The electrode potential is the difference of electrostatic potentials between the electrolyte and the electrode. The appearance of such a potential is due to the spatial separation of charges, which have opposite signs at the phase separation boundary with the formation of an electric double layer. Spatial separation of charges at the boundary between the metal electrode and the electrolyte solution is associated with such phenomena as the transfer of ions from the metal into the solution during the establishment of electrochemical equilibrium, and also the adsorption of ions from the electrolyte to the surface of the electrode; Displacement of gas beyond the ion positively charged crystal lattice; Noncoulomb adsorption of ions or molecules of a liquid on an electrode. Due to the last two phenomena, the electrode potential is never zero, even if the charge of the metal surface is zero. The absolute value of the potential of a single electrode is not determined, for this purpose the method of comparing the reference electrode and the electrode to be tested is used. The electrode potential is equated to the magnitude of the electromotive force (EMF) obtained in the electrochemical circuit. For aqueous solutions it is customary to use hydrogen electrodes. Standard elements of this type are used as standards in a variety of electrochemical measurements, as well as in galvanic devices. A hydrogen electrode is a wire or plate made of metal that absorbs gaseous hydrogen well (palladium or platinum is often used). Such a wire plate is saturated at atmospheric pressure with hydrogen, after which it is immersed in an aqueous solution rich in hydrogen ions. The potential of such a plate is proportional to the concentration of ions in the solution. The element is the standard, relative to it the electrode potential of the chemical reaction is counted.
When assembling galvanic cells on the basis of hydrogen and detectable devices, a reaction (reversible) takes place on the surface of the metal of the platinum group , which means either the reduction or oxidation process. The type of process depends on the potential of the ongoing reaction of the element being determined. The potential of the hydrogen electrode is assumed to be zero when the hydrogen pressure is one atmosphere, the proton concentration of the solution is one mole per liter and the temperature 298 K. If the element under study is in reference conditions, that is, when the ion activity affecting the potential is unity, and The gas pressure is 0.101 MPa, the value of such a potential is called standard. By measuring the EMF of the galvanic electrode under standard conditions, the standard electrode potential of the chemical reaction is calculated. Usually this value is measured under conditions when all the thermodynamic activities of the potential-determining reaction are equal to one, and the gas pressure is 0.01 * 105Pa. The potential of the element being tested is considered positive if in the "current source" mode in the external circuit from the left to the right electrons move, and in the electrolyte - positively charged particles.