Potential Measurement Reference Electrodes and Electrometers Ref

The potentials, relative to the standard hydrogen electrode, of several half cells used as reference electrodes are given in Table 6.1 (the section Examples of Half-Cell Reactions and Nemst-Equation Calculations in Chapter 2 provides discussions of half-cell-potential calculations). There are a number of factors that contribute to the selection of components and to the design of a satisfactory reference half cell  

It is noted in Fig. 6.1 that the experimentally measured potential, as measured against any given reference electrode (e.g., the saturated calomel electrode, SCE), is denoted as Eexp meas ref. When converting this potential to the standard hydrogen electrode scale (SHE), the following relationship applies  

In Chapter 4, in the section Relationship between Interface Potentials and Solution Potentials, E (or EM) is defined relative to the potentials, ( ), as follows  

Obviously, if the RE could be placed at the WE surface, then 4 s,re = 4 s,we and E = Eexp- otherwise, E Eexp. 

Now consider the relationship between ( )s RE and ( )s WE. With reference to Fig. 6.7(a), consider an anodic external current, Iex a. In the solution, this current flows from the higher solution potential at the WE surface, ( )s WE, past the RE, to the lower solution potential at the AE surface. The solution potential at the RE location is ( )s RE. A simple case is assumed in which the current distribution in the solution is uniform, leading to a linear solution-potential gradient. The potential difference in the solution between the WE surface and the RE position is Iex aR s where R s is the solution resistance (ohms) between the WE and RE. From the geometry in Fig. 6.7(a)  

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