Page:The New International Encyclopædia 1st ed. v. 06.djvu/893

* ELECTRO-CHEMISTRY. 777 ELECTRO-CHEMISTRY. ence of poti'iitial is eslablished whenever two (iilferc'iil ini'liils are plaeeii in contact vitli eacli otlier; but lliis is now )jositivcly known to be wrong. If tlill'erenccs of potinlial arix' from the communication of the electrodes outside the solution, those dillVrences are certainly very small and need not be taken into account, in the case of cells with two dill'erent solutions (like the Daniel! cell), a dillVrence of jiotential is known to e.ist at the surface of contact of the two solutions; but this, too, is very slight and need not be considered here. The only ])ossible seats of the main dillerence of potential in a cell are, therefore, the surfaces of contact between the metallic electrodes and the electrolyte solu- tions — i.e. at the places where ions form and disappear. But how are dilierences of potential cstgiblished there? A clear answer to tliis ques- tion is furnished by modern electro-chemical ■theory. If a liquid is placed in a closed vessel, its vapor gradually attains a maximum pres- sure which balances the vapor-tension of the liquid at the given temperature. Again, if an e.KCCSS of sugar is placed in some water, the sugar dissolves, or. so to speak, vaporizes into the water, and the j5ortion dissolved gradually at- tains a maxinuim "osmotic pressure" which bal- ances the 'solution-tension' of sugar at the given temperature. Similarly, if a metal is brought in contact with water, its ultimate particles tend to rush into the volume of the water until a maxi- mum osmotic pressure has been attained. Since, according to the theory of electrolytic dissocia- tion, the free particles of a metal in solution are invariably charged with positive electricity, the osmotic pressure of a metal in solution can evi- dently be exerted only by its electro-positive ions. Supposing, now, that a bar of some metal is placed in a solution of one of its salts, and remembering that the salt is dissociated into metallic and acidic ions, we readily distinguish three possible cases, viz. the osmotic pressure of the metallic ions may happen to be greater than, less than, or equal to the maximum osmotic pressure that would balance the 'electrolytic solu- tion-tension' of the metal. In the tirst of these cases, some ions will tend to precipitate themselves upon the metal bar, just as some of the vapor of a liquid would tend to condense, if its pressure exceeded the vapor- tension of the liquid ; and the force driving the ions out of solution will evidently be the e.xeess of their osmotic pressure over the solution- tension of the metal. As soon, however, as a single ion is precipitated out, the metal bar is rendered electro-positive, while the solution is rendered electro-negative by the newly created excess of eleetro-negative ions; for before the immersion of the metal bar the electro-positivo and electro-negative ions in solution are pre- cisely equivalent. (See Dis.sociatiox.) But this means that a difference of potential will have been created between the metal bar and the solution. Henceforward, every eleetro-posi- tive ion driven toward the metal bar by the exces- sive osmotic pressure will encounter a double force driving it back into solution, viz. the electrostatic repulsion of the positive bar and the electrostatic attraction of the negative solu- tion. Finally, when a certain number of ions have been precipitated out, the double electro- static counter-force will have become eqial to the excess of osmotic pressure, and then, equi- librium ensuing, the maxinuim dillerence of po- tential possible under tlie given conditions will have been estalilislic<l between the metal bar and the solution. This will obviously be the greater, the givaler the osmotic i)ressure (i.e. the con- centration of ions in the solution), and the less the specific solution-tension of the metal. Passing now to the .second of three possible cases, viz. the ease in which the osmotic pressure of the metal- lic ions is less than the specific solution-tension of the metal, we see that in this case molecules of the metal will enter the solution in the form of positive ions, rendering the .solution electro- positive and leaving the metal bar electro-nega- tive. In this case, too, an electrostatic couiilcr- force will then come into play, and the diller- ence of [wtential at the surface of contact will be maximum when this force has become equal to the difl'erence between the solution-tension of the metal and the osmotic pressure of ions in the solution. The dilTerenee of potential will ob- viously be the greater, the greater the solution- tension of the metal and the less the osmotic pressure of its ions in the solution. Finally, in the third case, viz. the case in which the oemotie pressure of the metallic ions exactly equals the solution-tension of the metal, neither metal will dissolve nor ions precipitate, and consequently no difference of potential will exist at the surface of contact of the metal and the solution. Theprinciples just developed permit of gaining a clear insight into the mechanism of electrochem- ical action. Take, for instance, again the Daniell cell, which consists, as we have seen, of a con- centrated solution of zinc sulphate and a <'oncen- trated solution of copper sulpliate, separated from each other by a porous partition, with a liar of metallic zinc in the former solution and a bar of metallic copper in the latter (see Fig.). V- ZnSBi —a— Cu-sej: -^ DAXIELL CELL. It will be seen in a later section of this article that the solution-tension of zinc is very great, that of copper very small. Some zine will there- fore enter the solution in the state ot positive ions, which will render the zinc solution electro- positive and leave the zinc bar electro-negative. On the other hand, a number of copper ions will join the copper bar in the state of metallic cop- per, rendering the bar electro-positive and leav- ing the copper sulphate solution electro-nega- tive. Equilibrium will rtisue when the excess of solution-tension in the case of zinc and the excess of osmotic pressure in the case of co|)|)er arc exactly counterbalanced by the electrostatic forces as explained above. Now, if we should