Reference Electrodes
To the Editor: Reference electrodes of the kind described in "A Simple Procedure for Making Hg/HgO Reference Electrodes" [THACKER, R., J. CHEM.EDUC.,45, 180 (1968)l should have thc Pt-wire insulated through the electrolyte-mercuric oxide, and the wire should make electrical contact only with the mercury. Otherwise the cell is shorted out, and a true equilibrium potential cannot be est,ahlished.
To the Editor: I n answer to J. Engelhardt's letter to the editor concerning the article "A Simple Procedure for Making Hg/HgO Reference Electrodes" [THACKER,R., J. CHEM.EDUC.,45,180 (1968)], the author would like to make the following comments. The potential of a Hg/HgO reference electrode depends on the establishment of an equilibrium between Hg, HgO, and OH- ions a t a particular temperature. I n the absence of impurities this equilibrium would not he disturbed by the presence of an inert material, such as platinum, in contact with one or all of the three phases. Thus the potential of this reference electrode would be thesame regardless of whether the platinum wire lead makes contact with the mercury only, or whether it makes contact first with the electrolyte, then the mercuric oxide, and finally the mercury. I n this laboratory, it has been shown that by using pure materials, electrodes prepared by the two methods have the same stable potential of 928 + 1mv (23' C) versus a hydrogen electrode in the same solution, which is in good agreement with the equilibrium values quoted in Ref. (5) in the ahove article.
Another example of an electrode which is unaffected by platinum coming into contact with the various phases is the silver-silver oxide reference electrode. Here an intimate mixture of silver and silver oxide is supported in a platinum thimble which is immersed in an alkaline electrolyte [W. J. HAMER AND D. W. CRAIG, J.Electrochem. Soc., 104,206 (1957) I. These electrodes have a stable potential of 1 . 1 7 1 ~versus a hydrogen electrode in the same solution which is in good agreement with the equilibrium values quoted in Ref. (5) mentioned ahove. The author is not sure what J. Engelhardt means when he says that "the cell is shorted out" if the platinum lead wire makes contact with the electrolyte, HgO, and Hg. The Hg/HgO reference electrode is a single electrode, not a cell, and would require the presence of another single electrode in the same solution in order t,o short it out.
Hofmann Elimination
To the Editor: I n the "Textbook Error" discussion of the Hofmann elimination [BAUMGARTEN, R. J., J. CHEM.EDUC.,45, 122 (1968)1, Dr. Baumgarten probably should have indicated that while it is true that certain R groups such as the benzylic and allylic types preferentially give the SN2product, in most other cases methanol is the product since the SN2reaction would certainly involve the CH3 group instead of the RCH,-CH, unit.
Volume 45, Number 8, August 1968
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