Feb. 20, 1954
COMMUNICATIONS TO THE EDITOR
melting a t 164-167’. It is believed t h a t these preparations were less pure. The resolidified phase exhibited the X-ray powder diffraction diagram recorded by Werners for a-D-galactopyranose. It is probable t h a t an equilibrium between the two anomers was established in the melt. I n this equilibrium melt the a-D-pyranose anomer was capable of crystallizing, and as it crystallized the equilibrium in the molten part generated more of it, so that the whole melt solidified as a-D-galactopyranose, which then melted a t the higher temperature of 164-1G7’, slightly below that of the pure a-D-form, 168-170’. Equal parts of a-D-galactopyranose, m.p. 168170°, and a-L-galactopyranose, m.p. 168-170°,
1199
were crystallized by dissolving in water, concentrating to a sirup and dissolving the sirup in methanol. The resultant optically inactive, crystalline phase melted a t 145-14s’. Previously reported values are: 163’,’ 140-142°,8 143-144°.9 The X-ray powder diffraction diagram of this preparation was identical with t h a t of a-D-galactopyranose. This racemic phase is therefore a mechanical mixture of the a-Dand a-Lanomers. (7) E. 0. von Lippmann, Bcr., 55, 3038 (1922). (8) E.Fischer and J. Hertz, ibid., 25, 1247 (1892). (9) C. Neuberg and J . Wohlgemuth, 2 . p h y s i o l . Ch ?n (1902).
36, 239
DEPARTMENT OF CHEMISTRY THEOHIO STATE UNIVERSITY 10, OHIO COLUMBUS
COMMUNICATIONS T O T H E EDITOR DISCONTINUITIES JN THE ADSORPTION ISOTHERM OF %-HEPTANEON MOLYBDENUM DISULFIDE Sir :
were observed in the same pressure range. Numerous experiments with benzene and water adsorption on the same surface showed no vertical rises in the adsorption or desorption curves, but in experiments with toluene adsorbate a vertical rise a t p / p 0 = 0.003 was obtained when allowing five to ten minutes between doses. The amaratus used was similar to that described by Harkins and Jura.3
In the course of a recent investigation a t Walker Laboratory of Rensselaer Polytechnic Institute, certain interesting experimental results were found regarding the appearance of discontinuities in the adsorption isotherm of n-heptane on molybdenum disulfide. These results, heretofore referred t o in a discussion of observed transitions by Young, Beebe and Bienes,’ were not included in the original publication on water and benzene adsorption on molybdenum disulfide. Reproducible discontinuities in the adsorption b0. 0.4 M isotherm of n-heptane on molybdenum disulfide a t 26.0’ were observed a t a relative pressure near 0.01 / > o when five to ten minutes was allowed for equilibrium between doses. However, when more time was allowed after the initial and succeeding doses, the discontinuity disappeared, and a smooth isotherm, concave to the pressure axis, resulted. If the sample was reactivated and the shorter time be-0 /O i tween admission of doses resumed, the discontinuity again appeared. This was observed when the sample was previously degassed a t room temI perature as well as when “activated” a t 200’. At higher pressures (above p l p o = 0.05) the time after each dose could be shortened considerably 1 I I without effect. 0.01 0.02 0.03 0.04 The low pressure portion of an n-heptane adRelative pressure, PIP”. sorption isotherm showing the discontinuity as Fig. 1.-The absorption of n-heptane on molybdenum well as the low pressure portion of a smooth isotherm, in which several hours elapsed between the disulfide activated adsorbent 1: 0 , five to ten minutes beinitial and second dose, is shown in Fig. 1. The tween doses; 0, up to eight hours between doses. adsorbent is “activated adsorbent 1” of molybAlthough the appearance of a discontinuity in an denum disulfide, as previously described.2 The desorption isotherms showed the same appearance isotherm may be a non-equilibrium condition which of discontinuities, except t h a t several small steps disappears in sufficient time, this does not eliminate the reality of the observation. As physical ad(1) D. M. Young, R. A. Beebe and H. Bienes, Trans. Faraday Soc., sorption is a rapid process, an adsorbate which 49, 1086 (1953).
c
do I
(2) E. V. Ballou and S. Ross, J . Phys. Chcm., 57, 653 (1953).
(3)
W.D.Harkins and
G. Jura,THIS JOURNAL, 68, 1356 (1944).
C)OM~IUNIC.~TIONSTo THE
im
EDITOR
1101. i ( i
comes t o equilibrium slowly occupies some surface positions which are either relatively inaccessible or are associated with an appreciable activation energy. DIVISION OF APPLIEDSCIENCE ~JNIVERSITY HARVARD JASUARY RECEIVED
a-ketol-3-membered hemiketal system and differ only in the orientation of the hydroxyl group of the a-keto1 system as in I and 111. These experimental results (coupled with the results of bismuth oxide oxidation in the cevine seriesIc) thus establish E. VERNON BALLOU a deduction made by Dr. R. B. LVoodward in September, 1953. 11, 1951
ON THE NATURE OF T H E GERMINE-ISOGERMINE PSEUDOGERMINE ISOMERIZATIONS
.Yir: Shortly beiore discontinuance of research on the ester alkaloids oi germinc a t the Syuibb Institute, otic oi us J.F. I found that germine acetonide hydroc.hloride (1 consumes one mole oi potassium periodate and affords a crystalline oxidation product which reduces Tollcns reaxent, 111.1). 274 ?;ti3 dcc., +" c l..j4, d c . . Calcd. ior C.+,H;:,03S: i, ti.j.7!); H , \.2\. F o u n d : C , ti>.; 13, \.37. Further characterization of the substance a t Har\.rircl showed it to be a n altlehydo-r-l~icti~ne II . 0 2
0
-+ HO-I TOC 13
0-
'>
6
ox-CIIO
HO- HCOH
DEPARTMEST OF CHEMISTRY S. MORRISKUPCHAV HARVARD USIVERSITY CAMBRIDGE, MASS.,ASD MARYFIESER THESQUIBB ISSTITUTE C . R. SARAYASAY LOUISF.FIESER FOR MEDICAL RESEARCH JOSEF FRIED SEW BRUNSWICK, S. J. RECEIVED FEBRUARY 4, 1951
THE SIGN OF OXIDATION-REDUCTION POTENTIALS
Sir: The choice of sign for oxidation-reduction couples is indeed arbitrary, but i t is not generally recognized where the arbitrary choice is made. If' (1) the measured voltage of a cell is taken as positive and (2) the equation for the spontaneous cell reaction is written with the products on the right; then there is no choice of signs for the "half reaction." For example, consider the spontaneous reaction Zn
I
I1
I11
Thus inirared absorption bands a t :3.(j5 and 5.7!) p are attributable to a n aldehyde group, and a band a t .j.(.j2 p is indicative oi the ylactone iunction. Furtherinore, the substance ior:ns an oxime, n1.p. -.>; % < , i b j S \ I Kupchan D. L : ~ v i e .C. I' Deliwala a n d B . Y Andoh, i b t ! , 76, .;.5l't l ' t . 3 ; c S 11 Kiipchan anit D L a v i e , t b ~ '76, 314 ( 1 9 5 4 , 2 , 1) FI R B i r r o n And C J. \\' HroCk5, C i i w t i l i ? ill! 1 , 1 / ! . j ! r ) 51, I.ICC. I s i S i . \Ye are inde:,ted 11, 1)r U . ~ r r < ~f.ir ii ~,mmii~iicrt'n; thme resiiltr t o ( 1 3 ;>Ti( r :re pii')lich:i(,n ,i 1.. i: < m i d and \V .\ J3cobs. J . GI,/Cht,i,r , 148, 57 1 , 4 2 1
+ 2H+ = Zn++ + H2
E"
=
f0.75
(A)
one must then write H?
=
+ 2e+ 2e-
2H+
and
Eo
=
0.0
(R)
Zn = Z n r f E " = +0.76 (CJ This is necessary since the hydrogen a n d zinc couples must add up t o give the equation and the potential for the completed reaction A. U'e then must also write Zn++
+ 2e-
=
Zn
Ea
=
-0.76
(C,)
because the laws of thermodynamics require that the sign of the free energy of a reaction be reversed if the equation is reversed. The sign conversion of C1and Cs is, however, the sa-ne. There is general agreement throughout the world, with regard to the arbitrary choices (1) and ( , 2 ) . G. N. Lewis in his selection of thermodynamic conventions was therefore co;npelled to choose the signs of his "standard electrode potentials" or "half reactions" consistent with C1 and C,. Unfortunately the older European conventions were Zn
=
Zn++
Zn++
+ 2e-
+ 2e=
Zn
E" E"
= =
-0.76 +0.76
The resulting confusion has been most distressitly. I t was hoped that the Commissions of the Congress of Pure and Xpplied Chemistry meeting in Stockholm ( 1 9.53) had resolved the difficultv but such does not seem to be the case. Thus, J. 0'11.B.' has criticized my use of the Lewis conventions. T o quote, "The author can be parochial in important matters. The sign convention for electrode potentials is presented dogmatically without mention that the one used is not that of the majority and t h a t an International Commission has recently recommended its withdrawal." The facts are that the Commission voted to accept the arbitrary choices ( I ) and ( 2 ) and the (1) J . O'AI. B . , T i a n s . F a v a d n y S o r . , 49, 1.503 (1
