Correlations of the impedance parameters and conductivity and permittivity of liquid and gel phases in a series piezoelectric quartz crystal sensor

The impedance characteristics of a series piezoelectric quartz crystal (SPQC) sensor were investigated using a network/impedance analyzer. The correlations between the impedance parameters, including the magnitude and phase of the impedance and the resonant frequencies, and the conductivity and/or the permittivity of the test solutions were derived theoretically and in good agreement with the experimental results. It was shown that the value of the series frequency (fs) decreases with increasing conductivity and permittivity of the liquid. The parallel frequency (fp) is the constant. The values of the maximum impedance (|Z|max) and corresponding frequency (fmax,), are not related to the conductivity and permittivity of the liquid. The value of the minimum impedance (|Z|min) decreases with increasing permittivity and has a maximum in a solution of middle conductivity. The corresponding frequency (fmin) has the same value as the fs. The maximum phase of the impedance (θmax) increases slightly with increasing permittivity and has a minimum in electrolyte solution of middle conductivity. In the impedance–frequency curves, there is an iso-impedance point for the SPQC in electrolyte solutions. The SPQC method was applied to monitor the process of hydrogel formation.