Toward development of a humidity correction algorithm for surface acoustic wave sensors .4. The effect of coating mass, relative humidity and temperature on the isosteric heat of adsorption of benzene, toluene, 2-nitrotoluene and 2,4-dinitrotoluene o

摘要

The effect of coating mass of an electrolytically deposited Pt/Pb alloy onto the Au electrodes of a 15 MHz AT-cut quartz piezoelectric crystal has been studied to determine if the value of the isosteric heats of adsorption of benzene, toluene, 2-nitrotoluene and 2,4-dinitrotoluene were influenced. At the limit of zero surface coverage, the heats of adsorption were not affected in dry air (relative humidity (RH) <5%) within experimental error. The values obtained for the heats of adsorption extrapolated to zero coverage were 6.7, 9.8, 41.8 and 49.8 kJ mol(-1) for the four gases, respectively. In other experiments, RH over the range <5-72% in the temperature range 279-325 K was found not to influence significantly the values of the heats of adsorption. It was deduced that the observed effect of RH on the response of the Pt/Pb-coated piezoelectric crystal was due to site occupancy by water. From these conclusions and those of earlier work, an algorithm for the corrections of sensor response to a humid challenge gas containing any of the test analytes was derived, based on the fact that both water and those analytes yield Langmuir isotherms on the Pt/Pb surfaces. Error propagation analysis on the derived algorithms revealed maximum errors in the calculated frequency change (Delta f), of 20% for benzene and 27% for toluene and these were borne out in reconstruction of the isotherms over the range <5-72% RH at 280 K for benzene and 281 K for toluene and 303 K using tabulated values of molar heats of condensation and the empirically derived data for the heats of adsorption.