Measurement of the electrical conductivity of aqueous metal chloride solutions using a Phononic Crystal Sensor

Abstract

Abstract This work proposes a novel approach for the measurement of electrical conductivity in aqueous solutions of transition metal chlorides, specifically CuCl₂–water, FeCl₃–water, and NiCl₂–water using a phononic-fluidic sensor. Indeed, it establishes a direct correlation between conductivity and the frequency of a cavity mode in the phononic crystal as a function of concentration. The phononic structure consists of air holes periodically distributed in a stainless-steel matrix. Meanwhile, the designed phononic crystal structure possesses a wide phononic band gap of width of 145 kHz that could be of a distinct effect through the detection process. By introducing a Transition Metal Chlorides as defect in the central hole of the phononic crystal, the emergence of a resonant mode is expected which in turn represents the mainstay towards the detection of Metal Chlorides. In this regard, our findings investigated a significant shift in the spectral position of the resonant frequency with the concentration’s changes of Metal Chlorides, providing a superb sensitivity with respect to the concentration of 1.88×10−4 (kHz/ppm) for FeCl₃ solution, 1.80×10−4 (kHz/ppm) for NiCl₂ and 0.83×10−4 (kHz/ppm) for CuCl₂. In addition, the highest figure of merit (FOM) and quality factor values are obtained for FeCl₃, which are 767.28×10−7 ppm−1 and 162.75, respectively. Moreover, the lowest detection limits of the proposed sensor are 0.024 ppm for FeCl₃, 0.039 ppm for NiCl₂, and 0.061 ppm for CuCl₂. In parallel, we performed direct electrical conductivity measurements for the same concentrations. A clear and consistent correlation between the resonant frequency and the electrical conductivity was observed. Based on this correlation, a calibration curve was established, enabling the indirect estimation of electrical conductivity from resonant frequency data. The sensor sensitivity with respect to electrical conductivity was found to be 0.1889 kHz/(mS/cm) for FeCl₃, 0.2056 kHz/(mS/cm) for NiCl₂, and 0.1548 kHz/(mS/cm) for CuCl₂.