Electrochemical Engineering

the scientific and technical specialization dealing with problems in the study, development, and application of instruments and equipment for automation, measuring, and computer technology, whose operation is based on the electrochemical processes and phenomena occurring at an electrode-electrolyte interface during passage of an electric current. The phenomenon of electroosmosis, the change in the concentration of the active components of an electrolyte in the strata near an electrode, and other phenomena are also the subject of the discipline.

The simplest electrochemical instrument—an electrochemical cell—consists of a miniature, hermetically sealed glass bulb filled with an electrolyte, in which there are two electrodes. The electrolytes used include aqueous solutions of acids, salts, and alkalis. In order to impart special properties to the electrolytes, various agents are added; for example, organic solvents may be added to extend the range of working temperatures down to –60°C. Possible future applications in electrochemical instruments include the use of solid electrolytes that exhibit an anomalously high ionic conductivity, such as RbAg₄I₅ and Ag₃SI. The electrodes are made of Pt, Ag, Al, Zn, and other metals or their alloys; Hg is also often used for electrodes.

Electrochemical devices have been used as a basis for small amplifiers; rectifiers; timing relays; integrators; nonlinear function generators; transducer sensors for acceleration, velocity, and temperature; vibration meters; indicators; and other instruments and equipment operating in the frequency range from 10^–7 to 10 hertz. Such devices are superior to electromechanical, electromagnetic, and electronic devices in sensitivity (10^–3 volt and 10^–6 ampere), power consumption (10^–8–10^–3 watt), noise level, and reliability.

Figure 1. Two-electrode mercury-capillary tube coulometer: (1) and (7) leads to connect the coulometer with an electric circuit, (2) and (6) hermetically sealed caps, (3) sealed glass capillary tube, (4) electrolyte droplet, (5) mercury electrodes

Examples of electrochemical devices are the mercury-capillary tube coulometer and the threshold voltage indicator. In the coulometer (Figure 1), mercury is transferred in the presence of current from the anode to the cathode, and a droplet of electrolyte is shifted toward the anode in proportion to the time integral of the current. The integrated currents range from 10^–9 to 10^–4 ampere, and the integration time may be up to several years. Such coulometers are used to determine the accrued operating time of electronic equipment and individual equipment components.

Electrochemical color indicators permit very small voltage changes (0.1–1.0 V) to be detected visually or depicted with negligible power consumption (10^–4–10^–6 watt). The functioning of the electrochemical indicators depends on the property of certain substances (called electrofluorescent indicators) added to an electrolyte to change the color of the electrolyte near the electrodes under the action of an electric current. The color depends on the nature of the indicator. For example, n- and m-nitrophenols give a yellow tint, methyl violet gives a violet tint, and Phenolphthalein gives a red tint.

Figure 2. Threshold voltage indicator: (1) and (6) leads to connect the indicator with an electric circuit, (2) hermetic seal, (3) platinum electrode, (4) glass bulb (cell casing), (5) electrolyte

The low-level threshold voltage indicator (Figure 2) is filled with an electrolyte that is colorless when there is no voltage on the electrodes. When signals are fed to the electrodes at a level above the cell’s threshold voltage, the color of the electrolyte changes in the vicinity of one of the electrodes. The response time for such an indicator ranges from 10^–2 to 10 seconds. Similar types of cells are used to indicate failures.