Ag/Ag₂SO₄ Reference Electrode - a Green Alternative?

RESOURCES > REF ELECTRODES >  SILVER/SILVER SULFATE

Environmental concerns over mercury contamination prompted (or forced) many to look for alternatives to popular mercury-based reference electrodes. The SCE electrode is being replaced by the Ag/AgCl electrode. The silver/silver sulfate electrode is a possible replacement for the chloride-free mercury/mercurous sulfate reference electrode.

The sparingly soluble salt Ag₂SO₄ 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.

Ag₂SO₄ (s) == 2 Ag (s) + 2e^- +SO₄⁼    [Eqn 1]
E = E^o₁ - (RT/2F) ln ( a_SO4⁼ )    [Eqn 2]

where E^o₁ 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,Hg₂SO₄| H₂SO₄ (aq) |Ag,Ag₂SO₄    [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) + Ag₂SO₄ (s) == Hg₂SO₄ (s) + 2Ag (s)    [Eqn 5]
 E = E^o₅   [Eqn 6]

where E^o₅ is the Standard Potential for the reaction in Eqn 5. They calculated E^o₅ 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 Ag₂SO₄ electrodes in lead-acid batteries. They report that the Ag₂SO₄ electrode

"has a reproducible and stable electrode potential, situated 38.4 mV positive with respect to that of the Hg₂SO₄ couple."

The potentials of the Hg/Hg₂SO₄ 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 Ag-sulfate and Hg-sulfate 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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REFERENCES
(1) "Reference Electrodes, Theory and Practice", DJG Ives & GJ Janz, Academic Press, NY (1961).
(2) "The Behavior of the Silver-Silver Sulfate and the Mercury-Mercurous Sulfate Electrodes at High Temperatures," MH Lietzke, RW Stoughton, JACS, 75 (1953), 5226-5227. DOI:10.1021/ja01117a024.
(3)  "Silver-silver sulfate reference electrodes for lead-acid batteries" P Ruetschi, J Power Sources, 113 (2003), 363-370. DOI: 10.1016/S0378-7753(02)00549-9.
 

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