Selection of ORP electrode materials

ORP measurement electrodes can be made from various metals such as nickel, copper, silver, iridium, platinum, gold, etc. They consist of an ionic lattice structure within which electrons can move, and a potential difference is generated due to the presence of identical ions. The standard potential values for these 6 metals are as follows: platinum and gold have higher ORP values, which enables higher measurement sensitivity. Compared to other ORP electrodes, the redox potentials of the ion equilibrium activities of precious metals such as platinum and gold are extremely low; therefore, they have almost no impact on ORP measurements ； Platinum can form a purified surface, and an oxygen-containing layer is easily formed on this surface, thereby increasing the standard electrode potential ； This oxide/hydroxide layer is primarily composed of PtQ or Pt(OH)2; chemical adsorption of oxygen begins only when the ORP exceeds a certain critical value. As the potential increases, the thickness of this surface protective layer also increases, although in most cases it reaches only the thickness of a single molecular layer. The monomolecular \"oxide\" layer formed due to the chemisorption of oxygen on the platinum surface is conductive; it does not affect the sensitivity of the electrode’s ORP measurement and maintains a high electrode potential, even when the redox potential of the sample solution has decreased. However, the response is slow when measured in solutions with lower ORP ； Platinum electrodes with a rough surface can adsorb more oxygen than those with a smooth surface, and their response is slower; therefore, platinum electrodes with a smooth or polished surface are preferable. The surface oxygen absorption of gold is much lower than that of platinum, so gold is more suitable for certain applications. In saline concentrated solutions such as urban sewage and industrial wastewater, gold can form cyanides and halides; moreover, when oxygen is present in the solution, gold is also rapidly corroded, but platinum has a much greater resistance to corrosion than gold. Furthermore, platinum has a higher exchange current density than gold; therefore, it is a better choice for measuring the ORP of natural water. Platinum also possesses strong catalytic properties, which allow the measurement solution to reach equilibrium more quickly, resulting in more accurate readings. For example, a platinum/hydrogen electrode coated with sponge-like platinum can enable high-precision ORP detection at normal room temperatures. In general ORP measurements, gold electrodes are used for highly oxidizing solutions, while platinum electrodes are used for oxidizing solutions (containing fluorides), natural water (rivers, lakes, and ponds), and other types of solutions.

Thank you! Can it be understood that a gold electrode is chosen for strongly oxidizing media, while a platinum electrode is used for saline media? If it is a reducing medium, what electrode should be chosen?