Two different metals in contact with a polar or ionic fluid generate the flow of electrons. When touched simultaneously by two different metal­lic rods, muscles contract, a pioneering observation that gave birth to the study of galvanic, voltaic, and Daniel cells.

The potential generated depends on the energy of sublimation, the ion­ization potential, the electronic work function, and the energy of solva­tion of ions. The nature of the solvent influences the last factor. The electronic work function also includes several other conditions of ionic activity. As a result, a potential difference will arise out of a simple con­centration gradient, provided that anionic and cationic stoichiometry is maintained. A review of the existing knowledge is worthwhile here.

If two small baths, each having either Zn or Cu metal and correspond­ing dilute solutions of Zn2+ and Cu2+salts, are in electrical continuity— say through a capillary of a U tube or a Pt wire—then current will flow in the two metals when connected outside, with Cu behaving as a cathode and Zn as an anode (see Fig. 1.3). The setup can also be designed by sep­arating the two systems by a semipermeable membrane.

A similar experience is the cylindrical design of the commonly avail­able dry cells, where a graphite rod at the center serves as a reference

cathode surrounded by a paste of chemicals, usually NH4Cl, totally housed in a small cylindrical cup of metallic Zn as an anode.

In each case, Zn gets oxidized and changes to Zn2+, and Cu2+ is reduced and is deposited as Cu; in the graphite (carbon) electrode, the chemical change is not noticeable. (Theoretically, CH4 should be formed, but slow escape of NH3 takes place.)

The field of electrochemistry has progressed considerably. Standard electrode potentials and electrochemical charts with a fair degree of accuracy and reliability are available. Taking Pt (inert) electrodes, hydrogen gas at 1-atm pressure, immersed in a solution of hydrogen ion of unit activity is usually a reference or standard hydrogen electrode (usually referred as zero or standard scale). If an element goes into a solution, producing cation (Zn ^ Zn2+ = +0.761 V), the half cell will give an oxidation potential with a sign opposite to the potential when the cation of the same species is deposited as the element, giving rise to a reduction (Zn2+ ^ Zn = —0.761 V); the numerical values are expected to remain in the same order.

One may observe, on the other hand, that alkali metals have a ten­dency to become hydrated oxides in water, so they exhibit a tendency to offer oxidation potential with a + sign. When the element approaches nobility, then converts to the halogen (2X— ^ X2 + 2e—), the situation is reversed. A representative partial list of the standard electrode poten­tials is reproduced (see Table 1.3). So one may expect that in a chemi­cal cell with Zn/ZnCl2-CuCl2/Cu, the EMF will be +0.761 —(—0.340) = 1.101 V.

If the electrode pair is made of the same material in a system, and the concentration difference of electrolyte is maintained between the two electrodes, a standard potential difference is expected, at the rate of 0.054 V per each tenfold rise in ionic concentration (referred to as con­centration cells).

TABLE 1.3 Standard Electrode Potentials at 25°C

If Zn is used as a common electrode, or better inert-metal electrodes are used (e. g., Pt) and immersed into NH4Cl or HCl solutions, say 0.1 and 1.0 N, a potential difference of 0.054 V will be experienced. The effect of temperature and other factors which affect ionic activity will definitely alter the values of EMF. The strength of the current will depend, expect­edly, on the total surface area or participation of the total number of ions and their charge-carrying capacities.

Electrochemical behavior of certain elements, e. g., carbon and silicon, must be determined indirectly. Only graphite exhibits direct application in a chemical cell, but other forms of carbon or silicon do not play any significant role at this state of knowledge (see Fig. 1.4).