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01
0 0
20
40
60 TIME,
80
100
120
140
Figure 3. Changes of potential difference ( A e ) between water and ether phases (crosses) and concomitant increase it1 abqorption, &OD (integrated over the band) in water phase (circles); squares show the concomitant increase i n ab3orption in reoxidized ether phase.
tion of known concentration was connected by an agar bridge saturated with KCl. Beckman platinum electrodes were used, and the difference of potential (A€) between the half-cells was measured by a Leeds and Northrup universal pH indicator assembly. The results are shown in Figure 3 (crosses, scale at right). The curve indicates growing accumulation of the reduced dye in the ether phase. (3) Reoxiclation of the Extracted Leuco Dye. The separated ether phase was exposed to air after dilution by methanol (to prevent precipitation of MB). The increasing amount of 3IB found in this phase after prolonged illumination is shown in Figure 4. (Recoloratioii could be observed upon exposure to air also in methanol-free ether-although solid MB does not dissolve niarliedly in ether, and no hIB is extracted into puic ether from an aqueous solution.) These three sets of experiments indicate slowly growing extraction of the leuco form of JIB into ether during prolonged illumination. The maximum amount extracted i n sonic of our experiments, about 35% of the total dye present, seemed to be close to saturation. The whole process, bleaching and extraction, is fully reversible-at least, after a not excessively long illumination period. If both phases are mixed together again and the ether is permitted to evaporate, the absorption spectrum of the remaining aqueous solution is found to be the same as before the experiment. With larger concentrations of MB (25 X M), the shape of the absorption curve changed during illumination. This must be attributed to the dimerization of the dye,9 Since the total concentration of MB in water was reduced by the extraction of a part of it (as leuro dye) into ether, the dimerization equilibrium must have been shifted. These results confirm that a storage of photochemical energy can be achieved by dividing the photoproducts The Journal of Physical Chemistry
540
min.
500
620
660
700
mP Figure 4. Absorption by reoxidized leuco dye in the ether phase after different times of illumination of the two-phase system: 1, 10 min; 2, 35 min; 3, 95 min; 4, 120 min.
between two phases in an inhomogeneous system. This could be considered as a model of the storage of photochemical energy in photosynthesis.2 Experiments should be made on the change in effectiveness of the separation of leuco methylene blue from illuminated solutions as function of concentration of the components, the pH, and the interface area between the two phases. (Competition between homogeneous recombination in the aqueous phase and the removal of the reduction product into another phase, preventing this recombination, must depend on the diffusion path between the locus of the primary reaction and the water-ether interface.) I n order to reproduce better the situation in the living cell, one could prepare very thin layers-only a few molecules thick-alternately hydrophilic and hydrophobic, and study the separation process in such systems. Finally, one should try to find and use dyes in which a higher proportion of the leuco form is in the neutral state under the pH conditions used. (9) E. Rabinowitch and L. F. Epstein, J . Am. Chem. SOC.,63, 69 (1941).
The First Ionization Potentials of Samarium, Europium, Gadolinium, Dysprosium, Holmium, Erbium, Thulium, and Ytterbium by the Electron-Impact Method
by K. F. Zmbov and J. L. Margrave Department of Chemistry, Rice University, Houston, Texas (Receiaed April 1 , 1966)
During a mass spectrometric study of gaseous equilibria involving rare earth subfluorides, it was possible
NOTES
3015
L
-
IOOL
In
c
13 z
>
a
a e m
a
a
>-
e
g
10
c W
z
0-DY
z Q
IO
9
I1
I2
I
13
ELECTRON ENERGY (VOLTS)
Table I: Ionization Potentials of Rare Earth Atoms by Different Methods Ionization potential,
Sm Eu Gd DY
Ho Er Tm Yb
ev
(5.6)" 5.70 f 0.02 5.56 f 0.10 (5.67) 5.68 f 0.03 5.61 f 0.10 (6.16) 5.98 f 0.10 (6.8) 5.80 f 0.02 5.8 f 0.10 6.19 f 0.02 5.85 f 0.10 6.08 f 0.03 6 . 1 1 f 0.10 6.15 f 0.03 5.87 i 0.10 (6.2) 5.90 f 0.10
Method
Optical spectroscopy Surface ionization Electron impact Optical spectroscopy Surface ionization Electron impact Optical spectroscopy Electron impact Optical spectroscopy Surface ionization Electron impact Surface ionization Electron impact Surface ionization Electron impact Surface ionization Electron impact Optical spectroscopy Electron impact
6
7
0
9
ELECTRON VOLTS (CORRECTED)
Figure 1. Ionization efficiency data for Sm+, Hg+, and C a + using the semilog matching method. The Sm+ scale must be shifted 4.87 and 0.65 ev to give the indicated matchings with Hg+and Ca+, respectively.
Element
5
Ref
6
7 This work 6 7
This work 6
This work 6 7
This work 7 This work 8 This work 7
This work
6 This work
All values in parentheses indicate e s t i m a h .
to measure the ionization potentials of Sm, Eu, Gd, Dy, Ho, Er, Tni, and Yb atoms by the electron-impact
Figure 2. Ionization efficiency curves for Dy+, Ho+, and E r + ions.
method. The experiments were carried out with a magnetic mass spectrometer described previously. Optical spectra of rare earth atoms are complex and incompletely interpreted. Gaseous rare earth atoms were produced either by heating the metal in a tantalum Knudsen cell or by reduction of the various trifluorides and CaFz with Gd or Ho in the same cell. As a typical example, the ionization efficiency curve of Smf is compared with that of €I& ion from the background in Figure 1, curve I, or with Ca+ from CaF2 reduction in Figure 1, curve 11, by using the semilog matching method,3 which consists of measuring the voltage shift required to match the initial portion of the ionization efficiency curve of the rare earth ion with that of the reference ion. With the known ionization potentials of Hg (10.43 e ~ ) and ~ " that of Ca (6.11 e ~ ) , ~ * ~obtains o n e 5.56 -f 0.10 ev for the ionization potential of Sm. This compares ~~
(1) G. D.Blue, J. W. Green, R. G. Bautista, pnd J. L. Margrave, J. Phys. Chem., 67, 877 (1963). (2) (a) For example, C. E. Moore, National Bureau of Standards Circular 467, Vol. 1-111, U. 8. Government Printing Office, Washington, D. C., 1949. 1952,and 1958,has not yet issued a volume on rare earth spectra. (b) J. Sugar and J. Reader, J. Opt. SOC.Am., 55, 1286 (1965). (3) S. N. Foner and R. L. Hudson, J. Chem. Phys., 36, 2681 (1962). (4) (a) W. B. Nottingham, Phys. Rev., 55,203 (1939); (b) R. E.Fox, J . C h m . Phys., 35, 1379 (1961). (5) J. C. Boyce, Rev. Mod. Phys., 13, 1 (1941).
Volume 70, Number 9 September 1966
NOTES
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// Q
i
ii 0.01I
I
ELECTRON Figure 3. Ionization efficiency curves for
The Journal of Physical Chemistry
---I I
ENERGY
Tm+,Eu+, Yb+, and G d + with Hg+ as a reference.
well with the estimated spectroscopic value of 5.6 eve and the recently published surface ionization data, 5.70 ev.’ Direct studies of Gd atoms sublimed from a T a 5*98 o’lo ev “le ionization potential of Gd. Eu, Dy, Ho, Er, Tm, and Yb atoms were produced
*
+--lev
by heating a mixture of the corresponding trifluorides with Gd metal in the tantalum Knudsen cell. Ioni(6) W. E. Forsythe, “Smithsonian Physical Tables,” 9th revised ed, Publication 4169, Smithsonian Institution, Washington, D. C., 1954. (7) N, I. Alekseev and D. L. Kaminskii, Zh. Tekhn. Fiz., 34, 1521 (1964).
COMMUNICATIONS TO THE EDITOR
zation efficiency curves on a semilog plot are shown for Dy, Ho, and E r in Figure 2 and for Eu, Gd, Tm, and Yb in Figure 3. The electron-voltage scale was calibrated by the vanishing-current method, using the background mercury as a standard. This method yielded 6.11 i 0.1 ev for the ionization potential of Er, in excellent agreement with recently published data8 obtained by the surface ionization method (6.08 f 0.03 ev). By using the Er value as a standard, one obtains I.P.(Dy) = 5.78 f 0.1 ev and I.P.(Ho) = 5.85 f 0.1 ev, and the use of the Ho value as a standard, along with Hg, yields the ionization potentials of Eu, Tm, and Yb.
3017
Table I summarizes values for the ioniztion potentials obtained by the different methods. It appears that the general rule
I ( M ) / I ( M + ) = 0.50
Z!Z
0.01
holds for the rare earth atoms based on these data and those of Sugar and Reader.2b
Acknozuledgment. This work was supported by the
U. S. Atomic Energy Commission under Contract AT- (40- 1)-2907. (8) N. I. Ionov and M. A. Mitsev, Z h . Eksperim. i Teor. F i z . , 38, 1350 (1960)-
COMMUNICATIONS TO THE EDITOR
Comments on the Paper “Solubility of Hydrogen in Potassium Hydroxide and Sulfuric Acid ; Salting-out and Hydration” by P. Ruetschi and R. F. Amlie
Sir: Recently,’ Conway, Desnoyers, and Smith developed a detailed theory of salting-out of nonelectrolytes by simple salts and polymeric ions and showed that substantial improvements on previous continuousdistribution theories2* could be made by regarding the salting-out as arising from effects in two distinguishable regions near the ion: (i) that due to the loss of normal solvent function of the water molecules attached to the ion in the primary hydration shell4 of radius r h , where appreciable dielectric saturation occurs; and (ii) that due to the more continuous distribution of solvent and nonelectrolyte in the region of solution beyond the , ~ ’ the ~ different size hydration radius, r h , a r i ~ i n g ~from and polarization of the nonelectrolyte and solvent water molecules in the field of the ion. From the periphery of the ion of radius a to Th, the dielectric constant is quite low (ca. 2-6) and most nonelectrolytes (except those more polar than water and of comparable size) will virtually be completely saltedout.’ We wish to point out that in a paper recently published by Ruetschi and Amlie,6 a type of approach very similar to that which we published previously1 has
evidently been made independently and a relation for the salting-out involving two integrands from a to l’h and l‘h to R (cf. ref 1, 6) or 03 is given as in our earlier paper.’ A two-region model has also been used by Glueckauf’ in the interpretation of ionic dielectric decrements where a step-function for the dielectric constant was used with the discontinuity at Th. While Ruetschi and Amlie6 based their calculations of the contribution ii arising from the nonelectrolyte-solvent distribution between r h and R (or a ) on Debye’s theory2 in terms of dielectric decrements (which are not always experimentally available), we preferred to extend Debye’s theory to obtain the salting-out constants in terms of the more readily available partial molal volumes and dipole moments of the nonelectrolytes. In doing so, it was possible to show that an equation such as ours (or the similar one now given by the above authors) can account for the observed salting-out of a number of combinations of simple salts and simple nonelectrolytes. (1) B . E. Conway, J. E. Desnoyers, and A. C . Smith, Phil. Trans. Roy. SOC.(London), A256, 389 (1964).
(2) P. Debye and J . McAulay, Physik. Z., 26, 22 (1925); P.Debye, 2. P h y s i k . Chem , 130, 55 (1927). (3) J. A. V. Butler, J . Phys. Chem., 33, 1015 (1929); Proc. R o y SOC. (London), A122, 399 (1929). (4) J . O’M Bockris, Quart. Rev. (London), 3, 173 (1949). (5) J. O’M.Bockris, J. Bowler-Reed, and J . A. Kitchener, Trans. Faraday SOC.,47, 184 (1951). (6) P. Ruetschi and R. F. Amlie, J . Phys. Chem., 70, 718 (1966). (7) E. Glueckauf, Trans. Faraday Soc., 60, 1637 (1964).
Volume 70, Number 9
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