The Na E-pH Diagram

The E (vertical) axis is a reflection of the potential values in volts (v) of reduction half-reactions describing the conditions under which changes in the aqueous oxidation state of the element occur. These E values range from -1-3.00 V to —4.00 V. The pH (horizontal) axis gives pH values ranging from a pH of —1.0 (10 molar hydrogen ion) to a pH of 15.0 (10 molar hydrogen ion). The sloped dashed lines have to do with the behavior of the solvent water. This will be discussed in detail later. 

the question arises as to the origin of the horizontal line at about —2.7 V. This is obtained from the reduction half-reaction which relates the two species (Na+ and Na) on the two sides of the line. A reduction half-reaction is one in which the electron or electrons appear on the left side of the equation. 

In this equation, e stands for the electron, Na+ for the sodium ion, (aq) for the aqueous state, Na for elemental sodium, (s) for the solid state, and E° represents the standard electrode potential given in volts (v). The superscript ° on E indicates that the reaction is taking place under standard conditions (T = 298 K, Na+ concentration of 1.00 M). Values of E° may be readily obtained from tables in reference works such as A. J. Bard, R. Parsons, and 

Jordan D. R. Lide and J. A. Dean. The potential for a half-reaction, E° or E, may be thought of as the driving force for the electron or electrons in the reaction. The horizontal line at —2.71 v represents this reduction halfreaction. In the region below this line, at E° values more negative than —2.71 v, the half-reaction proceeds to the right such that the predominant species is Na(s). In the region above this line, at E° values more positive than —2.71 v, the reaction proceeds to the left. As a result, in the region above this line the predominant species is Na+(aq). 

00 M to 0.10 M. The E° value is therefore not applicable and must be changed to an E value. The half-reaction now reads 

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