The salt bridge

The double vertical slash ( ) indicates the salt bridge, the contents of which are normally not indicated. Note that the double vertical slash implies that there is a potential difference between the salt bridge and each half-cell. 

A typical Ag/AgCl electrode is shown in figure 11.9 and consists of a silver wire, the end of which is coated with a thin film of AgCl. The wire is immersed in a solution that contains the desired concentration of KCl and that is saturated with AgCl. A porous plug serves as the salt bridge. The shorthand notation for the cell is... 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

The salt bridge allows ions to pass from one solution to the other to complete the circuit and prevents direct contact between Zn atoms and the Cu2+ ions. 

A voltaic cell using this reaction is similar to the Zn-Cu2+ cell the Zn Zn2+ half-cell and the salt bridge are the same. Because no metal is involved in the cathode half-reaction, an inert electrode that conducts an electric current is used. Frequently, the cathode is made of platinum (Figure 18.3, p. 484). In the cathode, Co3+ ions are provided by a solution of Co(N03)3. The half-reactions occurring in the cell are... 

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

FIGURE 12.5 The cell potential is measured with an electronic voltmeter, a device designed to draw negligible current so that the composition of the cell does not change during the measurement. The display shows a positive value when the + terminal of the meter is connected to the cathode of the galvanic cell. The salt bridge completes the electric circuit within the cell. 

The liquid junction potential from the organic side may be negligible, owing to the use of a nitrobenzene-water partition system containing tetraethylammonium picrate as the salt bridge. The mobilities of both ions in nitrobenzene are similar, and they have similar Gibbs energies of... 

D From the copper cathode to the salt bridge This question covers NSCS B3. This question tests the material that was covered in the textbook on pages 664-665. 

It is relevant to present here some preliminaries as regards the salt bridge, this being a traditionally used and more convenient way than the porous partitioning medium in setting up a laboratory assemblage of an electrochemical cell. In this premise, attention is focused on the line formulae of the two cells as presented below ... 

Fig. 2.13 Examples of liquid junctions (A) liquid junction with free diffusion is formed in a three-way cock which connects the solution under investigation with the salt bridge solution (B) liquid junction with restrained diffusion is formed in a ceramic plug which connects the salt bridge with the investigated solution...

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

Ans. No. If the Daniell cell were to be recharged, the Cu2t ions would get into the zinc half-cell through the salt bridge. There, they would react directly with the zinc electrode, and the cell would be destroyed. 

Ans. In a reaction at equilibrium, the ratio can have only one value at any given temperature. In the Nernst equation, the value can change, since the reaction can be stopped short of equilibrium simply by disconnecting a wire or the salt bridge. 

In the presence of bromide ions the electrode was subject to a drop in potential, (e.g., 1.5 to 5.7 mV at a Br iCl ratio of 2000 3) and to delayed response. A considerable hysteresis effect is also observed in concentrated solutions of chloride when the electrode is used in a 1M chloride solution and then dipped in one that is 0.02 M. Equilibrium is reached only after 10 min. The junction potential is minimised by diluting the test solution with the salt-bridge solution (10% aq. potassium nitrate). 

U. C. Singh, Probing the salt bridge in the dihydrofolate reductase-methotrexate complex... 

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