Raymond Thacker General Motors Technical Center 12 Mile and Mound Roads
Warren, Michigan 4090
A Simple Procedure for Making Hg\HgO Reference Electrodes
Mercury-mercuric oxide reference electrodes are often used for making single electrode potential measurements in alkaline solutions.' Unfortunately, however, these electrodes suffer from the disadvantage that mercuric oxide is appreciably soluble in these solutions. The solubility in 6M NaOH solution a t 2S0C, for example, is about 68 mg/L2 If the dissolved oxide were permitted to diiuse into the working electrode compartment of the electrochemical cell, it could lead to undesirable side reactions at a work'mg electrode as was observed in the case of silver oxide.a For instance, a t a hydrogen or hydrocarbon electrode a t rest, or a t an electrode that is polarized cathodically, the oxide would be reduced, thereby contaminating the electrode surface with mercury which is well known to- be a catalyst poison.' - PLATINUM WIRE A simple procedure is described for making mercury-mercuric oxide reference electrodes which eliminates, or a t least minimizes, this difficulty. The electrode consists of a piece of '/An. Poly-Flo (polyethylene) tubing, one 114" POLY-FLO TUBING end of which is closed by softening in a flame fol, ASBESTOS FIBRE lowed by crimping with tweezers, or by a plug of molten polyethylene, as KOH SOLUTION shown in the figure. Two MERCURIC OXIDE diameter holes are pierced across a diameter MERCURY of the tube about l-'/% in. from the closed end with C\ POLYETHYLENE PLUG a dissecting needle. A Mersurymersuris oxide reference strand of asbestos fiber, electrode.
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I80 / Journal o f Chemical Education
which is removed from a piece of asbestos cord, is then pulled through these holes with a short length of copper wire. The excess asbestos fiber on the outside of the tube is removed. Distilled mercury is then added to the tube to a depth of about in., followed by mercuric oxide (red or yellow) to a depth of in., and a sufEcient amount of the alkaline cell-electrolyte to cover the asbestos fiber. The asbestos fiber soaks up the elcctrolyte and thereby produces an ionically conducting path. It is important to use the same electrolyte in the reference electrode and the electrochemical cell to avoid the establishment of a liquid junction potential. The cell top consists of a short length of a/pin. Poly-Flo tubing through which a platinum wire is sealed by crimping one end of the tube in the manner described above. The platinum wire should he long enough to make contact with the mercury pool when the cell top is in place. Reference electrodes made in this way can be readily inserted through the top or side of an electrochemical cell made from polyethylene. Mercury-mercuric oxide electrodes prepared according to this procedure have a stable potential of 928 1 mv (23%) versus a hydrogen electrode in the same solution which is in good agreement with the values quoted in the lite~ature.~
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1 Ives, D. J. G., "Reference Electrodes," (Editma: IVES, D. J. G., AND JANZ,G.J.), Academic Press, NewYork, 1961, p.
335.
LINKE,W. F.,"Solubilities of Inorganic and Metd Organic Compounds," (4th ed.), American Chemical Society, 1958, vol. I, p. 1248. Qf. THACKER, R., Natwe, 198,179 (1963). 'BOND.G. C.. "Catalwk by Metals,'' Academic Press, New York, 1962,p. 99. DE BETEUNE, A. J., AND LOUD,N. A. S., "Standard Aqueous Electrode Potentials, and Temperature Coefficients at 25%" Clifford A. Hampel, Skokie, Ill., 1964.
