FIGURE 11.2 Pourbaix diagram for the iron-water system at 25°C (after [10]).

In Fig. 11.2 the potential scale (y axis) is given as a measure (in volts) versus the standard hydrogen electrode (SHE). It should be noted that all electrode potentials are quoted against a given reference electrode. The potential established when a metal is immersed in a solution arises from the release of positively charged cations together with the creation of a negatively charged metal. The potential difference created cannot be measured in absolute terms, but what may be measured is the potential difference between it and another electrode, the latter being a reference electrode (of designated electrode potential). The SHE is designated as having an electrode potential of 0.0 V. Other reference electrodes such as the silver/silver chloride and saturated calomel reference electrode have assigned potential values measured with respect to the SHE, in this case being 0.248 and 0.242 V at room temperature, respectively.

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