Zero-Current Electrochemical Cell Potentials—Convention

It is easy to go from one standard potential to the other via a calculation involving the free enthalpy of change in the physical state  

11 Zero-Current Electrochemical Cell Potentials—Convention 

By convention, the zero-current cell potential (formerly electromotive force) is equal, in sign and magnitude, to the potential difference between the cathode and the anode  

This is the potential difference between the electrode on the right and that on the left in the conventional diagrams given above since. By convention, they are written in such a manner that the electrons flow from the left to the right in the external circuit. The convention E = Ec Ea is in agreement with the free enthalpy principle, 

Applying Nernst s law immediately let us calculate the zero-current cell potential, provided the species activities are known. 

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Potentiometry is the measurement of an electrical potential difference between two electrodes (half-ceUs) in an electrochemical cell (Figure 4-1) when the cell current is zero (galvanic cell). Such a cell consists of two electrodes (electron or metallic conductors) that are connected by an electrolyte solution (ion conductor). An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions (see Figure 4-1). One of the electrolyte solutions is the unknown or test solution this solution may be replaced by an appropriate reference solution for calibration purposes. By convention, the cell notation is shown so that the left electrode (Mi,) is the reference electrode the right electrode (Mr) is the indicator (measuring) electrode (see later equation 3). ... 

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