Argentum Solutions, Inc.
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David C. Silverman |
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Graph of Potentials of Common Reference Electrodes Each reference electrode has its own equilibrium potential. Sometimes, required information is reported using different reference electrodes. The challenge is to convert the measured potential based on one reference electrode to that based on another. This figure shows relative position of the potential of each reference electrode discussed in this tutorial) .
The temperature is 25°C. Similar plots could be made at other temperatures.
A potential relative to a secondary reference electrode can be converted to the standard hydrogen electrode scale by adding the reference electrode voltage itself (assuming that value is relative to the standard hydrogen electrode). The equation is 
(29)
The equation to convert from one reference electrode scale to another is 
(30)Equation (30) says that the potential relative to the second reference electrode is equal to the potential relative to the first reference electrode plus the difference between the potentials of the two reference electrodes, the potential of the second reference being subtracted from the potential of the first. The way to look at the procedure is that a position on a line is being determined relative to another position on the same line. The procedure only works if all of the reference electrode potentials to be converted are versus the same reference electrode initially, for example the standard hydrogen electrode. The following examples show how to use the equations. Example 1 Suppose a potential of 0.1 volts is measured relative to the silver/silver sulfide reference electrode at sulfide activity of 3x10-4 at 25°C. That reference electrode has a potential relative to the standard hydrogen electrode of about -0.61V (SHE) accounting for the sulfide activity. To convert the measured 0.1 volts to the standard hydrogen electrode scale, the measured 0.1V is added to the -0.61V to obtain -.051V. The measured potential is about -0.51V relative to the standard hydrogen potential. Example 2 Suppose the desire is to convert the above measured 0.1 volts to the silver/silver chloride (saturated KCl) scale. That latter electrode has a potential of +0.199V relative to the standard hydrogen electrode. The answer is -0.709 V(vs Ag/AgCl(sat KCl)) (0.1 + (-0.61 - 0.199) = -0.709). If one measured a potential of 0.1 volts versus Ag/AgCl (saturated KCl), the potential would translate to +0.909V relative to the same silver/silver sulfide electrode (0.1 + (0.199-(-0.61) = 0.909). |
David C. Silverman, Ph.D. - Primary Consultant |