Calculations based on the Nernst equation

42 CALCULATIONS BASED ON THE NEAREST EQUATION Values of standard potentials are normally available for room temperatures (25°C), like those shown in Tables 1.16 to 1.18. Calculations based on the Nernst equations are therefore usually restricted to this temperature. The pre-logarithmic factor of the Nernst equation can thus be simplified, and 10-based logarithms can be introduced. With R = 8 3143 J K-1 mol-1, T = 298 15 K, 

The following examples may illustrate the ways in which oxidation-reduction potentials can be calculated. 

Example 31 Calculate the e.m.f. of a Daniell cell, made up by immersing a copper foil into a 0 1 5m CuS04 solution and a zinc rod into 0 25m ZnS04 solution and linking the two half-cells together. 

According to Table 16 the values of the standard potentials are as follows  

Thus first the two electrode potentials have to be calculated. These are 

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Detailed analysis of these data, and data collected at the same wells on different dates, will be published elsewhere. Data are presented here only to illustrate the kind of electrode response which can be expected in more complicated redox environments. The use of the two types of redox electrodes again illustrates the utility of a dual electrode approach to ORP measurements. Substantial differences between Pt and WIG electrodes indicate electrode interferences which may obviate calculations based on the Nernst equation. 

The half-cell equilibrium potentials are calculated based on the Nernst equation. 

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