Capacitively-Coupled Contactless Conductivity Detection (C4D) for In-Line Ionic Silver Monitoring



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2023 International Conference on Environmental Systems


Effective techniques for the in-line concentration measurement of ionic silver (Ag+) biocide are desired to ensure nominal dosing. The electrolytic conductivity change produced by chemical dosing of ca. 200-400 ppb Ag+ (~0.25-0.50 µS/cm) into highly-purified potable water is linearly related to the concentration dosed, and could serve as a useful proxy. In a previous work, the potential compatibility issues associated with conventional (wetted-electrode) conductivity measurement of dilute Ag+ solutions, as well as potential mitigations were discussed. An alternative approach, capacitively-coupled contactless conductivity detection (C4D), was also proposed. C4D uses electrodes placed outside an inert, insulating material, with dielectric polarization, enabling the production of an electric field and resultant current across the analyte solution that is monotonically related to its electrolytic conductivity. In this work, a systematic investigation of the performance of a COTS C4D instrument with 1/16” (1.59 mm) outer diameter flow tubing (eDAQ Pty. Ltd., Australia) for electrolytic conductivity measurement in the range of interest (~0.1-6.0 µS/cm) is reported. A series of test apparatuses were constructed to flow analyte solution of well-controlled conductivity through the C4D instrument by pumping of a mixture of two electrolyte solutions. The effects of excitation frequency and analyte conductivity range on instrument response were predicted based on theory and characterized experimentally. Analyte conductivity changes on the order of 0.05 µS/cm (equivalent to 40 ppb Ag+) were reliably resolved, but experimental difficulties prevented completion of satisfactory calibration curves spanning the entire 0.1-6.0 µS/cm range. Instrument response was found to be similar with both NaCl and AgNO3 solutions. A passive side-stream system for integration with a practical spacecraft water processing system was demonstrated, and the potentially deleterious effects of two-phase flow (entrained bubbles) on flow reliability and measurement accuracy were considered.


John Vance, KBR / NASA Ames Research Center, USA
John Abdou, KBR / NASA Ames Research Center, USA
Lance Delzeit, NASA Ames Research Center, USA
ICES303: Physio-Chemical Life Support- Water Recovery & Management Systems- Technology and Process Development
The 52nd International Conference on Environmental Systems was held in Calgary, Canada, on 16 July 2023 through 20 July 2023.


silver, silver ion, biocide, concentration, sensor, monitoring, detector, potable water, conductivity, C4D, Capacitively-Coupled Contactless Conductivity Detection, electrode, electrochemistry