A positive potential is indicative of a spontaneous reaction. The positive potential indicates that the cell reaction is spontaneous in the direction shown. If the conditions of the cell are not standard, the potential of the cell will differ from the standard potential. The most common variation is in the concentrations of the ions. The Nernst equation is used for calculating the potential of a nonstandard cell.
E o is expressed in volts.
The derivation of the Nernst equation is in your text. The Nernst equation can be simplified by combining constants. From the Nernst equation, it can be seen that if the concentration of a reactant is increased, the potential becomes more positive. If the concentration of a product is increased, the potential becomes less positive. This type of cell, where the potential depends on a difference in concentration, is called a concentration cell .