ENTROPY AND ENTHALPY OF ADSORPTION OF GASEOUS METAL CHLORIDES AND OXYCHLORIDES ON QUARTZ GLASS AS FUNCTIONS OF ADSORBATE COMPOSITION

Microamounts of radionuclides have been used for studying adsorption o f chlorides of Cs, Au, Tb, Tm, Pb, Zr, Hf, Sb, Bi, and Nb, as well as of oxychlorides of some of these elements, in a quartz thermochromatog raphic column with SOCl2 as chlorinating component of the carrier gas. Based on the temperature dependence of the bulk-to-surface concentrat ion ratio, enthalpy and enthropy changes in the course of adsorption ( for a certain standard state) have been determined for a number of com pounds. The Delta S-a(o) values fall within a fairly narrow range and correlate with the number of atoms m in the sorbate molecule: Delta S- a(o) [J mol(-1) K-1] = -(184 +/- 3) + (8.5 +/- 0.2)m. Thus, a simplifi ed equation has been derived for estimating Delta H-a(o) based on the adsorption constant measured for only one temperature: Delta H-a(o) [k J mol(-1)] = [(8.5 +/- 0.2)m - (184 +/- 3) - R In K-a(o))]T. Reproduci ble Delta H-a(o) values have been obtained only in the case when the i nert component of the carrier gas was thoroughly predried and the colu mn was pretreated with SOCl2 vapor for a long time, i.e., in the case of the high degree of Cl- modification of the quartz surface. The para meter \Delta H-a(o)\ I usually approximates the enthalpy of vaporizati on of the adsorbate substance.