ELECTRO-CIIEMICAL SERIES. The potential-dif ferences established between metals and their salts are now known for a number of different metals. A detailed explanation of the principles on which they are determined would carry ns somewhat beyond the scope of this article. Suf fice it therefore to state that the starting-point is a determination, by an extremely ingenious method, of the potent ial-differenee between metal lic mercury and solutions of salts, and that the potential-difference lietween mercury and a nor mal aqueous solution of one of its salts has thus been shown to lie 0.99 volt. the metal being electropositive with regard to the solution. (By a 'normal' solution of a salt is meant a solution containing the molecular weight of the salt in grains. See SOLVTION ) . This fact known, there is no difficulty in determining the potential ditTerenee for any other metal and its solutions. Indeed, let the problem be to determine the po tential-difference established when metallic zinc' is placed in a normal solution of zinc. sulphate. To do this, we may construct a cell consisting of a mereury electrode in a normal solution of mercuric sulphate and a zin• electrode in a normal solution of the sulphate of zinc, the solutions separated by a !Porous partition and the two metals connected by a CI ire. By direct determination. the electro-motive force of this cell may be shown to he 1.514 volts; and as the potential-difference between the mercury and its solution amounts to 0.90 volt, it is evident that the potential difference zinc and the normal of its sulphate must be 0.524 volt. In this calculation. the slight potential differen•e between the two solution. has been ne glected. In calculating the figures of the following
table, however, that difference. too, has been taken into account ; and so the figure for zinc, 0.51. is somewhat more precise than our figure rhe table shows the potential-differences between metals and their salt solutions of normal concentration. the plus sign showing that the solution is electropositive with regard to its metal. and the minus sign showing that the solution is electronegative with regard to its metal.
The series is similar to part of the old electro chemical order of Berzelius. (see historical sec tion under CHEMISTRY.) Only. again. while that order was purely qualitative, the modern series represents an exact qualitative expression of the electrical properties of metals and besides refers to a definite concentration of their solutions. zilice the potential-difference between a metal and a solution depends upon the solution-tension of the metal and the osmotic pressure of its ions in the solution. and since both the potential difference and the osmotic pressure can be deter mined, it is evidently possible to find also the solution-tensions. The following table shows the results of such calculations based on the table given above: The figures represent the solution-tensions in terms of atmospheres and may be changed to pounds per square inch by multiplying by 15. The solution-ten:ion of iron, for example. equals a pressure of 15.000 pounds per square inch. The enormous ten-ion of magnesium may serve to indicate how great may be the electrostatic counter-forces long before the amount of metal in solution has Iwcome appreciable.