COMMUNICATIONS TO THE EDITOR
2137
Electrical Conductance of Concentrated Aqueous Solutions and Molten Salts: Correlation through Free Volume Transport Model Sir: Following a successful correlation of electrical conductance data for nitrate melts,l both glass-forming and otherwise, by means of the equation A
=
AT-”z exp(-k/(T
- To))
(1)
(derived from the free volume transport model of Cohen and Turnbull,2 Tobeing the temperature below which the liquid contains no free volume, A and k are constants), we have investigated the possibility of extending this type of interpretation of transport phenomena to concentrated aqueous solutions. Such an extension is strongly suggested by reports that, e.g., hot Mg(N03)2-H20solutions, when sufficiently concentrated, yield glassy solids on ~ o o l i n g . ~ . ~ We have measured the electrical conductance of Mg(N03)2-Hz0solutions as a function of temperature and water content in the range Mg:H2O = 1:6 to 1:2.9, and of molten Ca(NOJ2.4Hz0as a function of temperature, and find eq. 1 is satisfied within experimental error with the same limitations found in the case of the water-free Ca(N03)z-KN03me1ts.l The To values (theoretical glass transition temperatures) obtained are, not surprisingly, dependent on the charge concentration, ranging from 201’K. for Ca(NOa)z.4Hz0to 244OK. for Mg(N03)~.2.9H20;cf. 306-330’K. for KN03-Ca(N0J2 glasses. The important finding, however, is that one can predict To for the 1:4 liquids,6 to within experimental error ( f5’), from the To 21s. cation charge-to-radius-ratio plot obtained experimentally for the water-free Ca(NO&KN03 melts by making the reasonable assumption that the predominant cations in the liquid at this compo& t i o n are quasi-spherical M(H20)2+ (M2+ = Mg2+, Ca2+) species with effective radii ( 2 ~ ~ ~ rMa+). 0 Furthermore, the constant k of eq. 1 for the aqueous nitrates is the same within experimental error as for the Ca(N03)z-KN03 melts. Finally, plots of E , vs. T (e.g., Figure 6 (ii) of ref. 1) for the water-free eutectic melts Li, Na, K, NOS6and Na, K, NOZ, NO8 form a natural extension to higher temperatures for E, os. T plots of the aqueous melts of about the same To value, thus linking the temperature dependence of conductance of the aqueous and nonaqueous glassforming liquids and emphasizing the lack of distinction between them. Thus, despite some ambiguity arising from the treatment of M(H20)d2+ species as spherical ions, these
+
observations strongly imply that both the electrical conductance behavior and the glass-forming ability of these solutions may best be interpreted by regarding the aqueous melts as molten salts of large (;.e., weak field) cations. This appears to be a fruitful concept, as it leads one to predict tetrahedral NiC12- complexes in, e.g., hot (100-150’) highly concentrated MgClz solutions by analogy with NiCLZ- in molten CsC1.’ The existence of these complexes, previously unobserved in aqueous systems, is readily demonstrated by test tube experiments and has now been verified by spectroscopic methods.*S9 The free volume approach offers an interpretation of both the temperature dependence of liquid transport properties and the phenomenon of glass formation, and, as glass-forming ability is apparently quite common among concentrated aqueous solutions, this approach may provide a useful new view of concentrated electrolyte solutions in general. (1) C. A. Angell, J . Phys. Chem., 64, 218, 1917 (1964). (2) M. H. Cohen and D. Turnbull, J . Chem. Phys., 31, 1164 (1959). (3) J. W. Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. IV, Longmans, Green and Co., London, 1940, p. 380. (4) We now 6nd that many solutions of highly soluble salts have concentration ranges which are glaa,+forming on sufEcient cooling (e.g., quenching into liquid nitrogen) although it seems that only concentrated Mg(N0s)rHzO solutions have high enough glass transition temperatures to yield brittle glasses at room temperature. (5) The 1:4 compositions are the simplest to treat since the higher water content solutions were not glass-forming and thus TOcould not be determined experimentally, and for lower water contents the choice of species and their effective radii raises difficulties. (6) L. A. King, C. C. Bissell, and F. R. Duke, J. Electrochem. Soc., 111, 720, (1964). (7) D. M. Gruen and R. L. McBeth, J . Phys. C h m . , 63, 393 (1959). (8) C. A. Angell and D. M. Gruen, to be published. (9) NiCl2- complexes are not found in concentrated CaClz solutions, however, because the Ni2+ competes successfully with the Ca*+for the hydration sheath. (10) Currently on leave at Argonne National Laboratory, Argonne, Ill.
DEPARTMENTOF METALLURGY UNIVERSITYOF MELBOURNE PARKVILLE N.2. VICTORIA AUBTRALA RECEIVED APRIL 9, 1965
Concerning the Reaction NO2 NO
+ Oz
C. A. A N G E L L ~ ~
+ NO* +
Sir: Kistiakowski and co-workers1s2have postulated this reaction as a result of their studies of the flash photolysis of nitrogen dioxide and the reaction of nitrogen atoms with nitrogen dioxide. Volume 69, Number 6 June 1966
2138
COMMUNICATIONS TO THE EDITOR
We have studied the photolysis of nitrogen dioxide in a flow system using polychromatic radiation of predominantly 3655-A. wave length. The apparatus and technique used will be described in a succeeding a r t i ~ l e . ~The kinetics of the photolysis (see Figure 1 and Table I) has led to the postulated mechanism
+ hv +NO + 0 NO2 + hv’ NO2* NOz* +NO, + hv’ NO2 + 0 +NO + Hg + hv +Hg* Hg* + NO +NO* + Hg NO* + NO2 N20 + NO2
(1)
(2)
--+
(3)
(4)
0 2
--+
(5) (6)
(7)
0 2
Reactions 1, 2, 3, and 4 have been studied by numerous investigator^.^^^ In order to ascertain if reaction 5 occurs, we photolyzed a mixture of nitrous oxide and neopentane (2,2-dimethylpropane) under the same experimental conditions as with nitrogen dioxide. The products of the photolysis of this mixture are nitrogen dioxide, neopentyl alcohol, and the reaction products of neopentyl alcohol and nitrogen dioxide. Since nitrous oxide itself is not decomposed under these conditions, this experiment demonstrates the presence of sufficient mercury vapor in the reaction vessel during the course of a run to initiate the photosensitized decomposition of nitrous oxide. Reaction 6 has been shown6 to be a resonant reaction; ie., there is essentially no potential energy barrier encountered in going from reactants to products. The excited nitric oxide is in the % state.’ A transition
2oo
7‘ 160 140
.g.
120
r
-
I
1
I
Table I
Run
1 2 3 4 5 6 7 8 9 10 11 12 13
Moles of N90 Moles of NO produced/ produced/ Contaat time min. min. X 10s min. X 107 x 107
124.0 63.3 11.6 29.0 9.38 25.8 8.62 39.0 49.5 140.0 26.8 31.6 76.5
29.2 36.4 59.2 54.3 13.8 52.1 11.3 0.12 4.16 146.0 4.50 2.57 0.46
... 3.20 19.3 1.02 0.76 41.6 93.0 20.8 20.8 26.1 94.9
x
107
154.0 83.5 29.5 32.0 35.7 27.6 6.81 62.5 150.0 123.0 33.7 40.2 14.3
to this excited state would result in an excited molecule with a long lifetime, since the radiative transition probability of returning to the ground state is exceptionally low.6 Consider reaction 7 in detail
This reaction can occur only when the nitrous oxide molecule approaches the nitrogen dioxide molecule in a nitrogen to nitrogen orientation. The nitrogen t o nitrogen bond and the oxygen to oxygen bond must be formed and two nitrogen to oxygen bonds broken. Since N203is stable in the solid phase, it is reasonable
to assume that
I
93.4 32.4
Mole8 of 02 produced/ min.
1
O=N-N
L
