Batteries standard equilibrium potential

PbO and Pb304 are oxides of lead that are important primary products. Under certain circumstances, PbO is also observed in lead-acid batteries (cf. Sec. 4.4.5.1). Fig. 1 shows that these oxides are only stable in a neutral or alkaline environment. Their equilibrium potentials are represented by curves C-F and their standard values are compiled in Table 3. 

This equation has a standard electrode potential of -0.447 V. Thus, the solution containing mercuric and chloride ions in contact with iron forms a battery. The reduction of the complex ions to metallic mercury is the cathodic reaction. The dissolution of iron is the anodic reaction. The overall reaction in the battery is given by the addition of Equation (13.42) and Equation (13.43). Due to the high value of its reversible cell voltage under standard conditions (0.85 V), it is expected that a very low equilibrium concentration of the complex ion can be achieved. 

All equilibrium potentials are referred to a standard hydrogen electrode. The Hg Hg2S04 electrode is widely used in lead-acid battery investigations. This electrode has a potential which is 0.620 V more positive than that of the standard hydrogen electrode at pH = 0 [18]. 

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