Galvanic Cells without Liquid Junctions

Some galvanic cells without liquid junction... 

Standard Electrode Potentials from Galvanic Cells without Liquid Junctions... 

The Standard Potential of Chlorine. Measurements of the potentials of galvanic cells without liquid junctions from which the standard potential of chlorine may be deduced have been made by Lewis and Ruppert40 who used, as one electrode, platinum over which a mixture of chlorine and nitrogen was bubbled, and, as reference, a calomel electrode and hydrochloric acid as electrolyte. The arrangement may be represented by... 

The Variation of the Standard Potentials of Some Electrodes with the Temperature. In a number of cases the standard potentials of galvanic cells without liquid junctions have been determined over a range of temperatures. From these determinations it has been possible to prepare Table V, which gives the standard potentials of a number of electrodes at intervals of 12.5° from 0° to 50°. Some slight adjustments, of the order of 0.2 millivolt, of the original data have been necessary to bring the figures into accord with the Ho values at 25° adopted in this book. A more complete table of standard potentials of the elements at 25° will be found at the end of Chapter 14. 

Because of the necessity, with galvanic cells without liquid junctions, of finding two reversible electrodes, one for a positive and one for a negative ion constituent, most of the recent work in this field has been carried out with cells of the type ... 

Table Vll. A Comparison of Ionization Constants, Kq, at 25°, as Determined from Conductance Measurements and from the Potentials of Galvanic Cells without Liquid Junctions...

The theory developed here has its most important application in the study oi reversible galvanic cells. Galvanic cells may be separated into two classes those with and those without liquid junction. In Sec. 13-2, we consider a galvanic cell without liquid junction using the necessary criterion for equilibrium [Eq. (13-23)]. In Sec. 13-3, we discuss a cell with liquid junction in terms of the complete conditions for heterogeneous equilibrium. 

In this section we apply the general results of Sec. 13-1 to a treatment of galvanic cells without liquid junctions. A representative example of cells without liquid junction is the following one, written in terms of conventional notation ... 

In Eqs. (122) and (123), M(Hg) is an alkali metal amalgam electrode, MX the solvated halide of the alkali metal M at concentration c in a solvent S, and AgX(s)/Ag(s) a silver halide-silver electrode. Equation (124) is the general expression for the electromotive force " of a galvanic cell without liquid junction in which an arbitrary cell reaction 0)1 Yi + 0)2Y2 + coiYi + , takes place between k components in v phases. In Eq. (124) n is the number of moles of electrons transported during this process from the anode to the cathode through the outer circuit, F the Faraday number, and the chemical potential of component Yi in phase p. Cells with liquid junctions require the electromotive force E in Eq. (124) to be replaced by the quantity E — Ej), where Ey> is the diffusion potential due to the liquid junction. The standard potential E° for the cell investigated by Eq. (122) is given by the relationship... 

Figure 14.1 A galvanic cell without liquid junction.

E ceii, eq Can be measured with great precision. If a reaction can be carried out in a galvanic cell without liquid junction, Eq. 14.3.13 provides a way to evaluate ArG under given conditions. If the reaction can only be carried out in a cell with a liquid junction, Eq. 14.3.14 can be used for this purpose provided that the liquid junction potential Ej can be assumed to be negligible or can be estimated from theory. 

The state of a galvanic cell without liquid junction, when its temperature and pressure are uniform, can be fully described by values of the variables T, p, and Find an expression for dG during a reversible advancement of the cell reaction, and use it to derive the relation ArGceii = -zFEceiieq (Eq. 14.3.8). (Hint Eq. 3.8.8.)... 

Separate each of the following reactions into its half-reactions and in each case write down the schematic representation of a galvanic cell in which the reaction would take place. Wherever possible devise a cell without liquid junction potentials. 

In recent years much work has been carried out, particularly by H. S. Harned and his associates, on concentration cells without liquid junction for the purpose of obtaining ionization constants of weak electrolytes. The principle involved in these investigations is as follows. Galvanic cells are set up of the form ... 

In Chapter 10 standard potentials were obtained from measurements on galvanic cells involving only one electrolyte. These cells without transference thus do not involve surfaces between solutions of electrolytes, more commonly called liquid junctions. Although the measurements on cells without liquid junctions can be much more readily interpreted than the results from cells containing such boundaries most of the earlier work was carried out with the latter type of cell. Thus for instance instead of measuring the potential of the cell ... 

Now imagine a reaction vessel that has the same temperature and pressure as the galvanic cell, and contains the same reactants and products at the same activities as in the cell. This reacrion vessel, unlike the cell, is not part of an electrical circuit. In it, the reactants and products are in direct contact with one another, so there is no constraint preventing a spontaneous reaction process. This reaction will be called the direct reaction. For example, the reacrion vessel corresponding to the zinc-copper cell of Fig. 14.2 would have zinc and copper strips in contact with a solution of both ZnS04 and CUSO4. Another example is the slow direct reaction in a cell without liquid junction described on page 453. 

So far, a cell containing a single electrolyte solution has been considered (a galvanic cell without transport). When the two electrodes of the cell are immersed into different electrolyte solutions in the same solvent, separated by a liquid junction (see Section 2.5.3), this system is termed a galvanic cell with transport. The relationship for the EMF of this type of a cell is based on a balance of the Galvani potential differences. This approach yields a result similar to that obtained in the calculation of the EMF of a cell without transport, plus the liquid junction potential value A0L. Thus Eq. (3.1.66) assumes the form... 

FIGURE 22-2 A galvanic cell without a liquid junction. 

One often wishes to describe a galvanic cell without taking the trouble to draw an actual picture of it. The notation used is a rudimentary diagram of the cell. Phase boundaries (between solid and liquid, liquid and gas, or solid and gas) are indicated by a liquid junction is represented by, a salt bridge by. The cells previously discussed may be represented thus (with spaces for filling in the concentrations) ... 

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