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MICHAELO'KEEFFEAND WALTERJ. MOORE
Vol. 65
for H20 (0.0025) at 126', and (a -1) for HC1 is twice the value for HzO (0.0035) a t 110'. The value of a for "3,does not differ from that for n = A - BL (4) HzO, as expected from the Rayleigh distillation. Here A and B are constants, A = 99; B = 0.36 ? It may be of interest to calculate the equilibrium 0.01 ml.-1 hr. Equation 4 is valid for flow rates coiistant K for the exchange of oxygen isotopes between approximately 20 and 100 ml./hr. between hydrated and solvent molecules from the The value of n calculated according to equation measured data. Using equation 2 , the measured 4 (col. 6) at the flow rate. L, (col. 3) and the over- values for a' and the data for HC1 ( g = 1. m = all separation, S, (col. 4) measured for the acid 6.8, and a = l.O07), the equilibrium constant at solutions, has beeii used to calculate the single 110' becomes K = 1.024 f 0.008. From the stage factor, cy for these systems. The factors data for HBr (u = 1, m = 10.9, a = 1.00'7) the are listed in col. 7 . The results for the average equilibrium constant at, 126' becomes K = 1.025 values of LY obtained in this way are: HC1 at l l O o , f 0.008. This compares to the value of 1.040 a = 1.0069 I 0.000'7; HBr at 126') a = 1.0074 $. calculated by Taube2 a t 25'. 0.0007; and HXO? at l % O , = 1.0028 2 0.0005. Since the separation factors observed for distilling The error is due>to the scattering of the measured azeotropic acid solutions are significantly larger values of n and the over-all separation. than those found for water distillation a t a comDiscussion.-Single stage factors a may be com- parable pressure, distillation of such solutions may pared with factors a' for water distillation at cor- be useful for coilcentrating oxygen isotopes. Exresponding temperatures6 Thus the increase for periments are in progress on oxygen exchange in a the HC1 solution is a - a' = 0.0034 =t 0.0007 packed column, between concentrated aqueous at 110' and for the HBr solution, ac - a' = 0.0049 lithium chloride aiid water vapor. f 0.000'7 a t 12,j'. These values are in agreement Recent resulJs by K. E. Holmberg (Acta. Chem., Scand. 14, with those obtained by Rayleigh distillation (Table 1660 (1960)), just called t o our attention, on physical propI). They indicate that (a -1) for HBr solution erties of isotopic forms of certain inorganic acid-water azeois three times as large as the corresponding value tropes, confirm our findings within experimental error.
lationship between the number of stages, n, and the flow rate, L , in ml./hr. can be obtained
DIFFUSIOIK OF OXYGEN IN SINGLE CRYSTALS OF NICKEL OXIDE BY
MICHAEL O'KEEFFEAND
n'.4LTER
J. M O O R E '
Chemical Laboratory, Indiana University, Bloomington, Indiana Received Aprzl 19, 1961
The diffusion of 180has been measured in monocrystalline S i 0 by following the exchange of gaseous oxygen enriched exp( -54 in l80with the NiO crystals. At an oxygen pressure Poz = 50 mm., from 1100 to 1500°, D'N,o = 1.0 ,X kcal./RT) cm.2 sec The D'N,o increases with PO?. The most reasonable mechanism for the oxygen diffusion is believed to be by way of interstitial oxygen atoms.
Several aut,hors have suggested that diffusion of oxygen may be a contributing mechanism in the growth of oxide films on iron2 and nickel.3 No experiment'al da,ta have been available on the diffusion of oxygen in cryslals of FeO, X i 0 or COO. The present work provides some information on the '*O-KiO system and allows us to conclude that Dgi0, the diffusion coefficient of oxygen in NiO, is at' least several orders of magnitude less than D$io, t,he diffiision casefficieiit of nickel in nickel oxide, at all temperatu:res. Experimental Methods
which was connected via a Kovar-glass seal t o a conventional vacuum syatem. After installation of the sample and pumping out, enriched oxygen was admitted to a known pressure, and the tube closed off by means of a vacuum stopcock. The volume thus sealed off was approximately 50 cc. After heating for a known time a t a constant temperature between 1100 and 1500" a sample of oxygen was drawn off and analyzed mass spectrometrically . If A Band A t are the abundances of oxygen-I8 in the gas phase initially and a t time t , respectively, the ratio of the uptake by the solid a t t to the uptake a t infinite time is given by
The diffusion coefficients were measured by following t h r oxchange of oxygen in the sample with a surrounding atmosphere of oxygen enriched in 1 8 0 . The enriched oxygen was obtained by electrolysis of water obtained from the Weissmann Institute and containing 0.437% 1 7 0 and 6.6270 180. The nickel cixide sample was in the form of a cube lvith (100) faces and 3 mm. cube edge cleaved from a single crystal boule grown by the Verneuil method a t the Tochigi Chemical Industry Company. To eliminate the possibility of exchange of oxygen with the container, the NiO sample was supported on platinum foil in a 90% Pt-lO% Rh tube
where 0 = X g / N s , the ratio of numbers of atoms of oxygen in gas and in crystal, and A is the natural abundance of oxygen-18 in the nickel oxide. The diffusion coefficient, D, is then given by
(1) Work done under Contract AT-(ll-1)-250, U. S. Atomic Energy Comniission. (2) H. Pfeiffer and B. Ilsohner, 2. Elektrochem., 60, 424 (1956). 13) R. Lindner and A. Akerstrom, Disc. Faraday Soc., 88, 133
(1957).
(2) where 01 = @/3and e erfc Z = eZ2(1 - erf a). The righthand side of equation 2 is the solution for a slab of thickneKq the cube of this expression is the corresponding solutio11 for a cube .s (4) J. Crank, "The Mathematics of Diffusion." Clarendon Press. Oxford, 1856, p. 54. (6) S. C . Jrtin, PTOC.R o g . SOC.(London), A243, 359 (1957).
DIFFUSION OF OXYGENIN NICKEL OXIDESINGLECRYSTALS
August, 1961
For small values of Dt (
