Textbook Errors, R. 1. Venable and D. V. Roach University of Missouri-Rolla
94
The Standard Electrode Potential
Rollo 65401
of the Silver-Silver Bromide Electrode The absolute magnitude of the standard electrode potential for the silver-silver bromide electrode relative to the standard hydrogen electrode is !isted in several textbooks' as 0.095. If one writes the Nernst equation for the cell using the usual conventions for reduction potentials, one gets E
=
E'
- 0.059 log a*'
=
Eo
- 0.118 log u,m,
-
where the cell reaction is written as AgBr + ' / a Ha Ag + H + + BrSuggestions of material suibable for this column and guest columns suitable for publication directly should be sent with as many details as possible, and particularly with reference to modern texlbooks, to W. H. Eberhardt, School of Chemistry, Georgia. Institute of Technology, Atlanta, Georgia 30332. ' Since the pmpose of this column is to prevent the spread and cont,inoation of errors and not the evaluation of individual texts, the sources of errors discussed will not be cited. In order to be presented, an error must occur in a t least two independent recent standard books. HARNED, H. S., KESTON, A. S., A N D DONELSON, J. G., J . Am. Chem. Soc., 58, 989 (1936). TOWNS, M. B., GREELER, R. S., A N D LIETZKE, M. A,, J . Phys. Chem., 64, 1861 (1960). ' LATIMJIR, WENDELL M., "The Oxidation States of the Elements and Their Potentials in Aqueous Solutions," Englewood Cliffs, New Jersey, Prentice-Hall, Inc., 1952, p.190. LATIMER, op. tit., p. 59. LATIMER, op. tit., p. 30. From references in footnotes 2, 3, and 4 (p. 354)
'
When one substitutes the experimental electrode potential? of 0.2005 V at a HBr concentration of 0.1 m along with the above E n value and calculates the mean ionic activity coefficient for the HBr one gets the rather surprising result that r+= 1.26. A quick check of the original literature2J shows that reported values of the standard electrode potential are 0.07103 and 0.0716. The standard electrode potential can be calculated from the AG" for the above reaction which can in turn be calculated from the free energies of formation of the various species involved as tabulated below. Substance AgBr Br-(as) H + (ad AG" reaction = -24.574 =
AGIO kcal
-22.930' -24.574'
o8
+ 0 - (-22.930)
kcal
- 1,644 cal = -nFEo
in very good agreement with the experimental values. Further if the value 0.071 is used in the Nernst equation to calculate the mean ionic activity coefficient of 0.1 m HBr the value of y+ is found to be 0.798, which of course is the literature value7 and seems much more reasonable than the 1.26 calculated above for this concentration. This leads to thc conclusion that the value of the standard electrode potential of 0.095 cannot be correct,.
Volume 46, Number 1 I , November 1969
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