Metals {electrode}| can contact solutions. Corrosion, electrolysis, electroplating, and batteries involve electrodes.
oxidation
At anode, ions enter solution, so anode is negative, and solution is positive. Oxidation is at anode surface, and reduction is at cathode surface. Opposite charges surround electrode charges, and solution ions solvate, with high attraction at surfaces. Charge gradient is higher for higher concentration and higher ion mobility. Higher temperature reduces attraction, by breaking up surface layer. Applied electric force reduces attraction.
current
Ion formation or discharge rate is current density, which is 0 at equilibrium. Ion far from electrode feels net force. At 10^-7 meters, ion sees widely distributed charges, as it enters ion layer around electrode and feels constant voltage. When ion reaches electrode surface, voltage changes rapidly to opposite sign. Finally, ion reaches electrode pure metal.
At high current, potential can be constant, such as at hydrogen electrode or calomel electrode. If high overvoltage causes high current density, diffusion can be too slow, and electrode can become polarized. Adding extra potential or moving electrodes reduces polarization. Solution friction causes slower cation flow than electron flow, causes ohmic resistance, and decreases current. Power generation maximizes if concentration polarization is just below limiting current.
Reference electrodes {calomel electrode} can use mercury and saturated mercury chloride. Potassium chloride reduces variation with temperature and can be salt bridge.
Electrodes {glass electrode} can pair with calomel electrodes or silver/silver chloride electrodes. Glass electrodes have silver and silver chloride in 0.1 M hydrochloric acid, in thin glass membrane. Temperature, acidity, and sodium contamination affect it. It requires storage in 0.1 M potassium chloride. It requires cleaning.
Reference electrodes {hydrogen electrode} can use platinum electrodes, with hydrogen gas at one atmosphere and hydrogen ion at one-molar concentration. Hydrogen electrodes can have platinum-surface changes and can vary hydrogen-gas pressure and hydrogen-ion concentration, so they are hard to control.
Silver and silver chloride reference electrode {silver electrode} has saturated silver chloride on silver surrounded by saturated potassium-chloride solution as salt bridge. It is stable at high temperature. It can be internal reference for glass electrode.
5-Chemistry-Analytical Chemistry-Electricity
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Date Modified: 2022.0225