The stability field of water

Key point: The stability field of water shows the region of pH and reduction potential where couples are neither oxidized by nor reduce hydrogen ions. A reducing agent that can reduce water to H2 rapidly, or an oxidizing agent that can oxidize water to O2 rapidly, cannot survive in aqueous solution. The stability field of water, which is shown in Fig. 5.3, is the range of values of potential and pH for which water is thermodynamically stable towards both oxidation and reduction. The upper and lower boundaries of the stability field are identified by finding the dependence of E on pH for the relevant half-reactions. As we have seen above, both oxidation (to O₂) and reduction of water have the same pH dependence (a slope of 0.059 V whenE is plotted against pH at 25C) and the stability field is confined within the boundaries of a pair of parallel lines of that slope. Any species with a potential more negative than that given in eqn 5.9 can reduce water (specifically, can reduce H) with the production of H₂ ; hence the lower line defines the low-potential boundary of the stability field. Similarly, any species with a potential more positive than that given in eqn 5.10 can liberate O₂ from water and the upper line gives the high-potential boundary. Couples that are thermodynamically unstable in water lie outside (above or below) the limits defined by the sloping lines in Fig. 5.3: species that are oxidized by water have potentials lying below the H₂ production line and species that are reduced by water have potentials lying above the O₂ production line.

The stability field in ‘natural’ water is represented by the addition of two vertical lines at pH=4 and pH=9, which mark the limits on pH that are commonly found in lakes and streams. A diagram like that shown in the illustration is known as a Pourbaix diagram and is widely used in environmental chemistry, as we shall see in Section 5.14.

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مواضيع ذات صلة

Symmetry operations, elements and point groups

Standard potentials and spontaneity

Heterogeneous acid–base reactions

Superacids and superbases

Acidic and neutral solvents

Basic solvents

Solvents as acids and bases

Interpretation of hardness

The classification of acids and bases

Halogens as Lewis acids

Group 14 Lewis acids

Group 13 Lewis acids