XOTES
410
as solvents. From the temperati-e dependence of the solubilities given in Table I the heats of solution in methanol, hexane, and water are 16.5, 10.3, and 6.9 kcal./mole, respectively.
Acknowledgments. R. Kuntz wishes to thank the Petroleum Research Fund for supporting the allied research which led to the measurements reported here. Special thanks are due Miss Pamela Worniington and Mr. Jonas Dedinas for determination of the absorption spectra of mercury in water, methanol, and perfluorodimethy lcy clobu tane .
Ionization of S o m e Weak Acids in Water-Heavy Water Mixtures'
by Pentti Sa10maa,2 Larry L. Schaleger, and F. A. Long Department of Chemistry, Cornell University, Ithaca, N . Y . (Received August 26, 1.963)
In connection with a recent study on acid-base equilibria and kinetics in mixtures of light and heavy water3 it was established that the glass electrode method of measuring acid dissociation constants permits precision determinations in these solvents. In this paper three additional sets of equilibrium data obtained by this method are reported. The results are collected in Table I. The pK values determined for the three acids in the solvent water, given in the table, are in good agreement with previously recorded values. * The relative ionization constants KH/KD reported earlier for chloroacetic acid, uiz., 3.0g4 and 2.76,5 compare well with the value 2.69 of Table I . On the other hand, the earlier values for hydrazoic acid,6 2.14 (at 20°), and for ammonium ion,' 3.1, are in poor agreement with the present data. According to the equilibrium theory of solvent isotope effect,s developed originally by Gross and co-workers* and revised recently by a number of author^,^-'^ the relative ionization constants K H / K , in mixed HZO-D20 solvents can be expressed in terms of the fractionation factors 1 and cp as
The Journal of Physical Chemistry
Table I : Observed and Calculated Values for the Relative Ionization Constants K H / K , of some Weak Acids in H20-D20Mixtures a t 25" ClCHiCOOH ~ K E = 2.809 Obsd. Calcd.
0.099 197 ,296 ,395 ,494 ,593 ,692 ,792 ,891 ,990 1.000"
1.07 1.16 1.27 1.39 1.56 1.71 1.95 2.17 2.40 2.66 (2.69)
1.08 1.18 1.29 1.41 1.55 1.71 1.90 2.12 2.37
NHP+
"S
PKH
3
4.694
PKH = 9 . 2 6 4
Obsd.
Calcd.
Obsd.
Calcd.
1.10 1.20 1.32 1.46 1.62 1.82 2.03 2.23 2.54 2.86 (2.89)
1.09 1.19 1.31 1.44 1.60 1.78 2.00 2.23 2.52
1.11 1.24 1.41 1.62 1.88 2.19 2.49 2.91 3.39 4.00 (4.06)
1.13 1.27 1.45 1.65 1.89 2.18 2.51 2.92 3.42
Extrapolated values.
in which the value of 1 is approximately 0.67.9j13 The calculated values given for chloroacetic and hydrazoic acids in Table I are based on these equations. Agreement between the observed and calculated values is excellent. It has recently been shown that the application of the simple Gross theory is not justified in cases in which the acid and its conjugate base contain several exchangeable hydrogens.a General equations, which depend on the number and character of exchangeable hydrogens in these species, were shown to be derivable without assumptions about fractionation factors. Ammonium ion provides an interesting application of the general equations since the number of exchangeable hydrogens in the acid and its conjugate base are four The support of the Atomic Energy Commission is gratefully acknowledged. On leave from the University of Turku, Turku, Finland. P. Salomaa, L. L. Schaleger, and F. A. Long, J . Am. Chem. Soe., 86, l(1964). A. 0. McDougall and F. A. Long, J . Phys. Chem., 66, 429 (1962). D. C. Martin and J. A. V. Butler, J . Chem. Soc., 1366 (1939). D. Dunn, F. S. Dainton, and 9. Duckworth, Trans. Faraday SOC.,57, 1131 (1961). G. Schwarzenbach, A. Epprecht, and H. Erlenmeyer, Helu. Chim. Acta, 19, 1292 (1936). P. Gross, et al., Trans. Faraday Soc., 32, 877, 883 (1936). E. L. Purlee, J . Am. Chem. Soc., 81, 263 (1959). (10) V. Gold, Trans. Faraday SOC.,56, 255 (1960). (11) C. G. Swain and R. F. W. Bader, Tetrahedron, 10, 182 (1960). (12) E. A. Halevi, F. A. Long, and M.A. Paul, J. Am. Chem. Soc., 83, 305 (1961). (13) According to a personal communication of Dr. V. Gold, n.m.r. studies lead to essentially the same value for 1 as that derived from other souroes.
NOTES
41 1
and three, respectively. In this particular case, the equations take the form
The fractionation factor for ammonia, pal can be estimated independently. The equilibrium constant for the reaction
+
T\",< HDO
=
NHzD
+ Ha0
in the gas phase a t 25' is 1.61,14from which the equilibrium constant for the liquid phase is
(-)(-) PHDO
K = 1.61 PH,O
PNHs
pXH,D
where the P's are the vapor pressures a t 25' of the species involved. By the rule of the geometric mean16 p2
2K/3
=
This agreement does not firmly establish eq. 3 and 4 as correct, the reason being that for ammonium ion, with the large KH/,KDratio of 4.06, the simple Gross equation gives nearly as good fit to the data as does the general equation. (So also does the equation for 1% "linear" theory.12) However, since data on other polybasic acids, e.g. H3P04 and H3As04,confirm the correctness of the general equations, the point of particular interest is that the general equation w h e n combined with a n independently estimated fractionation factor gives an excellent fit. (14) I. Kirschenbaum, "Physical Properties and Analysis of Heavy Water," McGraw-Hill Book Co., Inc., New York, N. Y., pp. 58-59. (15) J. Bigeleisen, J. Chem. Phys., 23, 2264 (1965). (16) I. Kirshenbaum, ref. 14, Chapter 1. (17) I. Kirshenbaum and H. C. Urey, J . Chem. Phys., 10, 70r3 (1942). (18) This can be shown to be true of the isotopically different waters. From the known values of P H ~ O / P H D a tOdifferent temperatures" and the values of P H ~ o the , normal boiling point of HDO calculates to be 100.71O , which is just the average of the boiling points of HzO and DzO, 100.00 and 101.43'. (19) The experimental value for the entropy of vaporization" of NDa, 23.9 oal. mole-' deg.-', does not materially differ from that of "3, and the difference between NHa and NHzD must therefore be even smaller.
1.004(PNH8/PPI",D)
in which the known value'8 of PH,O/PHDO, 1.069, has been introducedl. The vapor pressure of NH,D has not been measured directly, but its value relative to that of NH3 can be computed in the following way. The boiling points of NH3 and ND3 at atmospheric pressure are -33.48 and -31.11°, re~pective1y.l~It is reasonable to assume that the successive substitution of protium by deuterium atoms in ammonia effects approximately equal elevations in the boiling points18; therefore, the boiling point of KH2D cannot kgnificantly differ from -32.69". The entropy of vap~rization'~ of NH3 a t its boiling point is 23.3 cal. mole-' deg.-l, and using the same value for XHzD,19i.e., adopting Troutonls rule, one can calculate the vapor pressure ratios for different temperatures from the Clausius-Clapeyron equation. In this way, a value of 1.032 is obtained far PNH~/PI\",D at 25') which gives 9 2 = 1.04. Combiniing this value with the known values of I and K H / K D a, value of 1.083 is obtained from eq. 3b for the fractionation factor of ammonium ion, 91. With the fractionation factors available, KH/K, values for all intermediate solvent compositions can be calculated from eq. 3a. The results are given in the last column of Tablie I. The values thus obtained are in excellent agreement with those directly measured.
Particle to Particle Migration of Hydrogen Atoms on Platinum-Alumina Catalysts from Particle to Neighboring Particles
by S. Khoobiar Esso Research and Engineering Company, Process Research Division, Linden, New Jersey (Received August 49, 1963)
The dissociation of H2 to H atoms on noble metal wires at high temperatures has been suggested by many authors. The removal of H atoms from a hot wire is widely observed and accepted. This note reports results of a simple and direct experiment which strongly indicates that Hz dissociates on Pt/A1203 (0.5 wt. % Pt) catalyst at room temperature and that active H atoms migrate readily from parent particles to neighboring particles and initiate chemical reactions. These findings have not been observed previously.
Experimental WO3 and MnO2 are known to be good indicators for detection of H atoms because H atoms readily reduce these oxides with a collision efficiency of unity to Volume 68, Number 4
February, 1964
