





RESOURCES > REF ELECTRODES > SILVER/SILVER SULFATE Environmental concerns over mercury contamination prompted (or forced) many to look for alternatives to popular mercurybased reference electrodes. The SCE electrode is being replaced by the Ag/AgCl electrode. The silver/silver sulfate electrode is a possible replacement for the chloridefree mercury/mercurous sulfate reference electrode. The sparingly soluble salt Ag_{2}SO_{4} can serve as the basis for a tolerable reference electrode. The electrode reaction is shown in Eqn 1 and the corresponding Nernst equation is shown in Eqn 2. Ag_{2}SO_{4}
(s) == 2 Ag (s) + 2e^{} +SO_{4}^{=}
[Eqn 1] where E^{o}_{1} is the Standard Potential for the reaction shown in Eqn 1, and a_{SO4=} is the activity of sulfate. Ives and Janz (Ref 1) mentions this electrode, but does not give its potential. They do remark, though, that the solubility is rather high, and that its solubility must be taken into account if dilute sulfate solutions are used as filling solutions. 



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Lietzke and Stoughton ( Ref 2 ) examined the cell Hg,Hg_{2}SO_{4} H_{2}SO_{4} (aq) Ag,Ag_{2}SO_{4} [Eqn 4] for sulfuric acid concentrations between 0.05 and 0.5 M. The alert reader will notice that for the reaction shown in Eqn 5 the sulfate concentration does not appear in the Nernst equation (Eqn 6)! All of the reactants and all of the products are present at unit activity since they are either elements or solid precipitates in equilibrium with the aqueous phase. 2Hg (s) + Ag_{2}SO_{4} (s) == Hg_{2}SO_{4} (s) + 2Ag (s)
[Eqn 5] where E^{o}_{5} is the Standard Potential for the reaction in Eqn 5. They calculated E^{o}_{5} from the tabulated Gibbs Free Energies of the reactants and products (E=nFΔG) and arrived at +0.042V. More recently, Ruetschi (Ref 3) suggested the use of Ag_{2}SO_{4} electrodes in leadacid batteries. They report that the Ag_{2}SO_{4} electrode
The potentials of the Hg/Hg_{2}SO_{4} reference electrode can be combined with these calculated/reported differences to arrive at the potentials listed in the table, below. Because the sulfate activity does not affect the difference between Agsulfate and Hgsulfate electrodes (see Eqn 6), the same 0.04 V difference can be used for all filling solutions. Because of the uncertainties of the corresponding Hg electrode and the difference between the two authors' calculated/reported values noted above, the values in this table have been rounded to the nearest 10 mV.





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