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textbook forum Edited by RALPH K. BIRDWHISTELL University of West Florida Pensacola, FL 32504 Outmoded Terminology: The R. W. Ramette Carleton College, Northfield, MN 55057 In reading textbooks and even papers in ACS journals I note that some authors report their electrode potentials "versus the NHE." This is incorrect because NHE means the "normal hydrogen electrode", which was used temporar- ily as a reference electrode in the early days of electrochem- istry. Workers would immerse a platinum electrode into a solution of 1 N strong acid and bubble hydrogen gas through the solution at about 1 atm pressure. In this solution the activity of hydrogen ion is approximately 0.8 due to ionic interactions. The NHE terminology may be traced back to the classic article by Joel Hildebrand, "Some Applications of the Hydrogen Electrode in Analysis, Research and Tea- ching." l It may have originated with Nernst, long before there was an accepted distinction between concentration and activity. However, our internationally adopted electrode potential scale is based on the standard hydrogen electrode, SHE, a hypothetical electrode containing 1 m hydrogen ion having unit activity and no ionic interactions. Obviously this elec- trode cannot be made in the laboratory because finite con- centration and ideal behavior are mutually exclusive. The (approximately) calculated difference in potential between the NHE and the SHE is quite significant. From the Nernst equation we see that for the half reaction 2H+(1 N, activity = 0.8) + 2e- = Ho(l atm) ENHE = EO - 29.6 log (1 atm)/(0.8)~ - 5.7 mV We still see the "normal" terminology, with respect to the various calomel electrodes for example. The "normal calo- mel electrode" made with 1 N (M) KC1 has a potential vs. SHE of +0.280 V, while the "tenth-normal" version has a value of +0.336 V. The "saturated" calomel electrode has a potential of +0.244 V. All are "real" electrodes whose poten- tials are measured directly versus a working hydrogen elec- trode (see below), unlike the hypothetical "standard calomel electrode" which has a potential of +0.268 V. I believe teachers and authors should make this signifi- cant distinction. "Normal", which refers to an actual con- centration, should not be confused with "standard", which refers to unit activity and ideal behavior. The case is well made by Biegler and Woods2,who conclude that "the N.H.E. has neither fundamental nor operational significance and should be regarded as of historical interest only". What is of operational significance is the typical working hydrogen electrode, which comprises a P t electrode im- mersed in, say, 0.01000 m HC1 that is also in equilibrium Electrode with purified hydrogen gas bubbled through the solution at whatever pressure is permitted by ambient conditions. The great importance of this practical electrode is that its poten- tial can be accurately calculated from the Nernst equation, with the activity coefficient for hydrogen ion reliably esti- mated by the Davies equation to be 0.902 at this low ionic strength. To illustrate, if the barometer reading is 745.0 mm of Hg for a hydrogen electrode at 25 OC with the vapor pressure of water at 23.8 mm of Hg, we find for the calculat- ed potential Ew = 0 - 29.6 log [(745.0 - 23.8)/760]/[0.01000 X 0.9021~ = -120.4 mV vs. the SHE Such working electrodes are the practical primary standards for the determination of standard potentials for other cou- ples and for the determination of pH and acid dissociation constants. For example, suppose we want to determine the standard potential of the silver couple by using the galvanic cell Pt/HCl(O.01000 m), H2(721.2mm of H~)/A~No.,'(~.o~ooo m)/Ag The working hydrogen electrode on the left is the same as the one above that has a calculated potential of -120.4 mV vs. SHE. Suppose we measure the voltage of this cell at 25 OC and find a value of +798.3 mV. By convention we have and from the Nernst equation we have = EO Ag - 59.2 log [1/(0.01000 X 0.902)] - (-120.4) whence the standard potential for the silver couple is +799.0 mV. This calculation has ignored the correction that could be applied to include the liquid junction potential between the two solutions. The NHE could not be used reliably for this determination because of the uncertainty in the activity coefficient of the hydrogen ion at such a high ionic strength. Thus, as educators we should not confuse the NHE with the SHE. We can still learn from Confucius: "If the termin- ology is not correct, then the whole style of one's speech falls out of form; a gentleman never uses his terminology indiscri- minately." J. Am. Chem. Soc. 1913, 35,847-871. J. Chem. Educ. 1973, 50, 604. Volume 64 Number 10 October 1987 885
