ELECTROLYSIS Electro lysis of solutions in con tact. Electro chemical equiva lents. Faraday s law receives striking confirmation from the electro lysis of several solutions arranged in series in contact with_ each other by means either of porous septa, asbestos wicks, or siphon tubes. *Each liquid then acts as an electrode to the adiacent ones, and so at the junction we have separated the anion of one electro lyte and the cation of the next. These in general _unite, and if the resulting compound be insoluble, a precipitate is thrown down. Faraday thus precipitated magnesia from its sulphate by electrolys ing a solution of that salt in contact with water, the current passing from the salt solution to the water. Now, in all cases in which the ions unite at the junction, and do not appear free at all, the amount of the cation of one liquid must be chemically equivalent to that of the auion of the succeeding one, and hence obey Faraday s law. Many of the decompositions and combinations thus effected are very interesting, a list showing in a tabulated form the results of experiments by Hisinger and Berzelius, Davy, Daniell, Miller, and others will be found in Wiedemann (Galv., 13d. i. 368). We can only mention one example which is of theoretical importance. If the positive electrode be in solution of iodic acid which is in contact with dilute sulphuric acid containing the cathode, then at the sur face of separation there will be formed I and S0 4 , or H and S0 4 , according as the I observed at the negative electrode in the elec trolysis of HI0 3 solution is an ion or due to secondary action. By the union of the two ions at the junction the latter is shown to be the case; therefore iodic acid is electrolysed as H 2 + (I 2 5 +0). We gather at once from the truth of Faraday s law that we can assign to each ion an electrochemical equivalent (which may be referred to as E.C.E.), which will enable us to determine at once the amount of the ion which will be separated by a given quantity of electricity. With the notation already used the E.C.E. of an ion =J mt. The value of t the amount of water decomposed by one C.G.S. electromagnetic unit of electricity from experi ments of Weber, Joule, Bunsen, Oasselmann, and Kohlrausch is 00093 gramme (Wied. Galv., Bd. iii. 1077-1079). The quantity m is one of the chemical equivalents of the ion, usually that de duced from its most stable salts ; some metals, indeed, with two series of salts have two E. C. E. s. The following table of the elements gives the values of m and the E.C.E.s in absolute units, as far as they have been experimentally determined. Since m bears a simple ratio to the atomic weight, its value can be corrected by the results of chemical analysis. Table of Electrochemical Equivalents. 4J
| 5 a I V a c t* p ^ S x ~ J? s 3 .t: I 5 "Oo * ^ ? - 1 B 2-- o 1 ^ e . ^ lo c
O c?^ w ^ m I a IH ". <u tz II W P. c i B W-3 q,| II 1 W 2 f3 w :? 3 3 2 K w | Al .... 27-5 9-1(2) 00094 Mo.... 96 (*) Sb 122 40-6(2) 00420 Ni 59 29-5(1) 00305 As.... 75 25 (2) 00258 Nb.... 97-5 ( 3 ) Ba.... 137 68-5 00708 N 14 ( 3 ) Bi .... 210 71(1)(2) 00733 Os 199 ( 3 ) B 10-9 ( 3 ) O 16 8(M 00083 Br 80 80(1) 00826 Pd ... 106 ( 3 ) Cd .. 112 66(M 00578 P 31 ( 3 ) Cs. .. 133 01374 Pt .... 197 98-5(1) 01018 Ca. .. 40 20 00206 K .... 39-1 39-1(1) 00404 C 12 ( 3 ) Ro ... 1043 ( 3 ^ Ce .. 92 ( 3 ) Rb.... 85 85( 4 ) 00878 Cl . .. 355 355(1) 00366 Ru.... 1042 ( ) ... Cr .. 52-5 17-5 00181 Se..... 79-5 ( 3 ) Co ... 59 29-5(1) 00305 SI 28 ( 3 ) Cu ... 63 leso 00326 ) 00651 f Ag.... Na... 108 23 23(>) 00237 D 96 Sr .... 87-5 437 00452 F...... 19 19(i) 00196 S 32 l(i( 3 ) 00165 G.... 93 (3) Ta.... 138 ( 3 ) Au ... 1966 65-5(2) 00677 Te 129 61-5(2) 00666 H 1 1 000103 Tl 204 204(2) 02108 In .. 1134 ( 3 ) Th .... 119 ( 3 ) I.. .. Ir ... 127 197 127(1) 01312 Sn 118 j 29 5H ( 59H2) 00305) ooeiof Fe ... 5fi 28(i)( 7 ) 00289 1 Ti.... 50 La .. 92 (*) W ... 184 ( 3 ) Pb .. 207 103-5(1) 01069 U 120 ( 3 ) Li .. 7 7 00072 4t.. 137 ( 3 ) Mg.. 243 12( )(2) 00124 7.r... 65 32 5(l)(2) 00336 Mn.. 55 27-5(1) 00284 Zr ... 89-5 ( ) Hg.. 200 f200(i)( 2 ) 1 100(2) 020G6 ) 01033 | Every complex ion has also a definite electrochemical equivalent, usually coinciding with its chemical equivalent. The E.C.E. of an electrolyte is the sum of the. E.C.E.s of its component ions. 1 Faraday, Exp. Kef., ser. vii. 2 Renault (I.e. infra). 3 Either these elements have not been obtained as ions by electrolytic action, or quantative experiments are wanting.
- Bunsen.
Renault 5 determined the E.C.E.s by an inverse method. He observed the amount of the metal which, forming the negative pole of a battery with various electrolytes, gave a current equivalent t<> that produced by the dissolution of a definite amount of zinc in a ZnPt cell, the two currents passing through a differential galvano meter, and thus compared the amounts of elements which generate the same quantity of electricity in combining. It is perhaps neces sary to observe that the electrolytic reactions taking place in a gal vanic cell which generates a current are in every way identical with those due to a current from an external source sent through the electrolyte. In the former case, the energy of chemical affinity at the electrodes is transformed into the energy of electrical separa tion, and in the latter the converse is the case. The Electrochemical Scries. It is evident from 11 the examples we have given that it is Electr not an accident whether anion will appear at the anode or cathode ; chemic the cations have been all more or less similar in character, and series, were either metals or more allied to the metals than the corre sponding anions, which were bodies like Cl, Br, I, CN, 0, &c. Faraday (Exp. Res. , 847) was accordingly led to consider that an element or radicle was unalterably either an anion or a cation ; this, however, was contradicted by the fact that the same element may act as an anion in one solution and a cation in another, as is the case with iodine, which in KI is an anion, but from a solution of iodine bromide (IBr) appears at the cathode. The electrolysis of alloys 6 points in the same direction, so that the conclusion is suggested to us that "anion" and "cation" have only relative meanings, and that we might arrange the elements in a series such that, in a compound of an element A with any one of those above it, A would appear as a cation, but in a compound with any of those below, as an anion. To do this by purely electro lytic means is out of the question, as binary electrolytes do not exist for each pair of elements. As far, however, as the series can be thus made out, it is found that, as a rale, if two elements A and B, such that A is above B in the series, be immersed in a simple electrolyte, as dilute H 2 S0 4 , and connected by means of a wire, the current flows from B to A through the liquid. Hence in unknown cases we may observe the direction of the cur rent when the two elements are immersed in an electrolyte, say H 2 S0 4 , and determine the relative position in the series. 7 With the series thus roughly formed, it is observed that the wider two elements are apart the greater is the chemical affinity between the two, and thus that if we have a compound MR, where M is the electro-positive element, a more electro-positive element M having a greater affinity for R than M tends to replace M from the com pound, and a more electro-negative element R tends to replace R as iron replaces copper from CuS0 4 , and chlorine iodine from Kl. This further assists us in forming an electrochemical series of the elements, but it is still not very strictly arranged, and many of the members of the series are placed by their analogy to elements whose positions are known. Moreover, it is supposed that the relative position of two elements may vary with the temperature. Thus carbon which is used in batteries as the negative element, is at a full red heat electro-positive even to potassium, or at least reduces the carbonate of that element. Jablochkoff (Comptes Rcndus, Dec. 3, 1877) describes a cell of which the positive element is coke. The electrolyte is fused sodium or potassium nitrate, and the negative element is a cast-iron vessel containing the fused salt. The current is from coke to cast-iron through the nitrate, and the electromotive force 2 to 3 volts. Berzelius s final series stands thus : Electro-negative. Thorium. Zirconium. Aluminium. Didymium. Lanthanum. Yttrium. Glucinum. Magnesium. Calcium. Strontium. Barium. Lithium. Sodium. Potassium. Electro-positive. Oxygen. Boron. Mercur Sulphur. Carbon. Silver. Selenium. Antimony. Copper Nitrogen. Tellurium. Bismut Fluorine. Tantalum. Tin. Chlorine. Titanium. Lead. Bromine. Silicon. Cadmii Iodine. Hydrogen. Cobalt. Phosphorus. Gold. Nickel. Arsenic. Osmium. Iron. Chromium. Indium. Zinc. Vanadium. Platinum. Manga Molybdenum. Rhodium. Uraniu Tungsten. Palladium. Cerium 5 " Verification expdrimentale do la nJciproque de l.-i loi de Faraday, sur la decomposition des electrolytes," Paris, 1867; Ann de C/iim. [4] xi. 137. 6 Alloys of tin and lead, potassium and sodium, sodium amalgam, gold araal gam, and fused cast-iron have all been shown to suffer chemical decomposition on the passage of the electric current (Wied. Galv., i. 328). 7 This is not always conclusive evidence, as the direction of the current for tho same two elements sometimes varies with the electrolyte employed, as will bo seen by referring to the list of chemico-electric series in Gore, Electro-metal-
lurjy, p. 66. The boracic acirt !<, j 9 peculiarly anomalous.
