ELECTROCHEMISTRY, that branch of the science in which chemical reactions are brought about by the agency of an electric current. The fact that common salt can be split up into its elements, sodium and chlorine, by passing an elec tric current through its solution, was discovered in the early years of elec trical research, and during the last cen tury the decomposition by electricity of all solutions of salts, acids and bases, and the laws under which that decom position takes place, have been very thoroughly investigated. It has been discovered that when a solution is elec trolyzed, it acts as a conductor, at the same time decomposing in such a man ner that the metallic radicle is produced at the negative pole and the acid or hy droxyl radicle at the positive pole. For instance, when an electric current is passed through a solution of common salt, sodium is produced at the kathode and chlorine at the anode. The amount of chemical decomposition brought about is directly proportional to the amount of current passing through the solution. When this method of manufacture is employed on a commercial scale, there fore, it is customary to use a high am perage (or current) and a low voltage (or electromotive force). To obtain these conditions the resistance of the cell must obviously be kept as low as possible, and it has been found that this diminishes with rise in temperature and with increase in strength of the solution. The cell is therefore filled with a hot, concentrated solution and for a similar reason the electrodes are placed as close together as conditions permit, and are made large in area.
The most common application of elec tricity to industrial chemistry is prob ably in the manufacture of bleach and soda from common-salt. As has been stated, when brine is electrolyzed, sodi um and chlorine are produced. Sodium, however, reacts violently with water, and cannot exist, as such, in aqueous solution. As soon as formed, it com bines with water to form sodium hy droxide, more familiarly known as caus tic soda or lye. Unless special means are taken to prevent the soda and the chlorine from coming in contact with one another, they will combine to form hypochlorite of soda, sometimes called soda bleach. Electric cells to produce this soda bleach are in use to-day in laundries, but they are inefficient and require frequent repairs.
Special types of cells have therefore been designed to prevent the combina tion of the soda and chlorine, so that the soda can be drawn off, as and the chlorine gas collected and combined with lime to form bleaching powder. Of from one compartment to another, or by means of an Archimedean screw which continually carries away the mercury amalgam, and brings fresh mercury to take its place.
The Hargreaves-Bird, Allen-Moore, and Nelson cells differ in detail rather than in principle. In these cells the chlorine and soda are prevented from re acting with one another by means of a diaphragm, which separates the anode compartment from the kathode. Into the Inner, or anode, compartment, flows continuously a concentrated brine solu tion, and on either side of the compart ment is a diaphragm made of an asbes tos compound, which becomes saturated with the brine and so permits passage of the current to the kathodes on the outer sides of the diaphragm. The soda pro duced on the kathodes is washed down by steam which is injected into the outer compartments of the cell. In the Har greaves-Bird cell, carbon dioxide is also injected into the kathode compartment, so that carbonate, instead of hydroxide of soda is produced. The advantage of the Castner-Kellner cell over the other these cells the best known are the Cast ner-Kellner, the Hargreaves-Bird, the Allen-Moore and the Nelson.
In the Castner-Kellner cell, electrol ysis takes place between an anode above and a kathode of mercury below. The chlorine gas rises to the top of the cell and is drawn off, while the sodium com bines with the mercury to form sodium amalgam. The latter is decomposed by water to form caustic soda, the mercury being liberated and recovered for fur ther use. The process is made continu ous, either by giving the cell a rocking motion, which causes the mercury to flow three is the high purity and concentra tion of soda produced, but owing to the large amount of mercury required the cost of installation is very high.
The production of chlorates is carried out in cells similar in type to those de scribed above, but no diaphragm is used, and the oxidation of the hypochlorite is promoted by the use of potassium chro mate. Oxygen and hydrogen are pro duced by electrolyzing acidulated water, cells fitted with iron electrodes being commonly employed for this purpose. Other chemical processes in which an electric current is employed are the pro duction of ozone, peroxides and sodium, and in the preparation of various or ganic compounds.
. The fixation of atmospheric nitrogen can be carried out by passing an electric spark through the air. The nitrogen and oxygen of the atmosphere combine, producing oxides of nitrogen, which are dissolved in water, nitric acid being the final product.
In the laboratory, the electric current is used in analysis for determining con centrations, for titration, a galvanome ter being used in place of a chemical in dicator, and for determining metals quantitatively, by deposition on a plati num electrode, and weighing.