Edited by RALPHK. BIRDWHISTELL
textbook forum
University of West Florida Pensacola. FL 32504
Outmoded Terminology: The Normal Hydrogen EJctrode R. W. Rametle 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 temporarily as a reference electrode in the early days of electrochemistry. Workers would immerse a platinum electrode into a solution of 1N strong acid and bubble hydrogen gas throngh the solution a t 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 hack to the classic article by Joel Hildebrand, "Some Applications of the Hydrogen Electrode in Analysis, Research and Teaching." I t 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 hvoothetical electrode cootaininp 1m hvdrogen . - ion having .. unit activity and no ionic interactions. Obviously this electrode cannot he made in the laboratory because finite concentration and ideal hehavior 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
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2Ht(l N, activity = 0.8) + 2e- = H,(1 atm)
EN,, =I?
with purified hydrogen gas bubbled throngh the solution at whatever pressure is permitted by ambient conditions. The great importance of this practical electrode is that its potential can he accurately calculated from the Nernst equation, with the activity coefficient for hydrogen ion reliably estimated hy the Davies equation t o be 0.902 a t 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 a t 23.8 mm of Hg, we find for the calculated potential E, = 0 - 29.6 lag [(745.0 - 23.8)/760]/[0.01000 X 0.90212 = -120.4 mV va. the SHE
Such u,orking electrudes are the practical primary standards fur the determination of standard ootentials ior other couples and for the determination of p~ and acid dissociation constants. For example, suppose we want to determine the standard potential of the silver couple by using the galvanic cell m)/Ag Pt/HC1(0.01000 m), HJ721.2 mm of Hg)/AgN0~(0.01000 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 thevoltage of this cell a t 25 OC and find a value of +798.3 mV. By convention we have
- 29.6 log (1at1n)/(0.8)~- 5.7 mV
We still see the "normal" terminology, with respect to the various calomel electrodes for example. The "normal calomel electrode" made with 1N (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 potentials are measured directly versus a working hydrogen electrode (see below), unlike the hypothetical "standard calomel electrode" which has a potential of +0.268 V. I believe teachers and authors should make this significant distinction. "Normal", which refers to an actual concentration, should not he confused with "standard", which refers to unit activity and ideal hehavior. 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 historiial interest only". \\'hat is of werational si~nifiranceis the typiral working hydrogen electrode, which comprises a P t electrode immersed in, say, 0.01000 m HCI that is also in equilibrium
and from the Nernst equation we have E,,, = +798.3 mV =PAS - 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 he applied to include the liquid junction potential between the two solutions. The NHE could not he 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 terminology is not correct, then the whole style of one's speech falls out of form; a gentleman never uses his terminology indiscriminately."
' J. Am. Chem. Soc. 1913,35,847-871. J. Chem. Educ. 1973,50.604.
Volume 84
Number 10 October 1987
885
Classroom Demonstrations of Polymer Principles Part II. Polymer Formation F. Rodriguez School of Chemical Engineering, Olin Hall, Cornell University, Ithaca, NY 14653
L. J. Mathias Department of Polymer Science, University of Southern Mississippi, Hattiesburg. MS 39406 J. Kroschwitz John Wiley & Sons, Inc., 605 3rd Ave., New York, NY 10156
C. E. Carraher, Jr. Florida Atlantic University, Boca Raton, FL 33431 I n the previous installment ( I ) , t h e size and structure of polymers (macromolecules) were considered. In the present Installment, t h e manner in which these large molecules can he assembled from subunits (the process of polymerization) will he illustrated. Although many important polymers are linear assemblies of simple repeat units, some are more complex. In the examples chosen here, both linear (acrylamide and methyl methacrylate) and branched or cross-linked (nhenolic resins and epoxv cements) are included. ' T h e equipment neebedfor the demonstrations should he available in most chemical laboratories. When monomers or sol\.ents that represent health hazardiare involved, theronditions of rhe lerture hall or latmra~oryshould he examined carefully. Adequate ventilation is a must. Participants must be informed of any risks, and fire extinguishers should he a t hand. Safety glasses and protective clothing (gloves, apron) should he used when appropriate. Photopolymerlzatlon ot Acrylamlde This nolvmerization uses a safe and odorless solvent. wa" ter, requires oxygen, is slow enough t o he observed a t various stages, b u t rapid enough t o get done in class time. Moreover, the polyacrylamide produced in this experiment can he used in suhseauent flow experiments t o illustrate non-Newtonian flow andadrag reductibn. Hazard: It should be noted t h a t t h e monomer is quite toxic if ingested (2).Fortunately, i t is not very volatile. T h e polymer is not toxic.
.
Advance Preparation A 50% (hy weight) solution of the aerylamide monomer in water is best made up before the lecture hour. Although the solution usually is stable in contact withair for days, it has been known to polymerize occasionally in shorter periods, especially if oxygen has been eliminated from the system. The dry monomer contains noinhihitors and can be stored for long times (years) ifrefrigerated. A 50% solution in water is availablefor industrial users. The solution is inhibited by 25 ppm of cupric ion which allows storage of the solution up to 12 months. The photopolymerization will proceed in the presence of the cuprie ion, but addition of about 0.05% of the sodium salt of ethylenediaminetetraacetic acid (EDTA) will speed up the reaction. The photoinitiator is riboflavin (Fig. 1). The sodium salt of rihoflnvin-5'-ohosnhate is somewhat more convenient because it is more soluble in water 11 gof thr wrliumderivotrve iiequivalenttoO.7 goi rihdavin). I n either casr, a eonrentratim of alwd ?U ppm of aolutiw givrsa palr yrliow solution uith the acrvlamide. ~~~
~
.~~.~
~~~
accelerate polymerization. The projector serves as the light source for the formation of a free radical from the rihoflavin-oxygen-water combination. The radical adds to the monomer creating a new radical that again adds manomer, and so on (Fig. 2). Even an approximate calculation shows that the heat of polymerization must exceed 10 kcallmol (it is, in fact, close to 20). Consider a temperature rise of 70 "C with a specific heat of 1 callg" in a system of 50 g aerylamide (0.7 mol) and 50 g water. The heat of polymerization, AHp, then is at least After a few minutes of steamine. - the olue . -can be removed from the beaker with a spatula. The water-plasticized polymer plug is elastic and hnm~resStndents rhonld not handle it since it is hot. and also ..---there may he small amounts of unreacted monomer, whichis a toxic hazard. The lecturer should wear (disposable) plastic gloves when handling thereaction mass. \\'hen it rs desired to put the polyacrylamide in solutim for further d~monstrations,the plug sh
