The electrode potential is the difference in electrostatic potentials between an electrolyte and an electrode. The occurrence 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.
The 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 in the process of establishing electrochemical equilibrium, as well as the adsorption of ions from the electrolyte to the electrode surface; gas displacement beyond a positively charged ionic crystal lattice; non-Coulomb adsorption of ions or liquid molecules on the electrode. Due to the last two phenomena, the electrode potential is never equal to 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, the method of comparing the reference and studied electrodes is used. The electrode potential is equal to the magnitude of the
electromotive force (EMF) obtained in the electrochemical circuit.
For water-based 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 of metal that absorbs hydrogen gas well (often using palladium or platinum). Such a plate-wire is saturated at atmospheric pressure with hydrogen, and then 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. An element is a standard; the electrode potential of a chemical reaction is measured relative to it.
When assembling galvanic cells based on hydrogen and analyte devices, a reaction (reversible) occurs on the surface of the platinum group metal , which means either a reduction process or oxidation. The type of process depends on the potential of the ongoing reaction of the element being determined. The potential of the hydrogen electrode is taken to be zero when the hydrogen pressure is one atmosphere, the concentration of solution protons is one mol per liter, and the temperature is 298 K. If the element under study is in reference conditions, that is, when the activity of ions affecting the potential is unity, and gas pressure - 0.101 MPa, the value of such a potential is called standard.
By measuring the EMF of a galvanic electrode under standard conditions, the standard electrode potential of a chemical reaction is calculated. Usually this value is measured under conditions when all the thermodynamic activities of the potential-determining reaction are equal to unity, and
the gas pressure is 0.01 * 105Pa. The potential of the element under test is considered positive if, in the "current source" mode, electrons move from left to right in the external circuit, and positively charged particles in the electrolyte.