FAQ
What does a REDOX/ORP electrode measure, and how is it measured?
How should one select the electrode material for redox measurements: gold, silver, or platinum?
Platinum is a universally applicable electrode material and is unsuitable for only a very few applications. In environments containing ozone (O3) or hydrogen peroxide (H2O2), platinum acts as a catalyst; consequently, at low concentrations, it is unable to establish an electrode potential that is compatible with O3 or H2O2 within the composite layer. Furthermore, platinum oxidizes in air, let alone in strong oxidizing agents. In the former case, the electrode indicates an excessively high redox value, whereas in the latter, a storage effect occurs. In both scenarios, the response is slow. As a general rule, platinum electrodes should not be used in applications involving ozone or peroxides. In such applications, gold electrodes can be employed; moreover, gold electrodes have a more positive potential than platinum electrodes. Gold or silver electrodes also offer advantages in the treatment of cyanide-containing wastewater. Cyanide exerts a mild corrosive effect on the electrode surface, which helps keep the surface clean. However, gold should not be used in highly chlorinated acidic solutions, as gold forms complexes with chlorine, rendering the measurement invalid. With regard to construction and measurement cables, the same principles that apply to pH measurements also apply to redox measurements: the measuring amplifier must have a very high input impedance (10^12 Ω). Redox values are typically displayed as absolute or relative “mV” readings.
What does a REDOX/ORP electrode measure, and how is it measured?
A redox electrode is a noble-metal electrode. It is used for potential measurement while remaining chemically inert—that is, it must be able to withstand chemical attack. For this purpose, only noble metals such as platinum, gold, or silver are suitable. As a reference electrode, the Ag/AgCl reference system is employed, just as in pH measurement. When a platinum-wire redox electrode is immersed in a chloride-containing solution, an interfacial layer forms at the interface between the platinum wire and the solution surface; this is known as the Helmholtz double layer. This interfacial layer functions like a capacitor, with one plate connected to the platinum wire and the other plate, as in pH measurement, connected to the reference electrode. The capacitance charges in response to the electrochemical potential difference between the platinum wire and the solution. The potential of the solution, in turn, depends on the logarithmic concentration ratio Log [COX]/[CRED] and the sum of the potential differences among all ions present in the solution.
How is the polarization resistance Rp formed?
Ions serve as charge carriers, transporting charge to the electrode surface. When the electrode surface is too small or the ion concentration is too high, this charge-transfer process cannot proceed rapidly, leading to the formation of an ion cloud at the electrode surface that obscures the electrode. In the case of direct current, the initial current will gradually decay to zero. However, since conductivity measurements are performed using alternating current, only a finite additional resistance is introduced.
What is pH, and what should you know about pH measurement?
In general, pH measurement is used to determine the acidity or alkalinity of a solution. When an acid is added to water, the acidity of the water increases and the pH value decreases. Conversely, when a base is added to water, the alkalinity of the water increases and the pH value rises. The pH scale is the unit used to express acidity and alkalinity. When we say that milk is “cool” or that an acid is “weak,” we are not providing a precise description of the condition, because we have not specified the measurement unit or the measured value. However, when we state that the temperature of the milk is 10°C, we are giving a definite and precise description. Similarly, when we say that the pH of a weak acid is 5.2, we are also providing a precise and unambiguous statement.
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What does a REDOX/ORP electrode measure, and how is it measured?
A redox electrode is a noble-metal electrode. It is used for potentiometric measurements while remaining chemically inert—i.e., it must be able to withstand chemical attack. For this purpose, only noble metals such as platinum, gold, or silver are suitable.
As a reference electrode, the Ag/AgCl reference system is used, just as in pH measurement.
When a platinum redox electrode is immersed in a chloride-containing solution, an interfacial layer forms at the interface between the platinum surface and the solution–water interface; this layer is known as the Helmholtz double layer. This interfacial layer functions as a capacitor, with one plate connected to the platinum electrode and the other plate connected to a reference electrode, as in pH measurement. The capacitance charges in response to the electrochemical potential difference between the platinum electrode and the solution. The potential of the solution, in turn, depends on the logarithmic concentration ratio Log [COX]/[CRED] and the sum of the potential differences arising from all ions present in the solution.
At the same time, platinum also undergoes oxidation, and depending on the concentration of the oxidizing agent, a platinum oxide layer 3 to 4 atomic layers thick forms on its surface. This oxide layer conducts electrons, thereby hindering the redox measurement process. However, it also functions as an oxidative memory element, causing a delay in the measurement when the chloride concentration decreases. The more dilute the solution being measured, the longer this delay becomes. Under conditions of high-concentration redox buffer, this effect can be neglected.
This effect can also be illustrated by the earlier example of the two tanks: one is filled with water, while the other is empty. If the diameter of the connecting pipe is small, the process of leveling the water levels in the two tanks will be slower; conversely, a larger pipe diameter will result in a faster equilibration. Similarly, surface roughness on the electrode can introduce the aforementioned measurement inertia. This is because the pits and valleys on a rough surface can act as storage sites, thereby degrading the response time during the exchange process.
The surface of the redox electrode should be kept as smooth as possible.
Because the Helmholtz double layer behaves like a capacitor, a charging current flows whenever the potential changes, continuing until electrochemical equilibrium is reached. If the measuring amplifier does not employ a zero-current method to measure the potential of this composite layer, electrochemical equilibrium will never be attained. Under such circumstances, the measured value will continuously drift, and, under certain conditions, chemical changes may also occur at the electrode surface.
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