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f erentiating evidence on this point has been obtained by competition kinetic studies for the precursor. Thus, experiments with NOz- and methanol give k(X NOz-)/ k ( X MeOH) = 400 where X represents the precursor. However, it is known that if X is OH (0-),then this . ~ it is concluded rate constant should be ~ 5 Hence, that reaction 2 does not represent the primary process of “direct action”; it is suggested that this primary process may be reaction 3. To summarize, NO3 radical has been detected in the radiolysis of concentrated nitrate solutions. It is characteristic of the “direct effect” but is not a primary species. This indicates strongly that excitation is an important primary process in the “direct effect.” A fuller account of these results and others will be presented later in conjunction with work currently in progress on the y radiolysis of these solutions.
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+
Aclznozolt?dgments, This work was carried out by the author at hrgonne Kational Laboratory while on leave of absence from the Puerto Rico Nuclear Center. Thanks are due to Dr. J. R. Bugher, Director, PRNC, and Dr. E. J. Hart, Argonne Xational Laboratory, for facilitating and encouraging these arrangements.
o
ai
,
I
0.2
0.3
1
I
0.4
0.5
0.6
0.7
0.8
H/Pd
Figure 1. The effect of absorption and desorption of hydrogen on the paramagnetic susceptibility of palladium.
without any change in the magnetic susceptibility. These results were not confirmed by Lewis, et u Z . , ~ using an electrolytic method for removing hydrogen but were partially confimed in this laboratory6 using the high-voltage method. I n the present series of experiments diamagnetic and (8) G. E. Adarns and J. W. Boag, Proc. Chem. Soc., 112 (1964); slightly paramagnetic samples of palladium hydride J. K. Thomas, Trans. Faraday SOC.,61, 702 (1965). were prepared from very fine (0.80-p) palladium metal ~ I A L C ODANIELS LM RADIATION CENTERASD powder, by alternately heating and cooling the metal CHEMISTRY DEPARTMENT in pure hydrogen (obtained by evolution from UHB), OREGONSTATEUNIVERSITY at a temperature which never exceeded 200” above OREGON 97331 CORVALLIS, which the powder sinters. These samples evolved hyRECEIVEDJULY5, 1966 drogen at room temperature when the hydrogen pressure above them fell below 18 mm. The magnetic susceptibility of samples of palladium hydride was continuously compared to that of a standard (hlohr’s The Magnetic Susceptibility of salt), while small measured quantities of hydrogen were Palladium Hydride removed from the sample, in an apparatus which has been described previously.6 The results are shown in Si?: The linear decrease of the paramagnetic sus- Figure 1. Curves 1 and 2 show that it is possible to ceptibility of palladium as it absorbs hydrogen is well remove a large fraction of the hydrogen from diamagknown.’ The portion of the density of states curve to netic palladium hydride and from palladium hydride the right of the Fermi level is usually drawn to indicate which is slightly paramagnetic owing to an initial lower that palladium lacks about 0.55 electron to fill the hydrogen content, without changing the susceptibility d level. It has been suggested2 that hydrogen donates (xg). Curves 3 and 4 for the absorption of hydrogen its electron to fill this d level so that when the spins are by palladium are taken from Smith.’ It thus appears paired, the substance should become diamagnetic. This latter is observed experimentally but a t a composition variously reported but close to PdH0.66rather than (1) D. P. Smith, “Hydrogen in Metals,” University of Chicago Press, Chicago, Ill., 1947. at PdHo.,5. If the above explanation is correct, it (2) N. F. Mott, Advan. Phys. 13, 325 (1964). should be possible to obtain a linear increase in sus(3) A. Michel and M. Gallissot, Compt. Rend., 208, 434 (1939). ceptibility as hydrogen is removed from PdH-0.66. (4) J. C. Barton, F. A. Lewis, and I. Woodward, Trans. Faraday This was attempted3 by a high-voltage method for SOC.,59, 1201 (1963). extracting hydrogen without heating the sample and (5) T. R. P. Gibb, Jr., t o be submitted. it mas claimed that all the hydrogen could be removed (6) W. A. Norder, Rev. Sci. Instr., 31, 849 (1960). The Journal of Physical Chsmktry
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0
that the band theory explanation is not tenable for the desorption of hydrogen from palladium hydride and it may be that lattice expansion plays a more important role than was formerly thought. Further experimental and theoretical work is in progress.
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Aclcnowledgment. The authors are indebted to the U. S. Atomic Energy Commission for financial support for this work. DEPARTMENT OF CHEMISTRY TUFTS UNIVERSITY MEDFORD, MASSACHUSETTS
THOMAS R. P. GIBB,JR. J. MACMILLAN R. J. ROY
RECEIVED JULY 18, 1966
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“I’ On the Validity of Single-Ion Activity in Polyelectrolyte Solution
-sa
Sir: As is well known, the liquid-junction potential problem was a focus of earlier studies in the physical chemistry of electrolyte solutions. Some theoretical formulas have been derived for the potential and have been believed to be fairly satisfactory. This success has furnished a basis for the determination of the single-ion activities (including pH) in electrolyte solutions, which have no unique and rigorous thermodynamic meanings.’ Though the electrochemical study has been gradually extended to polymeric electrolytes, the liquid-junction problem has usually been regarded as already solved approximately on the basis of the earlier work mentioned above. The main purpose of the present communication is to caution against the validity of this interpretation in highly charged polyelectrolyte solutions, using MacInnes’ way of reasoning.2 Consider the following concentration cell with transference
Figure 1. Test of equation for liquid-junction potential.
electrode reversible
If we denote the sum of the electrode potentials by E,, we have
polyelectrolyte
electrode polyelectrolyte reversible to (I) solution ( 2 ) gegenions The thermodynamic consideration gives for the Iiquidjunction potential E I
where (Y and t Z p are the effective valency and the transference number (assumed to be concentration independent) of the macroions, a is the mean activity of the
-loa
0.005
0.01
0.02
0.05
Concentration, equiv/1000 g of water.
polyelectrolyte, aZgis the single-ion activity of the gegenions, R, T , and F have the usual meanings, and the subscripts 1 and 2 refer to the two solutions forming the junction. The emf of cell I, E , is known to be given by
E, = E
-
(3) By making tentatively the further approximation of El
(4) ~~
(1) See, for example, D. A. MacInnes, “The Principles of Electrochemistry,” Reinhold Publishing Corp., New York, N. Y . , 193CJ; Chapter 13; S. Glasstone, “An Introduction to Electrochemistry, D. Van Nostrand Co., Inc., New York, N. Y.,1962, Chapter 6; R. G. Bates, “Determination of pH, Theory and Practice,” John Wiley and Sons, Inc., New York, N. Y., 1964, Chapter 3. The relevant references are given in these books. (2) See Chapter 13 of MacInnes’ work or Chapter 6 of Glasstone’s work cited in ref 1.
Volume 70,Number 9 September 1966
