Electrolysis

ELECTROLYSIS, the decomposition of a dissolved electrolyte by an electric current passing through it. In conductors of the second kind, which include solutions of electrolytes (see Electrical Conductivity), the conduction of an electric current is associated with the transfer of ions liberated at both electrodes. Under the influence of electrostatic forces, positive ions (cations) are attracted to and liberated at the cathode, and anions at the anode. Faraday, who first studied this phenomenon, believed that the very splitting of the dissolved substance into ions was produced by the electric current, as a result of which he named the substance decomposed by the current an electrolyte. Later, Arrhenius established that an electrolyte is dissociated into its ions to a greater or lesser extent even without the passage of a current (see Electrolytic Dissociation), and an electric current is required only to neutralize the ionic charges and liberate the neutral atoms formed from the ions at the electrode, i.e., for the phenomenon of electrolysis itself. To neutralize a specific amount of ions and liberate them at the electrode, a strictly defined quantity of electricity is required. Chemistry calls the gram-equivalent of an atom or atomic group that weight quantity of a given substance (in grams) which reacts with 1 g (more precisely, 1.008 g) of hydrogen or replaces the same amount of it in compounds. According to the law of electrolysis established by Faraday, the liberation of one gram-equivalent of any substance always requires the same amount of electricity, equal, according to the most precise determinations, to 96,490 coulombs. This number is called the "Faraday" and is denoted by the letter F. Since a coulomb represents the quantity of electricity passing in 1 second at a current strength of 1 A, 1 ampere-hour equals 3,600 coulombs, and 1 F = 26.8 ampere-hours. This amount of electricity is required to liberate 107.88 g of Ag from $ ext{AgNO}_3$, 1.008 g of H from $ ext{HCl}$, and so on. In the case of calcium or another divalent element, its gram-equivalent equals half of its atomic weight: 40.07 : 2 = 20.04 g. If one and the same element possesses different valencies in different compounds, as for example copper in $ ext{CuCl}$ and in $ ext{CuCl}_2$, the electrochemical equivalent equals 63.57 in the first case and 31.79 g in the second. However, such a liberation of neutral atoms represents only the primary process in electrolysis. The free atoms or radicals liberated at the electrodes usually enter into various secondary reactions. Thus, in the case of $ ext{AgNO}_3$, silver is liberated at the cathode, and the nitrate ion ($ ext{NO}_3$) dissolves the metal of the anode, newly forming a nitrate molecule, or decomposes water, forming a nitric acid molecule and liberating gaseous oxygen. Secondary reactions occurring during electrolysis can be quite diverse. By the amount of electrolytically liberated substance, one can determine the amount of electricity that has passed through the solution. Apparatuses serving for this purpose are called coulometers. In the silver coulometer, the amount of liberated silver is determined; from the figures given above, it follows that 1 coulomb deposits 1.1180 mg of silver on the cathode. In the mercury coulometer, the height of the column of mercury liberated during the electrolysis of a mercury salt is measured, and so on.