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 ONTO PLATINUM LEAD-COATED PIEZOELECTRIC-CRYSTALS/

The effect of coating mass of an electrolytically deposited Pt/Pb allo y onto the Au electrodes of a 15 MHz AT-cut quartz piezoelectric cryst al has been studied to determine if the value of the isosteric heats o f adsorption of benzene, toluene, 2-nitrotoluene and 2,4-dinitrotoluen e 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 adsorp tion 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 t he range <5-72% in the temperature range 279-325 K was found not to in fluence significantly the values of the heats of adsorption. It was de duced that the observed effect of RH on the response of the Pt/Pb-coat ed piezoelectric crystal was due to site occupancy by water. From thes e conclusions and those of earlier work, an algorithm for the correcti ons 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 propag ation analysis on the derived algorithms revealed maximum errors in th e calculated frequency change (Delta f), of 20% for benzene and 27% fo r 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 an d 303 K using tabulated values of molar heats of condensation and the empirically derived data for the heats of adsorption.