Electrode solution pressure

We now consider briefly the matter of electrode potentials. The familiar Nemst equation was at one time treated in terms of the solution pressure of the metal in the electrode, but it is better to consider directly the net chemical change accompanying the flow of 1 faraday (7 ), and to equate the electrical work to the free energy change. Thus, for the cell... 

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

The two electrode potentials Fi and F3 are according to the Nemst conception of electrolytic solution pressure given by the expression... 

An apparently unresolved issue from the above analysis is the breakdown of the analogy, say, between carbon and metal electrodes for the latter (e.g., ZnlZn2+), it is easy to define an electrolytic solution pressure and an osmotic pressure, as the pioneers of electrochemistry (Nernst, Ostwald, Arrhenius, and Le Blanc) have done (see books.google.com) more than a century ago but I am aware of no such discussion (e.g., CIC+) on carbons in which the exact nature of C+ would have been addressed in those terms. 

On the other (copper) electrode the electrolytic solution pressure is lower than the osmotic pressure of the cations in the solution and therefore cupric ions from tho solution are deposited, thus giving the metal a positive charge, while the solution becomes negative due to the excess of anions (SO ). Both kinds of charges Cu++ and SO - are attracted and form again the electrical double layer. In this case, however, the double layer has an opposite effect than at the zinc electrode as it facilitates the transfer of the cupric ions from the electrode to the solution and prevents them being transferred in the opposite direction. Equilibrium will be attained, when the electrostatic forces of the double layer and the solution pressure of copper together will counterbalance the osmotic pressure of the cupric ions in the solution. 

Such electrodes differ from metalic ones only in the fact that the solution due to the negative charge of the ions formed gets negatively charged against the electrode, when the electrolytic solution pressure of the element is greater than the osmotic pressuro of its anions in the solution. 

Assuming that (a) the rate of combination between hydrogen atoms in the adsorbed layer is proportional to the square of their concentration, cH there, and (6) the potential difference between electrode and electrolyte is a linear function of the concentration of hydrogen atoms on the surface, in lormal analogy with the Nemst solution pressure hypothesis, then... 

C is a constant which is characteristic of the metal of which the electrode is composed, and is sometimes called the electrolytic solution pressure. Its numerical value is equal to the ionic concentration of a solution against which the metal would have no difference of potential. This quantity is of the greatest importance for the electrochemical behaviour of the metal. It cannot be determined, however, by measurement with concentration cells, for the solution pressure C disappears from the sum of the various potential differences in equation (5). The calculation of C from the total e.m.f. would be possible if we could choose the ionic concentration of one solution, say Cg, so that the potential difference 3 would be zero. Numerous experiments have actually been carried out with the object of constructing an electrode which would have the absolute potential zero against the solution. These experiments, although in themselves interesting and important, are based on special electrochemical hypotheses and not on purely thermodynamical principles. They are therefore beyond the scope of this book. ... 

Electrochemistry of Dilute Solutions 1 Hernsl s Theory of the Solution Pressure of an Electrode. 

It is very important to keep valve (26) in Figure 12 tightly closed during the whole elution. Otherwise the acidic or basic electrode solution will leak out of the electrode compartment. This will destroy the possibility of accurate pH measurements of the fractions which come out of the column. To prevent the leakage of electrode solution out of the central tube (19), the contents should be sucked out of the tube with the aid of a syringe and a capillary tube in order to achieve a pressure slightly under atmospheric in the tube. 

The rate of an electrode process directly depends on magnitude, of the potential drop at the electrode-solution interface. Therefore, while determining the activation energy from the temperature dependence of the reaction rate, we would have to maintain constant not only such common independent variables as pressure P and concentration m, but also ... 

In 1889 Hermann Walther Nemst (1864-1941) gave a theoretical treatment of the electrode potential set up between a metal and a solution of its own ions. He regarded a metal electrode as exerting a solution pressure which is a measure of the tendency of the metal atoms to leave the electrode and enter the solution as ions. At equilibrium, this is balanced by the osmotic pressure of the ions in solution. Nemst derived an equation linking the magnitude of the electrode potential with the ionic concentration and the absolute temperature. 

---