CHAPTER XIIL Calculation of Electromotive Forces.
Introduction. — We have treated in the preceding chapters, with the aid of the theory of electrolytic dissociation, of the several physical and chemical properties of homogeneous electrolytic solutions ; and we have developed the laws which regulate the equilibrium which obtains between two phases of a heterogeneous system. We now pass on to the con- sideration of the free energy w^hich can be obtained when an electrolyte passes from one solution to another, or from one phase to another, and shall study particularly those cases in which the transport of material is associated with a transport of electricity. In such cases the whole of the mechanical energy may be transformed into electric energy, and the latter can be very easily estimated by measuring the electromotive force produced simultaneously with the mass transport.
This mass transport (or transport of material) may consist partly in removing ions from one solution to another, and partly in the separation of the ions 'at the electrodes. The ions are always accompanied by their electric charges, but when they separate at the electrode they are quickly trans- formed into uncharged molecules, and give up their electricity to the electrode. In practice, this latter process is by far the more important, although the theory has been most completely developed for the former.
The greatest progress in the theory of this subject has been made by Helmholtz and Nemst. By applying the second law of thermodynamics, Helmholtz showed the connection between
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