Half-cell potential significance

However, potentiostatic circuits are not always necessary. A common situation is found in the use of ultramicroelectrodes, UMEs, in which the cell current is in nanoamperes and smaller due to the small size of the WE [3]. With such small currents, even small reference electrodes are used as counterelectrodes without fear of significant perturbation of their half-cell potential. The potentiostat of Fig. 8 can accommodate this case by connecting both the AE and RE leads together. Note that this converts the potentiostat into a simple voltage follower. UMEs are considered further in Chapter 2.5 and Sect. 2.1.4.1 of Chapter 2.1. 

Corrosion rate measurement is slow compared with half cell potential measurement. This is because the concrete react.s sloivly to the electric field and changes must be reasonably slow to ensure that the electrochemistry in the concrete is changing linearly and without capacitance effects. However, the speed of the total operation varies significantly from device to device. The slowest devices arc manually operated and take 10-20 minute. to take a reading that then must be manually plotted to calculate the corrosion current. The fastest microprocessor controlled devices take less than five minutes and give the corrosion current directly. 

When using the half-cell potential criterion as developed through the E-log I method, there is a risk that there will be times when the cathodic system will not completely control the corrosion of the steel. For example, if the concrete is near saturation, the steel can usually be polarized with relatively small current densities. Then, if the rectifier is regulated by a halfcell potential and the concrete dries so that oxygen becomes abundant and the polarized potential drifts significantly less negative, it is likely there will be insufficient current capacity to raise the potential to the protective potential value. 

The half-cell potentials of metal ions, e.g. Cu/Cu, are of great significance in inorganic chemistry. If metals are arranged according to increasing potentials, the result is the familiar electromotive series. 

Saheb et al. described an AglAg+ reference electrode (for use in an imidazohum-based ionic hquid) in which the concentration of silver ion was estab-Ushed by addition of silver(I) nitrate in acetonitrile [27]. A problem noted with the use of this system is that the potential of the reference half cell varies significantly with the volume fraction of acetonitrile, which is likely to vary after extended... 

The first standard potential is for a solution saturated with I2, which has an 12(09) activity significantly less than 1. The second potential is for a hypothetical half-cell in which the 12(09) activity is unity. Although the second cell is hypothetical, it is nevertheless useful for calculating electrode potentials for solutions that are undersaturated in L. 

Five examples are given of the application of the Nemst equation to half-cell reactions. These examples illustrate the influence of ion concentration, pH, precipitate phases, and complex-ion formation on the electrode potential. All of these variables have significance in aqueous corrosion ... 

The calculated potential is reasonable given the potentials of the half-cells. E° is reported to the correct number of significant figures. 

You ve heard electrochemistry of corrosion as a lecture I shouldn t spend much time on it but I d like to describe some electrochemical effects for film formers. First the general principles. If you put a good electronic conductor (a metal) in an aqueous solution, you will typically find that an electrical potential is developed between the piece of conductor and the solution. When ions of the metal enter the solution and leave extra electrons behind a negative potential is developed. All oxidation reactions occurring on the surface are expected to produce this result. Similarly, reduction reactions that use electrons from the metal are expected to produce a more positive potential in the metal. The solution potential of the metal influences the rate of an electrochemical half-cell reaction in accordance with Le Chatelier s Principle, so it is possible to predict through the use of the Nernst Equation the potential that will exist when the only significantly rapid reactions are the oxidation and reduction parts of a reversible reaction. When more than one potentially reversible process occurs, the rate of oxidation will be expected to exceed the rate of reduction for at least one and the converse for at least one. At... 

As the technique is slower than taking half cell measurements it is important to take measurements at the mo.st significant locations on the structure, e.g. by follo ving up a potential survey with, strategic corrosion rate measurements. Rate measurements should be taken at the position. of the highest and lowest potentials and at the steepest potential gradient. . 

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