Residual free-chlorine concentration in water supplies is a key metric studied to ensure disinfection. High residual chlorine concentrations lead to unpleasant odours and tastes, while low concentrations may lead to inadequate disinfection. The concentration is most commonly monitored using colorimetric techniques which require additional reagents. Electrochemical analysis offers the possibility for in-line analysis without the need for additional reagents. Electrochemical-based detection of chlorine is influenced by the solution pH, which defines the particular chlorine ionic species present in solution. As such, controlling the pH is essential to enable electrochemical based detection of residual chlorine in water. To this end, we explore the application of solid state interdigitated electrodes to tailor the in-situ pH of a solution while simultaneously detecting free-chlorine. Finite element simulations and subsequent electrochemical characterization, using gold interdigitated microelectrode arrays, were employed to explore the feasibility of an in-situ pH control approach. In practice, the approach converted residual chlorine from an initial mixture of two species (hypochlorous acid and hypochlorite ion), to one species (hypochlorous acid). Chlorine detection was shown in water samples using this exploratory method, resulting in a twofold increase in signal response, compared to measurements without pH control. Finally, tap water samples were measured using the in-situ pH control method and the results showed excellent correlation (within experimental error) with a commercial instrument, demonstrating the efficacy of the developed technique. This work establishes the possibility of deploying an electrochemical based reagent-free, in-line chlorine sensor required for water distribution networks.
Silver ions, the most toxic form of silver, can be present in drinking water due to their release from silver nanoparticles which are widely used in consumer products. Due to their adverse health effects, a quick portable approach for detection in drinking water is needed. Herein we report on the development of an electrochemical sensor for silver ions detection in tap water using linear sweep voltammetry with in situ pH control; enabled by closely space interdigitated electrode arrays. The in situ pH control approach, allows the pH of a test solution to be tailored to pH 3 thereby eliminating the current need for acid addition. A calibration curve between 0.2 -10 µM was established for silver detection in sodium acetate when 1.25 V and 1.65 V was applied at the protonator electrode during deposition and stripping, respectively, as a proof of concept study. For the final application in tap water, 1.65 V was applied at the protonator electrode during deposition and stripping. The chlorine ions, present in tap water as a consequence of the disinfection process, facilitated the silver detection and no additional electrolyte had to be added. Combination of complexation of silver ions with chlorine coupled with in situ pH control resulted in linear calibration range between 0.25 and 2 µM in tap water without the need of acidification.
This work describes a flexible and portable data acquisition system that has been developed to interface to nano and ultra-micro scale electrochemical sensors at the point of use. It can perform a range of voltammetric tests, including Cyclic Voltammetry, Square Wave Voltammetry and Generator Collector Voltammetery. The data acquisition system interfaces to a smartphone, operates from a rechargeable battery and is of suitable form factor to ensure that it's fully portable. By utilising commercially available components, this system has been developed to lower the barrier for entry for the development of emerging portable electrochemical sensing technologies at micro and nano scale. To show the full range of functionality of the system, a use case involving river water quality monitoring is presented through generation of a calibration curve, using a recently developed Tyndall National Institute ultra-microband electrochemical sensor, for the detection of dissolved oxygen in river water.
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