Experimental Measurement of the Volta Potential Difference at Interfaces

7 Experimental Measurement of the Volta Potential Difference at Interfaces 

A Volta potential difference usually arises between two condensed phases when they come into contact. Because these phases have different chemical properties, charge may be redistributed at their interface with air, even though this interface carried no net charge before contact. Measurement of the Volta potential difference is possible in a high-impedance electrochemical cell which contains an air gap. The potential drop across this cell is determined under conditions for which the potential drop across the air gap is zero, and is known as the compensation potential difference. 

A schematic diagram illustrating the experimental determination of the Volta potential difference between two metals, a and p, is given in fig. 8.11. For this system, electrons are the species common to each phase. The experiment involves an arrangement in which there is an air gap between a and p. On the other side of P, there is a second portion of phase a which is designated ot, so that the system can be described as 

the vertical bars represent phase boundaries according to the usual conventions for describing electrochemical cells (see chapter 9). A variable dc potential 

Because of the electronic equilibrium between phases a and p, the electrochemical potentials of electrons in these phases, p i and are equal. Furthermore, the measurement of aA /ct) is made under conditions that aApv / is zero, so that 

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