Toward development of a humidity correction algorithm for surface acoustic wave sensors Part 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 onto platinum/lead-coated piezoelectric crystals

Abstract

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 ( Δƒ′) 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.