAN equations are applied to describe the chromatographically determined adsorption isotherms of propane. The agreement of Tóth and UNILAN equations for the the isosteric heat of adsorption at the limit of zero surface coverage, q0st is a sensitive test of the accuracy of the second virial coefficients. If there was a large discrepency between the two values, or if one of them was negative, the values had given for the heat of adsorption and the adsorption second virial coefficients should have been disregarded.
The adsorption second virial coefficients were found empirically to vary linearly with reciprocal column temperature, Tc . The coordinates of the graph were [ln(B01S/Tc)] versus [(103)/Tc]. The lines for Tóth and UNILAN equations were obtained by the least squares regression. The limiting value of the isosteric heat at zero coverage, q0st, was directly proportional to the slope of the line.
It is worth to note, that the Tóth equation always gives a larger value of B01S than the UNILAN one.
Acknowledgments
This work was partly supported by the Polish State Committee for Scientific Research, Grant No. 3 T09B 036 16.
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