WIENEFFECTIN URANYL ION SOLUTIONS
to ref. 4, and little change of ionization according to ref. 3, it is perhaps not remarkable that the highfield quotients are but little changed over the same range of temperature.
513
Acknowledgment. The support of the Office of Naval Research through an equipment loan contract and of the Atomic Energy Commission through Contract AT(30-1)-2890 is gratefully acknowledged.
The Wien Effect in Uranyl Ion Solutions. 111. Uranyl Sulfate from 5 to 65"
by Joseph F. Spinnler and Andrew Patterson, Jr. Contribution No. 1766 from the Sterliw Chamistry Laboratory, Yale Universzty, New Haven, Conneclicut (Received Augwt 24,1064)
The low-field conductance, pH, and high-field conductance of solutions of uranyl sulfate have been determined over a range of concentrations and temperatures from 5 through 65". Uranyl sulfate is found to be an associated electrolyte, although disagreement between lowfield conductance and ligand displacement determinations of the dissociation constant and ambiguities in the high-field conductance results make it impossible to place the dissociation constant precisely. The present results are consistent with the conclusion of low-field conductance measurements that the association of the electrolyte increases with increasing temperature.
We have measured the low- and high-field conductances and the pH of a series of solutions of uranyl sulfate over a range of concentrations and tempem tures from 5 to 65'. In a previous paper, we have reviewed the current understanding of the ionic species which are thought likely to be present in the partially hydrolyzed solutions of uranyl nitrate and perchlorate. I n contrast with these electrolytes, uranyl sulfate is appreciably associated, as shown by the measurements of conductance over a wide range of concentration and temperature made by Brown, Bunger, Marshall, and Secoy,2and by the ligand displacement measurements of Ahrland.* The high-field conductance measure ments are thus of interest to compare with those on the two relatively unassociated electrolytes of ref. 1.
Experimental The experimental procedure waa essentially the same as described in ref. 1. The sample of uranyl sulfate is taken from the same lot as that used by the authors of ref. 2, and was kindly provided by them through
the good offices of Prof. H. S. Harned. Since it was originally prepared for conductance measurements, it was not further treated before use.
Results Plots of the pH of the solutions vs. log c are.given in Figure 1. The equivalent conductances of the solute employed are shown in Figure 2. Plots of the highfield conductance quotients measured are given in Figure 3. Figure 4 shows the effects on the high-field conductance quotient of changing the concentration of the solution at two temperatures, 25 and 35 '.
Discussion The experimental evidence from measurements of conductance at low fields is that uranyl sulfate is an associated electrolyte. The conductance measure(1) J. F. Spinnler and A. Patterson, J . Phys. Cham., 69, 600 (1966). (2) R. D.Brown, W. B. Bunger, W. L. Marshall, and C. H. &coy, J . Am. Cham. SOC.,76, 1532 (1964). (3) S. Ahrland, Acta C h . Scad., 8 , 1907 (1964).
Volume 60, Number 2 Febrwrry 1966
JOSEPHF. SPINNLER AND ANDREW PATTERSON, JR.
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3'1 2.0
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ry
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Figure 1. Plots of the p H of solutions of uranyl sulfate vs. log c, molesb., as a function of temperature.
1
I
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100 FIELD, X V l C Y
150
ZOO
Figure 3. Plots of the high-field conductance quotient, A h / A ( 0), per cent, as a function of applied field in kv./cm. for different temperatures and concentrations. All concentrations are to be read as moles/l. X 10-4. The concentrations correspond approximately to the same speczc conductance for each determination.
I
Figure 2. Plots of the equivalent conductance of solutions of uranyl sulfate vs. c'", moles/l., as a function of temperature.
ments of Brown, et U Z . , ~ when plotted against cl" approach the conductance ads asymptotically; the average dissociation constant at 25' determined by these workers after making correction for hydrogen and other expected ions present in the hydrolyzed mixture is 5.9 X These authors comment that the significance of a dissociation constant of an associated higher valence type electrolyte determined by low-field conductance measurements is questionable for lack of a completely dissociated 2-2 electrolyte with which to compare it. It is their opinion that greater credence should be given to Ahrland's result, The Journal of Physical Chemistry
,.2s
1.11tY
Figure 4. Plots of high-field conductance quotient vs. field to show the effect of changing concentration a t two temperatures, 25 and 35".
0.02, because of inherent weaknesses in conductometric methods. Magnesium sulfate is another example of a 2-2 valence type associated electrolyte, for which low-field conductance measurements yield a dissociation constant of 6.3 X 10-3. Bailey and Patterson4 have shown that it is possible to combine the theories for weak and strong electrolyte high-field conductance in such a way as to permit computation of high-field conductance quotients agreeing satisfactorily with the experimental values obtained on magnesium sulfate, and Freitag and Pattersod have (4) F. E. Bailey and A Patterson, Jr.,
(1952).
J. Am. C h m . Soc.,
74, 4428
WIEN EFFECT IN URANYL IONSOLUTIONS
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refined this calculation considerably. The excellent agreement between experimental and theoretical highfield conductance results obtained by them for magnesium, zinc, and copper sulfates5 lends considerable support to the idea of ion pairs in such associated electrolytes and to the physical significance of the dissociation constant in predicting other solution properties. It is thus a disappointment to find the experimental high-field conductance measurements of uranyl and magnesium sulfates in some ways quite similar and in other ways not: at 25" for solutions of similar concentration, 1.4 X M , the conductance quotient is similar, 3.2%. The data on magnesium sulfate show quite a small variation with change in temperature from 5 to 55" in contrast to those of uranyl sulfate where the change is one unit in AA/A(O) over the same range. In view of the success of Freitag and Patterone would be led to assume the two electrolytes had similar association constants, which is quite in disagreement with the data quoted above. The high-field results are not consistent with the view that uranyl sulfate is associated to a degree that the equilibrium constant should be as small as 6 X Magnesium sulfate, for which such satisfactory highfield conductance calculations can be performed, has a dissociation constant of 6.3 X lo+, ten times larger, but the high-field conductances are similar. The first ionization constant of carbonic acid is of the order of and the results6 are entirely in magnitude of contrast with those obtained on uranyl sulfate, though it should be kept in mind that the valence' types in the two electrolytes may be different. Referring to Figure 3, we observe that the shapes of the high-field conductance curves are typical of those of "strong" electrolytes, in contrast with the results on uranyl fluoride,' in which a linear increase of conductance quotient was found. This latter indicates small ion-atmosphere effects, while the shape of the curves in Figure 3 are typical of significant ionic field influences. That the curves dip below the origin at low fields is neither experimental error nor evidence of negative Wien effects'; it is more probably due to the different rate of change of the ionic field effect in the uranyl sulfate and reference electrolyte solutions. To perform high-field conductance theoretical calculations on uranyl sulfate for comparison with the experimental results, we have assumed, as in ref. 1, that two principal hydrolysis reactions are possible
+ HzO = UzOs+' + 2H+ UOz+' + HzO = UOzOH+ + H+ 2U02+2
(1)
(2)
We have made no attempt to deal with mixtures of
these two reactions, but have computed mixed valences and limiting ionic conductances for the combined cations8 and have employed these in the theoretical calculations. Further, the calculations have been made ignoring association; this is forced upon us by the fact that the mathematical development of the combined theories embodying association corrections will admit calculations only on symmetrical electrolytes, although any valence type, including fractional valence types, can be accommodated so long as association is not involved. I n spite of this limitation, the results are of considerable interest, as may be seen from inspection of Figure 5. At 5, 25, and 65O, the results of calculations assuming both reactions 1 and 2 are shown. At the other temperatures, only r e action 1 is shown. Considering for the moment only reaction 1, the theoretical calculation reproduces the experimental results quite closely at 5", within 0.1 unit; this widens at 25O, and at 65" the disparity is 0.51 unit a t 200 kv./cm. Reaction 2 is in every case a poorer compromise. The validity of these theoretical calculations may be examined by making calculations in which only one parameter is varied. As in ref. 1, the concentration, limiting combined ionic conductance, and valence factor are the important variables. Using data of the
0 0
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I
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Figure 5. Plots of theoretically computed high-field conductance quotients us. field aa a function of temperature and for two choices of hydrolysis reaction of uranyl sulfate; see text.
(5) H.Freitag and A. Patterson, Jr., J . Electrochem. SOC.,108, 529 (1961). (6) D. Berg and A. Patterson, Jr., J. Am. Chem. SOC.,7 5 , 5197 (1953); E.F. Wissbrun and A. Patterson, Jr., J. P h p . C h . , 58, 693 (1954). (7) J. F. S p i d e r and A. Patterson, Jr., ibid., 69, 508 (19G.5). (8) J. F, Spinnler and A. Patterson, Jr., ibid., 69,658 (1965).
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JOSEPH F. SPINNLER AND ANDREWPATTERSON, JR.
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Figure 6. Plots of theoretically computed high-field conductance quotients us. field to show effect of varying the concentration as a parameter. The data used, aside from the concentration variable, pertain to the experimental determination on a solution of uranyl sulfate a t 25" found in Figures 3 and 4.
Figure 7. Plots of theoretically computed high-field conductance quotients us. field to show effects of varying the limiting ionic conductance of the cation. As in Figure 6, the data used, except for the ionic conductance variable, pertain to the same experimental determination a t 25" found in Figures 3, 4, and 6.
experimental determination found in Figures 3 and 4, concentration 1.515 X low4M , and temperature 25O, in Figure 6 is shown the effect of changing the mncentration, and in Figure 7 the effect of varying &,+ over quite a wide range in the calculation. A variation of 10% in the concentration results in a difference of approximately 0.1 in AA/A(O) while a variation of 15% in the limiting ionic conductance results in a difference of less than 0.05 in the same quantity. Since the concentration is known with much higher accuracy and since the limiting ionic conductance is estimated to be known within 15%, these influences are of lesser importance, leaving the valence factor, zf, which in these measurements depends upon the pH measurements and on the assumption of a given hydrolysis reaction, as the determining factor in the precision of the theoretical calculation. In Figure 5, the two curves denoted 25' I and 25' I1 differ only in the z+ employed, the values being 1.83 and 1.69, respectively. It can be seen that a decrease in z+ of 0.14 results in a diminution of the high-field conductance quotient by 0.39% at 200 kv./cm., corresponding to a hydrogen ion concentration change of 0.003 X mole/l. The pH measurements are made to 0.01 pH unit, corresponding to a variance of 0.007 X mole/l. The importance of the valence factor and consequently of the pH determination is thus emphasized, and it is clear that theoretical results agreeing within 0.2y0 with the experimental high-field measurements are in good agreement by this criterion. Calculations at lower fields or with a smaller conductance quotient, as with uranyl perchlorate and nitrate, will be less seriously affected by errors in the hydrogen ion
determination, and agreement a t the lower temperatures, at which the pH measurements are more precise, should be better, as observed. However, the gradual and increasing divergence between the experimental and theoretical results at higher temperatures suggests, as is known to be the case with unassociated uranyl salts, that the hydrolysis pattern is changing, the valence factors applicable to the theoretical calculation changing a t the same time, so that no one arbitrary choice of hydrolysis reaction, e.g., reactions 1 or 2, will be applicable. A comment in ref. 2 is also pertinent; there it is noted that plots of (AT.I),'/(A&,' over the wide temperature range studied yield curves with negative slope, indicative that fewer ions are present to carry the current a t higher temperatures, and that accordingly there is an increase in association with increasing temperature. Inclusion of a dissociation constant would improve the agreement at higher temperatures where the experimental results lie above the present theoretical calculations. The calculation has been performed for uranyl fluoride6 in which case the expected hydrolysis products are all univalent and the salt itself sdiciently associated that it is reasonable to consider the ionic environment made up of symmetrical valence types. Although this cannot be done for uranyl sulfate, our results at 5 and 15' indicate that the degree of association is small, and that at 25' Ahrland's value of 0.02 cannot be in error by so much as a factor of ten. It does seem quite clear that the low-field conductance-derived equilibrium constant result of Brown, et a1.,2 while entirely representative of the results of their careful measurements, is the consequence of
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WIEN EFFECT IN URANYL ION SOLUTIONS
0
50
ID0 FIELD, I;V/CM
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Figure 8. Plots of experimental high-field conductance quotient determination vs. field on solutions of uranyl perchlorate and nitrate, ref. 1, uranyl fluoride, ref. 7, and uranyl sulfate, this paper. The concentrations specified correspond roughly to constant specific conductance of the soluliions.
complexities of ionic interaction and association which we do not presently understand. In view of the decidedly unusual behavior of the other uranyl salts studied in this series of investigations, it is of interest to compare the results and to temper the conclusions reached in the previous paragraph in light of this comparison. The results for 25' and essentially constant specific conductance of the solutions have been brought together in Figure 8. The behavior of uranyl nitrate, perchlorate, and fluoride has been explained in terms of specific interactions under the influence of high fields, in which the hydrolysis equilibria are reversed and the conductance
quotient decreased below the value which would have been expected for a similar valence-type electrolyte. It is thus puzzling to understand why uranyl sulfate should be immune to these effects, if indeed this is the case. Thus, it is quite possible that the postulations of the preceding paragraph are in error, that uranyl sulfate is indeed as weak an electrolyte as a dissociation would indicate, and that the highconstant of 6 X 6eld conductance results are the consequence of specifk interactions which are simply different in magnitude from those observed in the other uranyl salts. The principal basis in the present investigation for not coming to such a conclusion is the shape of the high-field conductance quotient us. field curves. As noted earlier, the uranyl sulfate curves are indicative of an appreciable ionic atmosphere; the uranyl fluoride curves are not. Because of the orderly behavior of magnesium, zinc, and copper sulfates6on the one hand, and the unusual behavior of uranyl nitrate, perchlorate, and fluoride116 on the other, one is forced to conclude that uranyl sulfate is an associated electrolyte with an association constant in the range determined by Ahrland rather than that indicated by low-field conductance results, but that at high fields there is every possibility that specific interactions cause the results to deviate from what would be expected of an ordinary, well-behaved, hydrolyzed, 2-2 electrolyte.
Acknowledgment. This work was supported by the Atomic Energy Commission, in part under Contract AT (30-1)-1375 and in part under AT (30-1)-2890. An equipment loan contract with the Office of Naval Research has made available many of the high-field conductance bridge components.
Volume 69, Number 8 Feebruary 1966
