Toward development of a humidity correction algorithm for surface acoustic wave sensors. Part 3. Water, nitrobenzene and 2-nitrotoluene adsorption on platinum/lead coated piezoelectric sensors

Isotherms for water, nitrobenzene and 2-nitrotoluene adsorbed onto a platinum/lead coated 15 MHz AT-cut piezoelectric crystal are reported. Both the nitrobenzene and 2-nitrotoluene isotherms were BDDT type 1 (Langmuir) in shape, as were the water isotherms, and showed predictable behaviour over temperature and relative humidity (RH) ranges 6–47 °C; < 5 to 47% RH for nitrobenzene, and 8–94 °C; < 5 to 81% RH for 2-nitrotoluene. The isotherms were used to obtain values related to the nett heat of adsorption (qS-qL) and the isosteric heat of adsorption (qs) for all three compounds (qL is the molar heat of condensation). The results for water were complicated. For nitrobenzene and 2-nitrotoluene they were simpler yielding a value for nitrobenzene of qs = − 67 kJ mol−1 and for 2-nitrotoluene = − 41 kJ mol−1; it is to be emphasized that the interpretation of qs derived in this way must be cautious. The values were quite constant over all the RH range studied, whereas the values for qS − qL changed. This can be explained if one invokes site blockings as the reason for the reduction in sensitivity of the sensor with increasing RH. This would affect the value of qS − qL calculated, but would affect the value of qs, calculated differently, much less. It is clear that these empirical terms give a good insight into the behaviour of adsorption dependent sensors.