COMPARATIVE ANALYSIS OF CONTACT AND CONTACTLESS METHODS FOR DETERMINING THE ELECTRICAL CONDUCTIVITY OF AQUEOUS SOLUTIONS
DOI:
https://doi.org/10.17721/2519-481X/2026/90-02Keywords:
electrochemical impedance, electrical conductivity, contactless sensor, electrolytic cell, reference resistor, voltage divider, drift, electrode polarization, STM32G431, CD4051Abstract
The purpose of this study is a comparative analysis of the direct contact measurement of the electrical conductivity of aqueous solutions and the impedance-based approach, which is considered as the first stage in the development of a contactless sensor for determining the conductivity of aqueous solutions. A two-electrode contact conductivity sensor with a cell constant K = 0.993 cm⁻¹ was used in this work. For the same sample of municipal water, the long-term drift of the direct contact channel was analyzed, and the influence of the reference resistor value on the accuracy of determining the impedance modulus of the electrolytic cell in an AC voltage divider circuit with controlled selection of the reference element was investigated. The switching of reference resistors with nominal values from 1 kΩ to 220 kΩ was performed by a CD4051 analog multiplexer controlled by an STM32G431 microcontroller, while the input and output voltages were digitized by the built-in analog-to-digital converter. It was experimentally established that the direct contact channel demonstrates a relative conductivity drift of about 12.03 % for an unchanged water sample, which is caused by polarization processes at the electrode–electrolyte interface, the formation of the electric double layer and the slow dynamics of the near-electrode layer. It is shown that even a simplified single-frequency impedance channel significantly reduces the influence of slow drift processes compared to the direct contact channel, and the smallest error of determining the impedance modulus, equal to 0.21 %, is achieved when the reference resistor is matched to the modulus of the impedance under study, which fully corresponds to the theoretical minimum of the error coefficient. Based on the obtained results, the expediency of a further transition to contactless conductivity measurement through the analysis of the full shape of the electrochemical impedance spectrum in a wide frequency range is substantiated, which will make it possible to obtain not only an integral conductivity value, but also additional informative parameters related to the concentration and mobility of ions in the solution.
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