Electrode potential is the difference in electrostatic potentials between the electrolyte and the electrode. The emergence 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 a metal electrode and an 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 onto the electrode surface; displacement of gas outside the ionic positively charged crystal lattice; non-Coulomb adsorption of ions or liquid molecules on an 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 test electrodes is used. Electrode potentialequates 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 for various electrochemical measurements, as well as in galvanic devices. A hydrogen electrode is a metal wire or plate that absorbs hydrogen gas well (palladium or platinum is often used). Such a plate-wire is saturated with hydrogen at atmospheric pressure, 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 a standard, the electrode potential of a chemical reaction is measured relative to it.
When assembling galvanic cells based on hydrogen and detectable devices, a reaction (reversible) occurs on the surface of a platinum group metal, which means either a 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 taken equal to zero when the hydrogen pressure is one atmosphere, the proton concentration of the solution is one mole per liter, and the temperature is 298 K. If the element under study is under reference conditions, that is, when the activity of the ions affecting the potential is one, and gas pressure - 0, 101 MPa, the value of this potential is called standard.
Measuring EMFgalvanic electrode under standard conditions, calculate the standard electrode potential of the chemical reaction. Usually this value is measured under conditions when all thermodynamic activities of the potential-determining reaction are equal to unity, and the gas pressure is 0.01105 Pa. 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 move in the electrolyte.
