Question

mercury would seem to preclude its use, there are many who find it to be a much more reliable electrode, and may keep one around as a reference standard. When using a standard that is not the SHE, one would have to adjust for the potential difference. Often it's hard to remember whether the value should be subtracted or added. The easiest way to reference the potential vs. different electrodes is to imagine them on a thermometer like scale: +0.400 V +0.300 V SCE (+0.244) AgCl (+0.230) +0.200 V +0.100 V SHE (0.000) -0.000 --0.100 V -- 0.200 V So a compound that measures at -0.100 V vs. SHE would measure at -0.344 vs. SCE or -0.330 V vs. AgCl. And a compound that measures at -0.100 V vs. SCE would measure at +0.144 (0.244 - 100) vs. SHE. Determining Reduction potentials: In order to measure the reduction potential, we have to understand the relationship between amounts of oxidized and reduced forms of a compound and the overall potential of a system. With the thermodynamic relationships between Gibbs free energy, overall potential, and equilibrium, an equation can be derived called the Nernst equation: E = E° RT Ared nF Aox -In This looks a lot like the Henderson-Hasselbalch equation, and in fact it graphs similar (Figure 13.1) The determined e' is sometimes called the midpoint potential, as it is empirically determined under non- standard conditions (e.g. pH = 0). There is a way to linearize this reaction: if we know the value of Aox & Ared reliably, we can calculate the logarithm of their ratio to straight line graph (Figure 13.2)