FARADAY'S LAWS. The two laws of electrochemical change laid down by Faraday about 1835, and known by his name, are two of the most fundamental statements in natural science. Faraday's first law specifies that the amount of chemical action produced by an electric current in an electrolysis is directly proportional to the quantity of electricity which passes through the circuit. The second law specifies that the quantities of different sub stances that are produced by the same amount of electricity at the electrodes during the electrolysis of different solutions are directly proportional to the chemically equivalent weights of the substances concerned. The quan tities of material produced by an electrolysis are then affected by only three factors, the magnitude of the current, the time that it acts and the chemically equivalent weight of the substances being nrodneed at the electrodes. and are entirely independent of such factors as con centration or temperature of the solution, the size or distance apart of the electrodes and all other purely physical conditions. These laws hold with great exactness, not .only for ordinary aqueous solutions, but also for non-aqueous solutions and for fused salts, or, in other words, for all electrolytes. This, however, does not mean that in every electrolysis there is always obtained the yield of any particular product that is "called for by Faraday's laws, for this is not always the case: The amount of material actually produced from any given electrolysis is called the °current yield?' and the ratio of the current yield to the theoretical amount called for by Faraday's laws is the ecurrent efficiency.° When the current efficiency is less than 100 per cent, this is due not to the fact that Faraday's laws do not hold in this particular case, but to the fact that the physical conditions imposed on the electrolysis have caused supplementary reactions to take place simultaneously with the first, thus changing the results. These supplementary reactions may be electrochemical in character, or they may be purely chemical. For example, in the electrol ysis of a solution of copper sulphate, if the current density is too high, the amount of cur rent passing in any given instant will call for the deposition of a greater amount of copper than happens to be present in the electrolyte in contact with the electrode. Since the current
cannot pass without doing some work, that por tion of the current which cannot be satisfied by the deposition of copper will cause decom position of the water of the solution and will deposit hydrogen on the cathode simultaneously with the copper. Since the electrolysis is pri marily designed for the deposition of the cop per, the current efficiency for copper is then less than 100 per cent, but the current effi ciency for both reactions taken together will be 100 per cent. On the other hand, if the deposition were being made in a strongly acid solution, the acid present would cause a certain amount of chemical solution of copper while the deposition is going on. In that case, the current would be doing the required work, but the product would be lost in its original form and the current efficiency would again be less than 100 per cent. Hence the physical condi tions imposed on an electrolysis cannot change in any way the amount of decomposition effected by the current, but can only change the nature •of the product obtained. The quantity of electricity that is necessary to de posit the chemically equivalent weight of any substance is 96,500 coulombs (ampere-seconds) and this unit is known as a Faraday after its discoverer. Another way of stating this is to say that one Faraday of electricity, 96,500 coulombs, is required to make a unit change in valence in a gram-atom of any element or radi cal. The fact that 96,500 coulombs will deposit one chemically equivalent weight of any sub stance makes it possible to calculate from this relation the amount of any element that would be deposited by any given amount of current. This is known as the electrochemical equiva lent (q.v.). See also ELECTROCHEMISTRY; ELEC TROLYSIS.
G. A. Roush, Assistant Secretary, American Electrochemical Society.