1320
HARRYR. BRONSTEIN
Electromotive Force Measurements on Solutions
of Rare Earth Metals in Their Molten Halides. I. The Cerium-Cerium Chloride, Praseodymium-Praseodymium Chloride, and Neodymium-Neodymium Chloride Solutions by Harry R. Bronstein Chemistry Division, Oak Ridge National Laboratory Oak Ridge, Tennessee
(Received August 1 6 , 1968)
A recently developed metal cell technique eliminated the need of ceramic materials and permitted the investigation of the emf of the rare earth metal-rare earth halide solutions in a corrosion-free system. The emf of solutions of Ce-CeCla, Pr-PrCla, and Nd-NdCl3 was measured as a function of metal concentration in the cell
The metal cell containing the rare earth metal-halide solutions was a cylindrical tantalum cup with communication to the bridging liquid by means of a sintered tantalum pellet of extremely fine porosity welded into the side of the cup. The potential measurements indicate the presence of a subion M2+ from the reaction Mo 2M3++ 3M2+ in all three systems. Saturation concentrations determined by the present technique are in agreement with those obtained by conductivity and phase equilibria studies where the solutions were not in aontaat with ceramic material,
+
Introduction A unique type of solution-a metal d i d v e d in its molten halide-has been the subject of much research in recent years. The extent of our knowledge and conjecture concerning these solutions has been reviewed in recent articles.2 The variety in the nature of metal solutions as distinguished by electrical conductance and phase equilibria is exemplified by the rare earth metalmetal halide solutions in the series Ce-CeCl,, Pr-PrCla, and Nd-NdC13.x-s Cerium metal dissolved in cerium chloride imparts a sizable electronic component to the conductivity of the solution; no solid phase intermediate in composition between CeCl3 and CeO is observed. Praseodymium produces less electronic conductivity and its phase diagram shows a solid mixed chloride PrClz.3 stable in the rather small temperature range 590-659’. An electrically insulating solid compound of lower valent neodymium, NdC12, persists down to room temperature in the system Nd-NdCla. The stability of the lower valent cation is undoubtedly reflected in the electrical conductivity of the solutions of Nd in molten NdCla which remain essentially ionic. An emf study should provide more conclusive evidence as to the species present in solution. Unfortunately, previous such measurements in solutions of Ce-CeCls and La-LaCla 12~1*are of doubtful validity because of the corrosiveness of these solutions14 to the ceramic The Journal of Phwical Chemistry
crucibles in which they were contained. Figures 1 and 2 show the distortion of the conductivity data caused by the reaction of these solutions with the (1) Research sponsored by the U. 9. Atomic Energy Commission under contract with Union Carbide Corporation. (2) (a) M. A, Bredig, “Mixtures of Metals with Molten Salts” in “Molten Salt Chemistry,” M. Blander, Ed., Interscience Publishers Inc., New York, N. Y,,1964; (b) J. D . Corbett, “The Solutions of Metals in their Molten Salts” in “Fused Salts.” E. R. sundheim. Ed., McGraw-Hill Book Co. Inc.. New York, N, Y.,1964, (3) H. R. Bronstein, A. 9. Dworkin, and M. A. Bredig. J . Phys. Chem., 6 6 , 44 (1962). (4) A. S, Dworkin, H. R. Bronstein, and M. A. Bredig, dbld., 66, 1201 (1962). ( 6 ) A. S. Dworkin, R . R . Bronstein, and M. A. Bredig, Discussions Faraday SOC.,3 2 , 188 (1961). (6) G . W. Mellors and S. Senderoff, J . Phys. Chem., 63, 1111 (1969). (7) L. F. Druding and J. D. Corbett, J . Amer. Chem. SOC.,83. 2462 (1961). (8) L. F. Druding, J. D. Corbett. and B . N. Ramsey, Inorg. Chem., 2, 869 (1963). (9) 8. Senderoff and G. W. Mellors, J . Electrochem. Soc., 105, 224 (1968). (10) M . V. Smirnov and V. 9. Lbov, Electrokhimiya, 1, 833 (1965); English translation, Soa. Electrochem.. 1, 750 (1966). (11) M . V. Smirnov and V. 9. Lbov, T r . Inst. Electrokhtm., Akad. Nauk SSSR, Ural’sk.Filial, No.8, 3 (1966); Trans. Inst. Electrochem., 8, 1 (1966). (12) M. V. Smirnov, P. M . Usov, and T. F. Kharemova, Dokl. Akad. Nauk S S S R 151, 691 (1963);Dokl. Chem. Proc. Acad. Sci. U S S R , 191, 583 (1963). (13) M. V. Smirnov and P. M. Usov, Dokl. Akad. Nauk SSSR, 151, 862 (1968);Dokl. Chem. Proc. Acad. SCZ. U S S R , 151, 606 (1963). (14) H.R. Bronstein, A. 9. Dworkin, and M. A. Bredie;, J . Phys. Chem., 64, 1344 (1960).
EMFOF
0
RARE
4
EARTEIMETAL-HALIDE SOLUTIONS
8
12
20
46
MOLE PER CENT
24
28
Ce DISSOLVED
32
1321
36
IN Cccl,
Figure 1. Specificconductivity of d u t i o m of Ce in MIr.
ceramic containersLb1' when compared to the conductivity measured in an all-metal apparatusa-' free of corrosion. I n addition, the uncontaminated solutions clearly show the expected constancy of the conductivity after the well estahlished saturation concentrations have been achieved.bbJ8 With the recent development of a technique which eliminates the need of ceramic containers and insulators,'o a true measurement of the emf of the rare earth metal-rare earth halide solutions in a corrosion-free system has become possible.
Experimental Section The essential features of the apparatus are the electrodes shown in Figure 3 which when combmed in the manner to be described constitute the cell Ta I MCla(l - X ) , Mo(X)(dissolved)
11 KC1
I I Vycor glass
KCI I CL, C
where MCls is the rare earth trihalide and Mo the dissolved rare earth metal cnntained in a tantalum tube closed at the bottom. Communication to the bridging liquid, KCI, is established by means of a sintered tantalum pellet of extremely fine porosity welded into
4
8
12
16
20
24
28
MOLE PER CENT La DISSOLVED IN
32
Lei,
Figurn 2. Speci6c conductivity of solutione of La in LaClr.
36
the side of the tube. The symbol 11 represents this junction between the two dissimilar liquids. The tantalum electrode capsule serves two purposes: (1) as a physical harrier separating the alkali halide communication liquid from the rare earth halide-metal solution, and (2) as an inert electrode assuming the potential of the rare earth halidemetal solution. Since the stability of the alkali halides is of the order of approximately 10kcal/halogenm greater than that of the rare earth halide, reduction of the alkali ion would be negligible either by direct contact at the junction or by transfer of electrons through the tantalum metal. Even though the Ce-CeCb and h c P r C l r solutions can support an electronic current,L*zthe bridging liquid of pure alkali halide which conducts only by ionic transport eliminates any effect on the cell potential by electronic (16) €2. Sanderoff and 0. W.Mmors. J . Phvr. Chsm.. 61. 294 (1980). (18) M.V. Smlmov. P. M.Usov. V. 8. Lbov. and 0. M. Shubanor. Tr. Inst. Eledmkhfm.. Akod. Nouk SSSR. UIaI'Ik. Ftltal, No. 8. 67 (198.5): Trona. In&. Elearochem.. No. 8. 49 (1886). (17) M. V. Sm!mov and V. 8 . Lbov. TI. Insl. Eledrokhfm.. Akad. Nouk SSSR. UmI'sk. Ftltal. No. 8. 7 (1088): TmM. Insl. Eleurcchem.. NO.8 , 5 (1988). (18) F. J. Kmeshea. Jr., and D. Cublmlottl. J . Chcm. E w . Data. 6. 507 (1881). (19) E.R. B r o w l n . J . Eledrochdm. Sm..112. 1032 (1066). (20) C. E. Wlcks and F. E. Block. Bullem 806. Bureau of Mlnes. U. 8. printing OIEar. WsrhlnSton. D. C.. 1983.
Vol-
7S,Nmn(urb
Mou I S W
EARRY R, BRONSTEIN
1322
conduction. The magnitude of the liquid junction potential between the rare earth metal-metal halide solution and the KCI bridging liquid is immaterial for the present measurements as long as it is unaffected by changes in concentration of the rare earth metal-metal halide solution. In a previously reported experiment with the CdCdC12 systemlPusing these tantalum capsule electrodes and a bridging liquid of the eutectic LiC1-KC1, the emf data for the cell
Cd(satd) I Cd I Ta
a function of cadmium concentration were in excellent agreement with emf values obtained with cells of quartzl ooristruction and di€ferent liquid junctions a9
graphite I CdCL(1
- XI,Gd(X) 11 CdCL, Cd(satd) 1 Cd 1 graphitez1
glaphite I CdCL (1 - X I , Cd (X)II CdClz I I CdCL, Cd(satd) I CdZ2
It is therefore evident that a residual liquid junction potential i s absent in the metal capsule electrode cell with the LiCl-KG1 eutectic as the bridging liquid. The junction potentials at the boundaries of the various unsaturated Cd(X), CdClz(1 X) solutions and of the saturated Cd, CdClt solution with the LiC1-KC1 eutectic thus appear to be identical. The junction potential. haa thus been shown to be unaffected by varying the concentration of the metal dissolved in the salt. The drawing of the chlorine reference electrode in Figure 3 is self-explanatory. The use of a solid Vycor membrane for ionic transport instead of a porous barrier avoids leakage of chlorine into the apparatus, The Vyoor glass acts as a salt bridge since it contains very small quantities of alkali metal ions which at high temperature become sufficientlymobile to impart ionic conductivity. Since the molten salt on both sides of the Vycor barrier is pure KC1 this junction potential is negligible OF none~istent.*~-~6 For simplicity of illustration the following are not shown in Figure 3 : the 6 in, i.d. X 24 in. deep Marshall furnace, the stainless steel containment tank 5 in. i.d. X 14 in. high, the 3.5 in. high tantalum vessel fitted in the bottom of the tank to hold the molten communication liquid into which the two electrode assemblies are placed, the tank lid with entry tubes for the electrode assemblies, and the tantalum-jacketed thermocouple tube. Variation in vertical position of the above electrodes is permitted by the pressure-vacuum Teflon seal illustrated on the tantalum electrode assembly. In addition, the tank lid has an entry for evacuation 1-
Ths Journal of Phgeical Chemietry
and argon pressurization of the tank. The tantalum electrode tube 0.76 in. i d . X 3.5 in. in length was constructed as previously describedlgwith the exception that now the tube has a collar a t the top for attachment to the adapter as illustrated. Pressure equalization between the inside of the electrode and the tank is achieved through the coarse threads by which the adapter is attached to the tantalum tube. By means of suitable seals, the #-in. diameter tantalum center tube of the cell assembly is used bath as a stirrer and as a means of adding accurately weighed metal to the melt. The end of the center tube has a small hole in the bottom and slits on opposite sides. Small blades welded to the tube as illustrated provide stirring action when the tube is rotated. In operation the argon pressure is released, the center tube raised out of the melt, briefly uncapped, and an accurately weighed piece of rare earth metal dropped into it. After reinserting the tube into the melt, the apparatus is again pressurized with argon. The addition of metal in this manner avoids splashing which would cause a change in concentration. The solution can be stirred by slightly lossening the seal and manually rotating the tube. At termination of an experiment the center tube is raised out of the melt, and if the rare earth metal is solid at, the temperature of the experiment as is the case in the Pr-PrCla, the weight of the recovered excess metal in the tube is an additional check upon the saturation concentration. The tantalum electrode tube contained a weighed quantity of rare earth halide to give a liquid height as illustrated, The adjustment of the position of the tantalum electrode tube such that the levels of the two liquids are approximately the same is accomplished by touching the bottom of the containment vessel and raising the tube to the proper height as calculated from the weights of the salts, their densities, and the dimensions of the containers. Aided by frequent stirring of the solution, dissolution of the metal and the establishment of a steady potential was usually achieved within 1 hr. Data were not considered reliable unless the voltage remained constant within &0.001 V for at least 20-30 min. This procedure was followed for each metal addition. When the voltage showed no further change with addition of metal, saturation of the solution had been achieved. In the investigation of the Cd-CdCL system,19steady potentials for 2 and 10 mol % Cd solutions us. a saturated Cd-CdClz electrode were recorded for a period of 16 hr, demonstrating that drift of potential with time, (21) G. A. Crawford and J. W. Tomlinson, Trans. Faraday SOC., 6 2 , 3046 (1966).
(22) V. P. Mashovets and V. P. Poddymav, Zh. Prikl. Khimi, 37, 813 (1964); J. A p p l . Chem., 37, 815 (1964). (23) R. Littlewood, Electrochim. Acta, 3 , 270 (1961). (24) 0.G.Dijkhuis and J. A. A . Ketolaar, i b l d . , 11, 1607 (1966). (25) T.Farland and T. Ostvold, Acta Chem. Scand., 2 0 , 2086 (1966).
EMYOF RAREIEARTHM~TAL-HALIDB SoLuTroPI'e
1323
which would affect the potential of later additiona, does not occur within the elapsed time for performing the experiment. At termination of the experiments, examination of the communication liquid, of the melt of the electrode tube, and of sections of the porous tantalum diaphragm showed that the boundary between the two liquids was formed within the fine porosity tantalum pellet. The emf of the cells was measured in the Poggendorf manner using a potentiometer-voltmeter bridge in conjunction with a Hewlett-Packard Model 426 dc microvolt-ammeter and a recorder. High purity tank chlorine dried by passage through "anhydrone" was used for the chlorine electrode. A flow rate of 1 bubble/sec as measured by a sulfuric acid bubbler on the exit line was found satisfactory. Faster flow rates had a cooling effect on the electrode and produced oscillations of greater than =tO.OOI V when at null potential. Once the molten salt (KC1) was saturated with chlorine the flow could be stopped for short periods of time without changing the operating characteristics of the electrode. Constant temperature, &2', of the system was maintained by conventional control devices. The temperature of the system was measured by a chromelalumel thermocouple in a tantalum-jacketed stainless steel tube positioned in the communication liquid near the tantalum electrode assembly, The method of preparation of the anhydrous salts and the purity of the salts and of the metals have been reported previously.3 All operations in handling the anhydrous salts and metals were performed in a helium drybox. Dry, mass-spectrographically pure argon was used as an atmosphere in the apparatus.
r16,4mole% as Pro at Saturation I
e.30
-
Experimental Slope
0.225
O
-1
Figure 6, E in relation to log (Xpra+/xpr2+) (855")for the cell
Results and Discussion The results of the electromotive force measurements on the Ce-CeCls, Pr-PrCh, and the Nd-NdCls solutions are presented in Figures 4,5,and 6, respectively. The observed potentials are plotted against the logarithm of the concentration ratio using Ma+-M2+ as the potential determining couple, The metal-metal halide electrode was the negative electrode and the chlorine electrode the positive electrode of the cell in all the measurements and therefore the measured voltage of the cell as written, according to convention, is designated to be positive,
XCI bridplnp llquld
Oo2
Oo4
OB6 'Q8
*' 109
3+Ia2
3,
''4
'"
Figure 4. 8 in relation to log (XceS+/X&+) (853') for the cell
I.@ 2'o
Figure 6. E in relation to log (XNda+/XNa2+) (800') for the cells
Ta I NdCls(1 - X), NdCl(a-,)(X) / I KC1
Vycor
and
(b, series 1; 0,series 2.
AgCl(7 mol oJO)/Ag. Volume 75, Number 6 May 19B8
1324
BARRY
The saturation concentration of the metals in their respective molten trichlorides as determined by the present emf method 8.66 mol % Ce, 16.4 mol % Pr, and 25.5 mol % Nd are in good agreement with the values 9.0 mol % Ce, 17.8 mol % Pr, and 26.0 mol % Nd obtained by equilibration6-8 and electrical conductivitys-6 where the solutions were not in contact with ceramic material. For the Nd-NdCh solutions, an Ag-AgC1 (7 mol %) , LiC1-KC1 eutectic, Vycor reference electrode, and the LiC1-KC1 eutectic as the bridging liquid were used in one series of measurements. In another series, the assembly C, C12-KC1, Vycor was the reference electrode and KC1 the bridging liquid, The difference of 1.000 T7 between these two series is in agreement with the known potential between the two types of electrodes,28*26*27 This agreement not only reflects the experimental reliability of the system but also implies that the liquid junction potential at the boundary Ta I Nd(X), NdCls( 1 - X) 11 LiC1-KC1 eutectic is the same as that at the Ta Nd (XI,NdCls(1-X) I I KC1 boundary. However, if ionic mobilities are an important parameter in determining the magnitude of the liquid junction potential, the above result is not too surprising since the ionic mobilities of Lit and K+ in the eutectic mixture are approximately equa1.2*-30 As these solutions at saturation are in equilibrium with essentially pure liquid or solid metal, with the exception of the Nd-NdCh system where solid NdClz is present, the measured cell potentials can be compared with approximate cell potentials as calculated from estimated values of the standard potentials of the trihalides.81 The calculations are made with the assumption of unity for the activity coefficients of the trihalide at the saturation concentration, The calculated values 2,924and 2.913 V are in substantial disagreement with the experimental values 2.350 and 2,400V for the CeCls and PrCL solutions, respectively. Lacking a value of E o ~ ma ssimilar calculation for the NdC12-NdC13solution is not possible but a discrepancy of similar magnitude would be expected. These rather sizable differences, 0.5-0.6 V, in the observed potentials from those calculated must be attributed to surprisingly large liquid junction potentials existing a t the boundaries of the melts and the bridging liquid since to explain these differences otherwise would require a rather absurd activity coefficient of 107 for the various trihalides. A similar disagreement by 0.7 V, observed by Senderoff and Mellorse in their investigation of the cell
I
Mo I Ce(satd) , CeCls 11 CeCla I C t , C must be interpreted as resulting from an unsuspected reaction of the melt with the ceramic container and asbestos dia~hragm.'~Such an extreme junction potential is not likely for the rather similar liquids of this cell. The Journal of Physical Chemiatry
R. BRONBTEIN
For a direct experimental demonstration of the occurrence of a liquid junction potential of the rnagnitude estimated the following cells were examined Vycor I KC1 glass ~
Vycor ~
glass
KC1
The potential of cell A at 800" was 0,570 V and that These results demonstrate the existence of a rather sizable liquid junction potential and that the sign and the magnitude of the potential is of the order estimated, When compared to the value of 0.250 V for the cell of cell B a t 850" was 0.600 V.
C, Clz KC1
1
powdered 10 mol % KC1 (212, All08 190 mol % MgClg ~
C,825"
the potentials of cells A and B appear not unreasonable. It has been demonstrated previously that the potential of the reference electrode system including its junction with the metal-metal halide solution does not change within the time necessary to complete the experiment or with variation of the concentration of the studied sol~tion.'~Therefore, the magnitude of the liquid junction potential has no effect on the reliability of the present measurements as a true reflection of the properties of these solutions. The emf results may now be directly related to other observations on rare earth metal-rare earth halide solutions. Even though the cerium-cerium trichloride system does not exhibit a solid subhalide, results of cryoscopic studies in the dilute range are not far different from those obtained in the Pr-PrCls and Nd-NdCls systems2 where solid subhalides exist. The apparent number of (26) L. Yang and R. G. Hudson, Trans. A I M E , 215, 589 (1959). (27) M. Takahashi and Y. Amada, Denki Kagaku, 3 2 , 140 (1964). (28) J. Perie, M. Chemla, and M. Gignoux, Bull. SOC.Chtm. Fr., 1249 (1961). (29) M. Chemla, Discussions Faraday Soc., 3 2 , 63 (1961). (30) C. T. Moynihan and R. W. Laity, J. Phys. Chem., 68, 3312 (1964). (31) W.J. Hamer. M. 8. Malmberg, and B. Ruben, J. Electrochem. Soc., 103, 8 (1956). (32) W. K. Behl and J. J. Egan, J. Phys. Chem., 71, 1764 (1807). The value is incorrectly listed as 25 mV. Dr. J. J. Egan, oral communication to Dr. M. A. Bredig of this laboratory.
Em
OF
RAREEARTHMETAL-HALIDE SOLPTIONS
new particles, n, produced on dissolving one atom of metal is found to be approximately 3, which may be attributed to its reaction with the solvent salt a’ccording to Mo 2Ma++ 3M2+.sa We compare these results with the concentration dependence of the potentials obtained in the present measurements. Indeed in plots of E us. log ( X H ~ + / X N ~all* )three , systems yield straight slopes corresponding t o the oneelectron process implied by the abscissa (Figures 4,5,6). This result not only supports the existence of an ion of the type M2+but also satisfies the necessary condition for chemical reversibility of the electrode in that the Nernst equation is obeyed. With allowances for small deviations in experimental data, the rather surprising linearity of the plots implies that the ratio of the activity coefficients of the postulated species remains constant over a considerable concentration range. Similar behavior has been observed in the emf measurements of the mixtures of molten saltsa4 CeCls-CaC12 over virtually the entire range of composition, in the Sb-SbIa systema6 over a sixfold concentration range (0.4 to 2.5 mol % metal) for the species Sbz*+, in the Bi-BiCls systemaBover the concentration range 0.8 to 14.5 mol % Bi for the species Biz2+,and in the Cd-CdClz system21f22 for the species CdZ2+. It may be concluded that a subvalent ion of the type rrM2+” does exist in the solutions of Ce-CeCls, Pr-PrCls, and Nd-NdCla. The reason for the constancy of the activity coefficient ratio is open to conjecture but a ion is quite possible explanation may be that the ‘‘M2+11 similar to the ions already present in the parent halide solvent. While the emf results are most simply explained if the electrons introduced by the metal are localized on specific sites such as (Ms+e-)2+,the mode of electronic conduction in these solutions is still open t o conjecture. The difference in the electrical conductivity between the various systems might be explained by postulating that while in the Nd-NdC13 system the equilibrium M2+e Ma+ e- lies very far to the left, it is shifted in the Ce-CeCla system somewhat to the right, providing highly mobile electrons, in the “conduction band.” The number of such electrons in the cerium system can then still be thought to be small compared with the number of Ce2+ions and, while able to affect greatly the electrical conductivity, to have little or no effect upon the emf. This formulation, however, is unsatisfactory because it would seem to imply a large mean free path, of the order of that assumed in a pure metal for a relatively small number of mobile electrons. In light of current t h e o r i e ~ ~ ’and - ~ ~p o s t u l a t e ~regarding ~ ~ ? ~ ~ the conduction mechanism of the electronic component of the metalmetal halide solutions, it seems preferable to think in terms of a very small mean free path or of a “hopping” of an electron between ions of adjacent oxidation states. The magnitude of the electronic (not metal-like) con-
+
+
1325 tribution to the overall conductivity by this mechaniam would be due to the “intrinsic jump frequency”greatest for the electron in Ce-CeCls and decreasing to a negligible quantity in the Nd-NdCla solutions. Also in accord with this mechanism is that the barrier height, the energy required for delocalization of the electron into a conduction band, is apparently great enough to prevent thermal ionization. The temperature dependence of the conductivity of these solutions is that typical of ionic melts.2 Finally, we compare our results with those obtained by Russian workers. As has been demonstrated previously, solutions of the rare earth metal-metal halides are extremely corrosive to ceramics.14 Notwithstanding this fact, these workers in investigating the electrical c o n d u ~ t i v i t y ~and ~ J ~emfl0-l3 of solutions of Ce-CeCla and La-LaCla employed ceramic containers (BeO, AlzOa, BN49 and in some of the emf studies asbestos (Ca-Mg silicates) as a diaphragm material. The distortion of the data by the inevitable reaction of the solutions with the container and diaphragm material, which is also predictable from available thermodynamic data, is quite obvious. With concentration cells
I
Mo Ce(at saturation), CeCls boron nitride crucible
I
diaphragm asbestos
I
CeC13(1 - X ) , Ce(X) I Mo boron nitride crucible and Mo I La (at saturation) , LaCla alundum crucible
11 LaCls(1 - X),La(X) I Mo poroua Be0 crucible (33) However, since the electrical conductivities of the Ce-UeC1 solutions are relatively much greater than those of Pr-PrC18 and of Nd-NdCla it has been suggested that the cryoscopic effect could be due to “cryoscopically active electrons” Cea -+ Ces+ 3e-.g (34) 9. Senderoff, Q. W. Mellors, and R. I. Bretz, J . Electrochem. Soc., 108, 93 (1961). (35) J. D , Corbett and F. 0.Albers, J. Amer. Chem. Soc.. 82, 533
+
(1960). (36) Yu, K. Delimarskii and Yu. G. Roms. Ukr. Khim. Z h . , 30, 457 (1964). See, however, L. E. Topol and R. A. Osteryoung, (bid., 31, 998 (1965).
9. A. Rice, Discussions Faraday Soc., 32, 181 (1961). D. 0. Raleigh, J. Chem. Phys., 38, 1677 (1963). E. G. Wilson, Phys. Rev. Lett., 10, 432 (1963). J. Jortner, S. A. Rice, and E. G. Wilson, “Solutions MetalAmmoniac Proprietes Physiocochimiques. Colloque Weyl,” G . Le Poutre and M. J. Sienko, Ed., W. A. Benjamin, Inc.. New York N . Y., 1964. (41) M. Shimoji and K. Ichikwa, Phys. Lett. 20, 480 (1966). (42) K. Ichikwa and M. Shimoji, Trans. Faraday Soc., 62, 3543 (37) (38) (39) (40)
(1966). (43) K. S. Pitzer, J. Amer. Chem. Soc., 84, 2025 (1962). (44) M. A. Bredig, J. Chem. Phys., 37, 914 (1962). (45) Carefully dehydrated B N was found by us to be readily attacked
by these solutions. Volume YS, Number 6 May 1089
JOHNT. HERRONAND ROBRRT E. Hum
1328 zero emf was not attained until concentrations X , of 29 mol % Ce in CeC13 l o and 33 mol % La in LaClB,l2 far in excess of the actual saturation limit which is very firmly established as 9 and 10 mol yoin the Ce-CeCIB 318 and La-LaCla,6JS respectively. Also, the claim is made that the interchange of a NaC1-KC1 eutectic, Clz, C electrode for the CeC13,Clz, C electrode in the cell
Mo 1 Ce (satd) CeCL, boron nitride crucible
1 asbestos
1 diaphragm
1
CeCls 1 CIS, c
produces no change in potential of the cell."
The
present study shows that a sizable effect of 0.6 V should have been noted. It is nothing less than astonishing that solubility measurements made by the Russian workers10J2using molybdenum crucibles and sampling of the salt-rich phase a t the equilibration temperature as well as quenching experiments yielded the same results as they obtained in emf and conductivity measurements where reaction with the ceramic containers undoubtedly occurred. In a recent article PolyachenokP6also takes note of the discrepancies in the saturation concentrations as determined by Smirnov, et U Z . ~ O J ~ (46) 0. G.
Polyachenok, 2. Neorg. Khim., 13, 200 (1968).
Rates of Reaction of Atomic Oxygen (O'P). Experimental Method and Results for Some C1 to C, Chloroalkanes and Bromoalkanes by John T. Herron and Robert E. Huie Institute for Materials Research, National Bureau of Standards, Washington, D . C.
20.23~
(Receiued August 1 5 . 1 9 6 8 )
A method of measuring rate constants for the reactions of atomic oxygen (gap)with organic compounds i described, and rate constants are reported for the reactions of atomic oxygen with 12 chloroalkanes and bromoalkanes from 336 to 622'K.
Introduction
Experimental Section
The reactions of atomic oxygen (8P) are of importance in areas of atmospheric chemistry ranging from the properties of the upper atmosphere to air pollution. Although the number of reactions of known or probable importance is large, only a limited number of reliable rate constants have been reported. The only class of compounds which has been studied extensively is the olefins, for which relative rate constants have been reported by Cvetanovic and coworkers,l and absolute rate constants have been reported by Eliasa2 In the case of the alkanes reliable data exist for methane and ethanea and for n - b ~ t a n e . ~For other reactions the data come from a variety of sources, are often limited to one temperature or a small range of temperatures, or are simply unreliable, In view of the need for experimental data in this area, we have undertaken a survey of the reactivity of atomic oxygen with different classes of organic compounds, using a mass spectrometric method. In this first part we report results for a series of chloroalkanes and bromoalkanes.
The experimental apparatus has been described in detail el~ewhere.~It is basically an electrical dischargeflow system coupled to a mass spectrometer. Atomic oxygen was prepared by passing a mixture of 1-5% oxygen in an argon carrier through a 2450MHz electrodeless discharge. This resulted in the decomposition of about 20% of the molecular oxygen. In addition to ground-state atomic oxygen, the discharge may yield various metastable atomic and molecular species capable of initiating reaction. This point is discussed later. The mixed gases passed through a trap cooled with liquid nitrogen before entering the discharge zone, After leaving the discharge zone, the gas mixture flowed through the reactor which was a 20-mm i d . Pyrex tube with an overall length of about 40 cm.
The Journal of Physical Chemiatry
(1) R. J. Cvetanovic, Advan. Photochem., 1, 115 (1963). (2) L. Elias, J . Chem. Phys., 38, 989 (1963). (3) A. A. Westenberg and N.de Haas, i b i d . , 46, 490 (1967). (4) L. Elias and H. I. Schiff. Can. J . Chem., 3 6 , 1657 (1960). (5) F. S, Klein and J. T. Herron, J . Chem. Phys., 41, 1285 (1964).
