Theory of the surface acoustic wave/impedance sensor system and its application to the end-point determination in titrimetry

Theory and characteristics of the surface acoustic wave (SAW)/impedance sensor system in electrolyte solution are reported. This kind of sensor was constructed by a 62-MHz SAW resonator and a pair of conductive electrodes in series. By using the circuit network analysis theory, two oscillation equations were derived and used to explain the frequency-shift response to solution conductance. Results showed that the sensor possesses high sensitivity to the solution conductivity and can be applied to conductivity measurements in solution. Its sensitivity and accuracy could be improved in the presence of a large amount of unreacting foreign electrolytes. The sensor was successfully applied to four types of titration (acid-base, precipitation, oxidation-reduction and compleximetric titration) by monitoring the frequency-shift response. Factors influencing the frequencimetric titration curve were also investigated. Analytes at levels down to 1 × 10⁻⁵ M could be determined by the proposed titration technique. The total acidity in several food samples was determined and the results were consistent with those obtained by conventional methods.