FAQ
How should one select the electrode material for redox measurements: gold, silver, or platinum?
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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How should one select the electrode material for redox measurements: gold, silver, or platinum?
Platinum is a versatile electrode material and is unsuitable only in very few applications.
In an ozone (O3) or hydrogen peroxide (H2O2) environment, platinum acts as a catalyst; therefore, at low concentrations, the potential corresponding to O3 or H2O2 cannot be established in the composite layer.
Furthermore, platinum oxidizes in air, let alone in strong oxidizing agents. In the former case, the electrode displays an excessively high redox value, while in the latter, a memory effect occurs. In both scenarios, the response is slow.
In principle, platinum electrodes should not be used in applications involving ozone or peroxides.
Gold electrodes can be used in the aforementioned applications; moreover, gold electrodes have a more positive potential than platinum electrodes. Gold or silver electrodes also offer advantages when employed in the treatment of cyanide-containing wastewater. Cyanide exerts a mild corrosive effect on the electrode surface, which helps keep the electrode surface clean.
Gold should not be used in acidic solutions with high chloride concentrations, as gold forms complexes with chlorine, rendering the measurement invalid. The same principles that apply to pH measurements also apply to redox measurements, both in terms of instrument design and cable configuration: the measuring amplifier must have a very high input impedance (10¹² Ω). Redox values are typically displayed as absolute or relative “mV” readings.
Redox measurement is a bulk-parameter measurement and cannot be used to quantify a specific ion. Consequently, it is also impossible to determine the concentration of a particular species in a redox system, since all ions present in the solution participate in the redox process. Furthermore, the redox electrode should not be chemically cleaned in the medium being measured, because the cleaning solutions typically contain high concentrations of ions; such high ionic strength combined with the resulting potential shock results in a very slow return of the electrode potential to its original measured value.
If cleaning is necessary during the process, it must be performed mechanically or by ultrasonic means.
Before use, the electrode can be briefly immersed in a sulfite solution outside the medium being measured, followed by treatment with sulfuric acid or hydrochloric acid. Failure of a redox measurement chain typically occurs in the reference system, manifesting as zero-point drift. In such cases, zero-point calibration is required, for example using a 225 mV redox buffer solution. The slope of the redox electrode remains unchanged.
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