The Al E-pH Diagram

As one scans down the vertical line at a constant pH of 7.0, the predominant species Al(OH)3 is replaced by the predominant Al at an E value of about — 1.9 V. The reduction half-reaction along with its E° value is as follows  

Similar observation of the vertical line at a constant pH of 14.0 shows the transformation from Al(OH)4 to Al as the predominant species at an E of 

Values of E° cannot be used for the last two reactions as given because the H concentration (pH) in both cases is not the standard value of 1.00 M. Hence the Nernst equation must be used to ascertain the applicable values of E when [H+] is 10 ° M inthe A1(0H)3 to A1 reaction and 10 ° Minthe Al(OH) reaction. 

These values of —1.88 v and —2.33 v match the values of —1.9 v and —2.3 v which were estimated from the diagram. Please note also that the line between A1(0H)3 and A1 and the line between Al(OH)4 and A1 are both sloped. This behavior indicates that the lines are functions of both E and pH. This is obvious by virtue of the presence of [H ] in both equations. 

It is to be noticed that both of these equilibrium constants are protonation constants, since they apply to the addition of a proton (H+) to a given species. The pH values for which these reactions occur can be determined using equilibrium constant expressions and the appropriate equilibrium constant, K. Recall that equilibrium expressions take the general form of  

---