Pourbaix diagram for a metal showing two valence states, and

1 Pourbaix diagram for a metal showing two valence states and 

The upper left-hand corner of the diagram represents conditions that are oxidising and acidic. Under these conditions the higher valence state, is the stable form. The region over which this remains true is called the stability field of the ions. [Note that, in reality, the stable species is generally a hydrated ion such as M(H20)g (aq).] 

As the pH increases, that is, as the acidity decreases, one moves towards the right-hand side 

The change in oxidation state is revealed by the half-reaction, which also confirms that there is no change in acidity. At some point, a boundary between the stability field of and is crossed. The boundary between the two stability fields is horizontal. Horizontal boundaries separate stability fields that involve a change in oxidation state (e transfer) and no change in acidity (H or OH concentration). 

A continued reduction in the oxidising potential, moving further towards the lower left-hand side of the diagram, causes the A/ + ion to be replaced by more stable metal, M°. The boundary between the stability fields for M + and is horizontal, for the reason given above. 

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