189
NOTES
Jan., 1962
GASEOUS O S l D E S OF RHENIUM1 Results and Conclusions -The experimental potentials of the cell Ta/Ta(V), EF(1 m ClO*-)/ BYMARTIN H. STUDIER H2Pt as a function of log (F-) are plotted in Fig. 1. ArOonna National Labwotory, Argonna, IIItnnis The vertical scatter of t,hc potential is inherent in Received AugW 14. 1081 the behavior of solid metal electrodes, even a t the higher fluoride concentrations. A least squares fit Several gaseous rhenium oxides have been deof the 78 data points considered to be in the re- tected with a Bendix Time of Flight Mass Spectromversible or normal range was made t o a quadratic eter.2 During a study of surface ionization13 function. The resultant equation neutral species were volatilized (at temperatures below 500’) from rhenium surfaces on which samples in nitric acid had been evaporated. Ions EDoii= 0.760 + 0.182 log (F-) + O.O13[10g (F-)]* (4) produced by electron bombardment of the gaseous is shown as the curve plotted in Fig. 1. The first species were identified by their masses and corresponded to the empirical formulas derivative of equation 4 d E,,ir/d log (F-)= 0.182
+ 0.026 log (F-) (5)
yields an analytical expression for the slope which may be substituted directly into equation 3 to give values of f i as a function of log (F-). Such a formation curve is shown as an insert in Fig. 1 over the fluoride ion concentration range for which the data would appear to be most applicable. This formation curve for the tantalum fluoride system is seen to be about one-half of an fi unit higher in this fluoride region than the formation curve found previously.2 This agreement is satisfactory considering the difficulties of the system, and the assumption that the cell approaches reversible behavior appears justified. No new calculation of a set of formation constants was made, but the results did strengthen the evidence for the existence of the species TaFg-4, although in a statistical sense only. The solubility of Ta+& may be inferred from the studies by assuming from Fig. 1 that Eofor the reaction of equation 1 was near 0.35 v. a t the higher fluoride concentrations. Upon subtracting the tantalum fluoride half-reaction of equation 1 from Lati~ner’s~ half-reaction for the oxidation of tantalum to the pentoxide one obtains 2TaF,,+“
+ 51120 =
T d ) 6
+ 2nF- + lOII+, EO
0.4 v. (6)
The free energy change favors the reaction from left to right as written. This agrees with ohservation siuce the salt IC2TaT’7when dissolved in pure watcr is hydrolyzed to give an acid solution. Very thin oxide films arid freshly precipitated hydrated tantalum oxide dissolve in sufficiently strong hydrofluoric acid, but the massive crystalline oxide such as is formed after sintering the hydrate a t 1000°for several hours is hardly affected by the cold acid. This work was supported by a cooperative agreement between Oregon State University and the U. S. Bureau of Mines, Albany, Oregon. (5) W. M. Latimer, “Oxidation Potentials.” 2nd ed., Prontice HaU, Englewood Cliffs, N. J.. 1962.
Monomers
Dimers
Re +
Re+
Re0 + Reo*+ Reo: + Reo, +
Re20 Re202 %*O: Reno, R e z O a +,&%Ob + RerOs+ Re20T+,Re90~++ +
+
+
+
+
Gaseous oxides still were observed after they had been evaporated from the filament and the source had cooled to room temperature. As the source was warmed gradually the intensity of the oxide ion beams increased and the oxides were observed to fractionate with respect to each other. Since RezO, has the highest mass of any oxide observed, it must be a primary gaseous product. All the lower dimeric oxides are primarily fragmentation products of the ionizing electron beam. Relatively high electron energies are required to produce the lower mass dimeric ions. I n addition, a t a given electron energy the dimeric forms were found to remain in constant ratio to each other with large variations in time, temperature and vapor pressure. The highest mass monomeric oxide, ReOd, is also a primary gaseous product for it was frequently observed in the absence of oxides of greater mass. Although both Reo3+ and Reo2+ are formed in abundance by fragmentation of higher oxides, marked fractionation of Reo2+, Reo3+and Reo4+ with respect to each other and to the dimeric forms suggests that both Reoz and Reo3 have an independent gaseous existence. Reo+ and Re+ wcrc observed as fragmentation products only. It is of interest to note that the oxide ion Reoa+ appears a t masses 233 and 235. It is possible that, it may interfere with uranium isotopic aiialyscs when rhenium filaments are used in surface ionization sources. (1) Based on work performed under tho auspiees of the U. S Atomic Energy Commission. Presrnted in part at the Ninth Annual Meeting (June, 19611 of the A.S.T.M. Committee E14 on Mass Spectrometry. (2) D. B. Harrington. “Encyclopedia of Spectroscopy,” Reinhold Publ. Corp., New York, N. Y..1V60, pp. 628-1347, (3) M. 11. Studier, E. N. Sloth and L. P. Moore, J . Phys. Chem, 66, 133 (1962).
