ORP is defined as the electromotive force between a noble metal electrode and a reference electrode when immersed in solution [1]. The electrochemical setup typically involves a platinum wire as a working electrode (sometimes gold, rarely silver) and a well-controlled reference electrode. When these electrodes are immersed together into a solution, an electron migration is produced between the working electrode and the solution. Depending on the nature of the species diluted, the working electrode loses electrons if the media is an oxidant, so the potential measured becomes positive. If the media is reductive, the working electrode gains electrons, and the resulting potential will be negative. Voltage is measured between both electrodes (working and reference electrode), so the ORP value is usually given in millivolts.

ORP provides valuable information because it is a measure of sanitizer activity and water quality, due to the changes in ORP being associated with the existence of contaminants or biological activity. ORP measurements are also useful in several major industrial applications where maintaining potential in a production stage under control is a critical parameter or when it is necessary to assure the quality of water. Examples include cyanide oxidation or chrome reduction in wastewater treatment of metal-plating industry residues, bleach production and bleaching of paper, biological growth control in cooling towers and water quality monitoring in swimming pools, hot tubs, spas, or aquafarming.

Screen-printed electrodes (SPEs) offer several interesting advantages in industrial applications such as point-of-care testing, usability, and a wide range of possible customizable setups. However, most SPEs include a pseudo-reference electrode fabricated with silver or silver/silver chloride. Since the KCl electrolyte is absent, this pseudo-reference electrode is exposed to media implying that the reference electrode is not under control, especially in aggressive media. In this publication, an SPE with a platinum paste working electrode and a controlled solid-state reference electrode is presented. Several experiments are included to show that these platforms offer ORP monitoring in household, industrial or laboratory applications.

References:

[1] ASTM D1498 - 14 - Standard Test Method for Oxidation-Reduction Potential of Water