J. Phys. Chem. 1985,89, 1062-1064
1062
one has to go to very high current densities, where the ohmic drop corrections are large. In our experiments, the ohmic drop corrections are relatively small, since the current densities are low. Thus, the curvature of the Tafel lines which we observe at low temperatures in the solid phase could be an indication that the coverage becomes appreciable, and the current starts to be controlled by the recombination reaction alone. A rough estimate shows that even at the highest cathodic potential the concentration of protons at the electrode surface had dropped only by a few percent, so that a concentration overpotential can be ruled out. Also, the current densities recorded with different sweep rates converge at high cathodic potentials (cf. Figure 2), which is in line with a control by the recombination reaction; this behavior could not be caused by the generation of hydrogen, since the amount by hydrogen produced changes with the sweep rate. Formally, the curvature of the Tafel lines in the solid phase could also be accounted for by an uncorrected ohmic potential drop between the working and the reference electrodes. For this to be the case, the ohmic drop would have to be on an average about five times as large as the values that we actually measured. We see no reason why our measurements should contain such an error. The hydrogen evolution reaction has previously been investigated in liquid methanol and ethanol both on Hg and Pt over a broad temperature range.I5-l7 In these electrolytesa significant (15) Bockris, J. OM.; Parsons, R.; Rosenberg, H. Trans. Faraday Soc. 1951, 47, 766.
(16) Conway, B. E.; MacKinnon, D. J.; Tilak, B. V. Trans. Faraday Soc. 1970,66, 1203.
(17) Conway, B. E.; Salomon, M. J. Chem. Phys. 1964, 41, 3169.
variation of the apparent cathodic transfer coefficient a, with temperature was observed, which is basically still unexplained. In our work, a, has the value of two at all temperatures both in the liquid and in the solid phase (in the latter, of course, only at potentials before the curvature sets in). This is not really surprizing, since this value is determined by the thermodynamics of the adsorption reaction, not by the kinetics. Thus any significant variation of a, from two would not be in line with a Volmer-Tafel mechanism with ratedetermining recombination. In the alcoholie electrolytes, ac seems to be connected with the kinetics of a charge-transfer step and is thus much more difficult to interpret. Conclusion
The hydrogen evolution reaction was investigated on Pt both in liquid and in solid HC104*5.5H20.Although it is still difficult to determine absolute values of the current in the solid phase, both the shape of the Tafel plots and the temperature variation of the current are well reproducible. In both phases our results are consistent with a Volmer-Tafel mechanism with rate-determining Tafel reaction. In contrast to results obtained in alcoholic electrolytes, no significant variation of the apparent cathodic transfer coefficient with temperature was observed. In the solid phase the Tafel plots are curved at high cathodic potentials. This could indicate that the coverage of the electrode with the adsorbed intermediate hydrogen becomes appreciable, and the current begins to be determined by the recombination reaction alone, but a definite interpretation is not possible at this stage. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Transitlons In the Speed of Sound In Concentrated Aqueous Electrolyte Solutions Frank J. MiUero,* Marino Fernandez, and Faina Vinokurova Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 331 49 (Received: September 1 1 , 1984; In Final Form: February 5, 1985)
Precise sound speed measurements on concentrated aqueous solutions of LiCl, MgC12,CaC12,SrC12,and BaClz show positive transitions. These transitions occur respectively at 10.7 f 0.7, 3.8 f 0.6, 5.9 f 0.5, 2.1 f 0.2, and 0.9 f 0.4 m for LiCl, MgC12,CaC12,SrCI2,and BaC12solutions. No transitions were found in the densities of these solutions. These transitions may be attributed to changes in the hydration structure around the cations. Since the sound speeds increase, the adiabatic compressibilities and partial molal compressibilities will decrease atythe transition. This would lead to an increase in the hydration numbers, if one uses a simple model for the water structure around the ions.
Introduction
Recently' while studying the speed of sound in concentrated aqueous MgC12 solutions from 25 to 95 OC,we noted that an apparent transition occurred in the relative speed of sound (AU = U - V',where the subscript zero refers to H20). We decided to look at this transition more carefully and for a number of other chlorides (Li, Ca, Sr, Ba). In this Letter we present these results.
Experimental Section The densities and sound speed of the solutions were made by the methods described in our previous ~ t u d i e s . ~The . ~ relative densities (Ap = p - po, where the subscript zero refers to pure water) are precise to f3 X 10-6 g cm-3 while the relative sound (1) Millero, F. J.; Vinokurova, F.; Fernandez, M.; Hershey, J. P. J . Solution Chem., submitted for publication. (2) Millero, F. J.; Kubinsky, T.J. Acousr. Soc. Am. 1975, 57, 312. (3) Mitlero, F. J.; Ward, G. K.; Chetirkin, P. V. J. Acoust. Soc. Am. 1977, 61, 1492.
0022-3654/85/2089- 1062$01.50/0
speeds AU are precise to k0.03 m s-I.~ The flow densimeterS was calibrated with the densities for water6 and ~eawater.~The sound velocimeter was calibrated with the sound speeds for water.s All the measurements were made at 25.00 f 0.01 OC. The concentrated stock solutions were made with reagent grade (Baker) salts and ion-exchanged (Millipore Super Q) water. The concentrations of these stock solutions were determined by AgNO, titrations. These concentrations were checked by using densities and known equations of state. The agreement was within *0.01%. The sound speed measurements were made in a large thermostated cell (500 cm3) which could be used to dilute a stock solution by 50%. The experimental runs for a given electrolyte (4) Millero, F. J.; Lawson, D.; Gonzalez, A. J. Geophys. Res. 1976, 81, 1177. ( 5 ) Picker, P.;Tremblay, E.;Joliceur, C. J . Solution Chem. 1974, 3, 377. ( 6 ) KM, G. S . J. Chem. Eng. Data 1975, 10, 97. (7) Millero, F. J.; Poisson, A. Deep-sea Res. 1981, 18, 625. (8) Del Grosso, V.; Mader, C. V. J. Acousr. SOC.Am. 1972, 52, 961.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1063
Letters
SrCI,
Mg CI,
I
Run I Run 2 A Run3
0
I2O 1101
'"h.5
I
1.5
mi
2
2.5
aa
LiCl 58'00/
38.2
E
\
a' 36.2
I
t
21
57,34 E 5668 -
BaCI,
oRun I Run 2
\
3
a 56.02 . 5536 -
Figure 2. The values of AU/m plotted vs. ml/' for LiCl solutions at 25 OC.
Figure 5. The values of AU/m plotted vs. m1/2for BaCI, solutions at 25
OC.
1
CaCI,
BaCI,
I65
57
'
I30
0
viii Figure 3. The values of AU/m plotted vs.
for CaCI, solutions at 25
115
\
OC.
CaCl,
were all made with the same stock solution either by weight dilutions with water or weight additions of the concentrated stock. Results and Calculations The relative sound speeds AU = U - V' divided by the molality plotted vs. m 1 / 2for the MgClz solutions are shown in Figure 1. A number of runs were made for these solutions in an attempt to narrow the transition zone. As shown in Figure 1, the transition does not always occur at the same concentration. We have estimated that the transition occurs at 3.8 f 0.6 m. The results for the other salts are shown in Figures 2-5. The transition for these salts are estimated to occur respectively at 10.7 f 0.7, 5.9 f 0.5, 2.1 f 0.2, and 0.9 f 0.4 m for LiCl, CaClZ, SrClZ,and BaC12 solutions.
61
r
22 l5
t
b
-22
I
2
mi
3
'15
4
Figure 6. The values of A p / m plotted vs. m1l2for various salts at 25 OC.
J . Phys. Chem. 1985,89, 1064-1066
1064
The relative densities Ap = p - po for these solutions have also been measured. Although the values of A p / m for these systems shown in Figure 6 have some curvature over these concentration regions, they do not show any sharp density transitions. Concentration transitions in the properties of aqueous electrolyte solutions have been discussed by a number of workers."' The earlier volume work has been summarized by V a s l ~ w .More ~ recently Phutela and Pitzer'O have discussed a transition in CaCl, solution near 5 m using osmotic coefficient and apparent molal enthalpy data. Our value of 5.9 f 0.5 m determined from sound speed measurements is in reasonable agreement with this value. Phutela and Pitzer'O suggest that this transition could be related to 2Ca(H20)72++ 4C1-
-
Ca$14(H20)8
+ 6H20
(1)
Mahluddin and Ismall' I have also reported viscosity transitions in concentrated solutions of Ca(N0J2, MgC12, and NiC12. Their reported transition of 3.5 m for these systems is in good agreement with the value of 3.8 f 0.6 m found for MgC12. and earlier studiesgat least support our results and indicate that they are not related to some artifact of the experimental measurements, such as some sound adsorption effect (our system is run at 2.8 MHz). We have no complete answer for the cause of these transitions. They could be related to the hydration (9) Vaslow, F. J . Phys. Chem. 1969, 73, 3745. In "Water and Aqueous Solutions", Horne, R. A., Ed.; Wiley: New York, 1972. (10) Phutela, R. C.; Pitzer, K. S.J. Solution Chem. 1983, 12, 201. (11) Mahluddin, S.; Ismall, K. J . Phys. Chem. 1983, 87, 5241.
structure around the Mg2+and Cl- ions or related to the formation of various ion clusters as suggested by Phutela and Pitzer.Io Since the sound speeds increase for LiCl, MgCl,, and CaCl,, decrease for SrCl, solutions, and go through a near linear transformation for BaCl, solutions, each system may have difference causes. An increase in the sound speed would result in decreases in the adiabatic compressibilities, os, and apparent molal compressibilities, 4K,S.One would infer from a decrease in 4K,S that hydrated water molecules are increased at the transition. This follows from the simple hydration m ~ d e l ' ~that . ' ~ relates changes in the partial molar compressibility to changes in hydration numbers ( A h ) -&KO
Ah = os vs where @S and V , are the compressibility and molar volume for bulk water (&V, = 8.1 X lo4 cm3 mol-' bar-' at 25 "C). This is not what one would expect for reaction 1 which would release water molecules. Further studies of other properties of these solutions are needed to elucidate these transitions. Acknowledgment. The authors acknowledge the support of the Oceanographic (WE-8120659) and Earth Science (EAR-840759) sections of the National Science foundation and the Office of Naval Research (N0001480-G0042) for this study. (12) Millero, F. J.; Ward, G. K.; Lepple, F. K.; Hoff, E.V. J . Phys. Chem. 1974, 78, 1636. (13) Millero, F. J.; Masterton, W. L. J . Phys. Chem. 1974, 78, 1287.
Effects of Proton Exchange on Dlffuslon in Aqueous Solutions of Methanol Allan J. Easteal,* A. Vernon J. Edge, and Lawrence A. Woolf Atomic and Molecular Physics Laboratories, Research School of Physical Sciences, The Australian National University, Canberra, A.C.T. 2601, Australia (Received: October 30, 1984)
Values are reported for tracer diffusion coefficientsof H2I80and 14CH30Hand the average diffusion coefficient (4)obtained by using HTO as tracer in water, methanol, and three water + methanol mixtures at 278 K. Tracer diffusion coefficients of H2l6Oand '?H3OH in methanol at 298 and 323 K are also presented. The assumption that tritium (from HTO) is statistically distributed between H 2 0and CH30H, previously used to indirectlydetermine Dm from measurements of DT and DMCH,OH, has been tested. The diffusion data are consistent with either a statistical or near-statistical (*lo%) distribution of tritium. 0H increases with temperature but is significantly smaller than unity even at 323 K, in The ratio ~ 2 ~ s 0 / D ~ 4 C Hin3 0methanol conflict with approximate literature values.
Introduction In solutions comprising water and a lower monohydric alcohol ROH (e.g., methanol, ethanol) rapid proton exchange occurs between the two components. Only the hydroxyl proton of the alcohol can participate in the exchange although both protons are available from the water molecule. The conventional method of measuring the tracer diffusion coefficient of water, namely, use of tritiated water in a diaphragm,cell experiment, gives in these circumstances not simply the tracer diffusion of water but a weighted mean diffusion coefficient DT which is determined by the distribution of the label T between the H 2 0 and the ROH. The Fick's first law expressions for the fluxes of tritiated species are for concentrations ci mol dmm3 J, = -Di(aci/t3x)
(i = T, HTO, ROT)
(1)
because CT
(ac~/ax) =
= CHTO
+ CROT
+
(~CROT/~X)(~CHTO/~X)
(2) (3)
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= JHTO + JROT Combination of eq 1-4 gives JT
(4)
Because each H 2 0 has two protons available for exchange and ROH only one, tritium exchange occurring on a statistical basis will produce two HTO molecules for each H 2 0 molecule undergoing exchange and one ROT molecule for each ROH, consequently CHTO = ( ~ c w / ( ~ c+ wC A ) ) C T (6) CROT = ( C A / ( ~ C W + C A ) ) C T where the subscripts W and A denote the water and alcohol, respectively.
0 1985 American Chemical Society
