Breck (1972) has proposed a new approach for determining the  redox level  (Eh) in aerated natural waters and has described an electrode arrangement which, supposedly, is capable of accurately measuring Eh in such waters

Ben-Yaakov, Sam

Kaplan, I. R.

English

Yale University

      P.O. BOX 208118 | NEW HAVEN CT 06520-8118 USA | PEABODY.YALE. EDU   JOURNAL OF MARINE RESEARCH The Journal of Marine Research, one of the oldest journals in American marine science, published important peer-reviewed original research on a broad array of topics in physical, biological, and chemical oceanography vital to the academic oceanographic community in the long and rich tradition of the Sears Foundation for Marine Research at Yale University.  An archive of all issues from 1937 to 2021 (Volume 1–79) are available through EliScholar,  a digital platform for scholarly publishing provided by Yale University Library at  https://elischolar.library.yale.edu/.  Requests for permission to clear rights for use of this content should be directed to the authors, their estates, or other representatives. The Journal of Marine Research has no contact information beyond the affiliations listed in the published articles. We ask that you provide attribution to the Journal of Marine Research.  Yale University provides access to these materials for educational and research purposes only. Copyright or other proprietary rights to content contained in this document may be held by individuals or entities other than, or in addition to, Yale University. You are solely responsible for determining the ownership of the copyright, and for obtaining permission for your intended use. Yale University makes no warranty that your distribution, reproduction, or other use of these materials will not infringe the rights of third parties.  This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. https://creativecommons.org/licenses/by-nc-sa/4.0/   Comments on "Redox Potentials by Equilibration," by W. G. Breck' Sam Ben-Y aakov and I. R. Kaplan Department of Geology and l nstfrute of Geophysics and Planetary Physics University of California Les Angeles, California 90024 Breck ( 1972) has proposed a new approach for determining the "red ox level" (Eh) in aerated natural waters and has described an electrode arrangement which, supposedly, is capable of accurately measuring Eh in such waters. The apparatus described by Breck is composed of a solution of a known oxidation-reduction couple (0*/R *) connected to a test solution (0/R) by a graphite and saturated KCl bridge. It is claimed that, at equilibrium, the auxiliary solution 0*/R * will assume the Eh of 0 /R. Hence, the Eh of the test solution (0/R) can be determined by measuring the redox potential of O * /R * by means of a platinum electrode relative to a reference electrode. The schematic diagram in Fig. 1 represents the arrangement proposed by Breck as it depicts all the electrochemical half cells involved. Breck's basic assumption is that, at equilibrium (i.e., when the loop current drops to zero), the "redox level" of 0*/R * is equal to that of 0/R, and hence, E (Fig. 1) represents the Eh of 0 /R. However, Fig. 1 reveals that this assumption can be correct only if the graphite electrode is capable of measuring the "redox level" in both 0 /R and 0*/R *. This can be shown in the following way: Assume that the electrode arrange-ment proposed by Breck is indeed functioning as claimed. At equilibrium, the oxidation-reduction potentials of O /R and O * /R * are identical ( as h ypothet-ically assumed), hence: E' = Eo;R + Ejo;R = Eo• /R* + Ejo. /R*, or ( neglecting the junction potentials): E' = Eo;R = Eo• /R*• (1) ,. Accepted for publication and submitted to press 14 June 1972. Withdrawn and resubmitted 15 October 197 2 . 79 80 'Journal of Marine Research [31,1 Hence, the graphite electrode should be capable of directly measuring the redox potentials in 0 /R and 0*/R. If, however, the graphite electrode is not revers-ible to oxidation-reduction reactions in 0/R and/or 0*/R •, the auxiliary solu-tion 0*/R • will not reach the same "redox level" as that of 0/R. Breck's implicit a p1·iore assumption-that the graphite electrode can meas-ure redox potentials in aqueous solutions-is not supported by experimental or theoretical evidence. Furthermore, there is evidence that this assumption is probably incorrect. Berl (1943), Weisz and Jaffe (1948), and Yeager et al. graphite bridge ef Eo;R E E' 0/R Ejo,R* sa~C~ted bridge EjO/R I Figure 1. Schamatic representation of the cell arrangement proposed by Breck (1972). The half cells Eo JR and Eo* JR* represent the graphite half cells Ejo JR and Eio• JR*• the liquid junction potentials; Ept the platinum electrode half cell; a,1d ER the liquid junction potential of the reference electrode relative to which the platinum electrode is measured. (1964) found that the graphite electrode is reversible to the redox couple oxygen/peroxide (which, according to Breck, controls the "redox-level" in aerated natural waters) only at alkaline solutions in the range o. 1 to 5 M KOH. These alkaline conditions are clearly outside the range of natural waters (Baas-Becking, et al. I 960 ), hence the use of the graphite electrode for redox potential measurement in these waters is eliminated. In addition, proper opera-ti on of the Breck cell requires that the graphite electrode be thermodynamically reversible in a solution of an oxidation-reduction couple such as ferric-ferrous. No evidence of this effect is given, thus casting doubt on its applicability as a redox electrode. Incidentally, if the graphite electrode can indeed measure redox potentials in 0 /R (which it probably cannot), why bother with a complicated cell ar-rangement as described by Breck. It seems that the author has used ill ogical reasoning. In order to electrochemically equilibrate 0*/R • with 0/R (as pro-posed), one must assume reversibility of electrode reactions to both 0*/R • and 0 /R. If the graphite electrode is indeed reversible to oxidation-reduction re-actions, why not measure directly the potential EoJR relative to a conventional reference electrode ? r 973] Ben-Yaakov and Kaplan : Comments on Breck's Paper There are other diffi culti es with the cell arrangement proposed by Breck. For example, use of the KCl bridge as an electronic conductor may result in unpredictable large liquid junction potentials [ eq. ( r )]. An electrolytic solution is not an electronic conductor, but rather an ionic conductor. Hence, the mechan-ism of charge transfer through the K Cl bridge is a complicated process that may not only cause abnormall y large juncti on potentials but may also modify the compositi on of 0*/R *. H owever, unlike the basic problem regarding the applicability of the graphite electrode, these second-order difficulti es can prob-ably be overcome. In the second part of his paper, Breck discussed the problem of redox control in aerated natural waters and presented arguments that support the view that the o)l!ygen/peroxide couple controls the oxidation-reduction poise in these waters. Without going into the question of whether this view is acceptable (we believe that the arguments can be accepted only if supported by sound experimental evidence), we would like to comment on the concept of redox level of aerated natural waters. It is generally accepted (Stumm and Morgan 1970: 300-377, Broecker and Oversby 1971: 277-278, Berner 1971: 118-119) that aerated natural waters are not in thermodynamic equilibrium with respect to all their oxidati on-reduction couples. Therefore, there is no single redox level that would describe all the couples. Furthermore, it is highly prob-able that certain oxidation-reduction couples are more important than others in a given geochemical process. Some couples may be important in short-term processes whereas others may be important in long-term processes extending over geological t ime. It would appear, then, that the concept of "redox level of aerated natural waters" is hopelessly ambiguous and may not be rigorously defined. The platinum electrode has long been used as a rough indicator of oxidation-reduction levels in natural waters. Despite its many shortcomings, it has been a useful empirical indicator in numerous geochemical and hydrological studies. It is yet to be shown that the electrode proposed by Breck can match the questionable performance of the platinum electrode. Judging from Breck's data ( 1972: table 1 ), the cell does not seem to respond to a variety of conditions that may be met in natural-water measurements. The Eh range of nine surface saline-water measurements was 0.51 ,:Ir .02 V and the range of six nonsaline waters (including one hot spring) was 0.50 ± .01 V . However, a sulfide-smelling spring (Warner Hot Springs) yielded a value of 0.42 V, showing a potential shift of only 0.08 V between oxidizing and reducing waters. By comparison, the platinum electrode displayed a potential shift of 0.30 V. Although no pH data were provided, it would not be surprising if the total spread in pH between fresh water and seawater was larger than r .o pH unit. Assuming Breck's sug-gestion that the reactions described with eqs. (6) and (7) and Fig. 7 are the mechanisms for redox control of aerated water, it is obvious that H+ enters into the equation. The resultant Eh shift expected over a r-pH unit range should Journal of Marine Research be ± 0.06 V. The apparent lack of response to sulfide on the one hand and to pH on the other points to either a lack of reversibility of the graphite electrode or to polarization poisoning. REFERENCES BAAS-BECKING, L. G . M., I. R. K APLAN, and D . MOORE 1960. Lim its of the natural environment in terms of oxidation-reduction potentials. J . Geo!., 68: 243-284. BERL, w. G. 1943. A reversible oxygen electrode. Trans. electrochem. Soc., 83: 253-270. BERNER, R. A. 1971. Principles of chemical sedimentology. McGraw-Hill, New York. 240 pp. BRECK, W, G. 1972. Redox potentials by equilibration. J. mar. Res., 30: 121-139. BROECKER, W. S., and V. M. OVERSBY 1971. Chemical equilibria m the earth . McGraw-Hill, New York. 318 pp. STUMM, WERNER, and J. J . MORGAN 1970. Aquatic chemistry. Wiley-Interscience, New York. 583 pp. WEISZ, R. S., and S .. S. JAFFE 1948. The mechanism of the reduction of oxygen at the air electrode. Trans. electrochem. Soc., 93: 128-141. YEAGER, E., P. KRAUSE, and K. V. RAO 1964. The kinetics of oxygen-peroxide couple on carbon. Electrochim. Acta, 9: 1057-1070. 

Comments on  Redox potentials by equilibration,  by W. G. Breck

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Comments on Redox potentials by equilibration, by W. G. Breck

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