Potentials compensation

Contrary to reversible cells, this reaction proceeds, though at a very limited rate, even when the electrodes are not connected and no current is flowing through the cell. If its EMF is compensated in a potentiometric connection by an equal potential of opposite direction, the zinc dissolution continues. Should the compensating potential exceed the EMF of the cell, the above mentioned reaction will not proceed in the opposite direction but a new process appears instead, during which the copper is dissolved and the hydrogen is evolved on the zinc electrode ... 

If two metals of differing work function are connected electrically, at the same temperature and without a source of emf, the electrostatic potentials just outside the two surfaces are different. This potential difference V12 is known as the contact potential difference and is equal to the difference in the work functions of the two metals and < 2- compensating potential, equal and opposite, is applied... 

Farrell J. R. and McTigue P. (1982), Precise compensating potential difference measurements with a voltaic cell—the surface potential of water , J. Electroanal. Chem. 139, 37-56. 

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. 

Fig. 8.11 Experimental setup for determining the Volta potential difference between two metals a and p. The compensation potential is determined with potentiometer P by moving a with respect to P until there is zero current in galvanometer G.

A similar equation can be written for the calomel system on the right-hand side, which involves Hg and the HCl solution, which has the same concentration as the solution on the left-hand side. It follows from equation (8.7.11) that the compensation potential difference also measures the difference between the electrochemical potentials of the Cff ions in the two HCl solutions ... 

When the compensation potential is determined using the Kenrick apparatus, the mercury streams down the center of the vertical tube and the HCl solution down its walls so that the Volta potential difference across the air gap is eliminated. In this system both the concentration of HCl and the pressure of hydrogen gas can be varied. It differs from cells (8.7.8) and (8.7.16) in that no attempt is made to balance the charge carriers on opposite sides of the cell. The Galvani potential difference measured between the two copper leads can be related to the electrochemical potentials of electrons in the mercury and platinum as follows ... 

Equation (8.7.34) shows that the compensation potential depends on the hydrogen ion activity in the electrolyte solution. Strictly speaking, single ion activities are not subject to experimental determination. However, if the solution is sufficiently dilute, the single ion activity a + may be estimated by the mean activity of the electrolyte to a good approximation. 

Values of the work function for simple ions have also been measured in non-aqueous solutions [16]. These experiments involve measuring the compensation potential in a cell such as... 

Only the Volta potential difference sAmV / can be measured experimentally. For example, using the Kenrick experiment illustrated in fig. 8.12, one can measure the value of sA]ji / between liquid metals like mercury and electrolyte solutions. In the case of solid metals, the cell used to measure the compensation potential can have the same configuration as that for the Kenrick experiment (cell (8.7.9)). However, the air gap is formed between a metal disk placed a small distance above the solution phase. The potential difference between these phases is eliminated using ionizing alpha radiation from a radioactive substance placed in the gap. The end result is that the Volta potential difference is obtained for the cell. 

If the Volta potential gap across the air gap is maintained at zero, then the potential drop across the cell is a compensation potential as described in section 8.7. Thus, one may write... 

The other method used for liquid surfaces is the flow method of Kenrick (14) in which a jet of one solution is passed down the center of a tube whose walls carry a flowing layer of a second solution. The potentials between the flowing liquids are monitored with a quadrant or other electrometer. This method has been used with good results by Randles (15) and Parsons (16). Case and Parsons (17) compared the Kenrick and radioactive electrode methods for methanol-water mixtures. They found good agreement except at elevated methanol concentrations where methanol adsorption at the air electrode probably occurs. Measurement of the null current (compensation) potential in the Kenrick method is suitable for determining the surface potentials of solutions where rapid surface equilibrium occurs, but it is not convenient for spread monolayers or adsorbed films that have slow time effects. 

Case and Parsons measured the compensation potential between an aqueous solution and an organic solvent and directly obtained ""A ai where... 

The Donnan equilibrium allows the evaluation of the distribution of M and X" over both sides of the semipermeable membrane. If electrodes responding to either M+ or X" were inserted at either side of the membrane there would be no potential difference between them. This is a consequence of the system being in equilibrium which implies that no work can be performed. Nevertheless, because of the different ion concentrations, the potentials at the respective electrodes i/o,i and /o n are not equal. Consequently, there must be a compensating potential difference across the membrane, Axi/. If the electrodes respond reversibly to the ion concentrations so that Nernst s law (Equation 9.14) applies, it follows for the membrane potential... 

The earthworm Eisenia foetida) is definitely not suitable for determining the ecotoxicity of materials that are digestible by the animals. The feeding effect will result in an increased growth, which may more than compensate potential inhibition effects [65]. 

---