Oct., 1952
ISOTOPE EXCHANGE REACTIONS OF FLUORINE WITH HALOGEN FLUORIDES
885
ISOTOPE EXCHANGE HEACTIONS OF FLUORINE WITH HALOGEN FLUORIDES BY RICHARD B. BERNSTEIN' AND JOSEPH J. KATI Chemistry Division, Argonne National Laboratory, Chicago, Illinois Received March Id, 196s
The gas phase exchange of radioactive F18between elemental fluorine and the halogen fluorides ClFa, BrF6 and IF7 has been investigated in cop r, aluminum and nickel reaction vessels. The rate of isotope exchange a t temperatures below 100' was essentially zero in afcases. Complete exchange occurred within a few minutes in the region of 300",and conveniently measurable rates were observed at intermediate temperatures. To evaluate the heterogeneous contribution to the gas phase exchanges, the behavior of fluorine and the halogen fluorides with AlFa, CuF2, NiFz and CaFz was examined. Fluorine exchange with the metal fluorides was slow; the halogen fluorides exchanged rapidly, but the extent of exchange decreased with increasing temperature. These exchanges are not significant in the temperature range where exchange between fluorine and the halogen fluorides is extensive; consequently the latter exchanges are considered primarily homogeneous. A mechanism for the exchange baaed on the well-known reversible dissociation of ClFa and IF7R i plausible in the li ht of the data. Since quantitative data were lacking for I F p l i m i n a r y measurements of the temperature de endence of &e dissociation constant were made, which indicate that the O and AS' for dissociation are comparable with t f e values known for CIFs. However, in the case of BrFS,no dissociation was observed u p to 400', and the possibility of an aRsociative mechanism for this exchange is suggested.
Introduction The radioactive nuclide Fi8 which has a halflife of 112 min. and emits a 0.7 MeV. positron is a convenient tool for the investigation of the behavior of fluorine and its compounds. Dodgen and Libby2 first employed Fi8 in a study of the exchangeability of fluorine between HF and Fz. Subsequent studies have been made of the fluorine exchange between HF and the halogen fluoride^.^ The two systems were strikingly different. Whereas halogen fluorides were found to exchange very rapidly with HF a t room temperature in the gas phase, the corresponding HF-Fz exchange occurred a t a negligible rate below 200". In both cases it was considered necessary to postulate an exchange mechanism which proceeded via molecular complex formation. It was thought of interest to extend this work, and the present investigation is concerned with fluorine exchange between gaseous halogen fluorides and elemental fluorine.
Experimental Materials.-Anhydrous HF and ClFs (Hamhaw) were purified by distillation. BrFI was purified by a vapor phase fluorination to remove BrF (or Brz) before final distillation. If was prepared by the fluorination of IFs (General Chemical) according to the method of Schumb and Lynch.' Fluorine (Pennsylvania Salt) containing approximately 1%inerts was used without further purification. Vapor density determinations of purified IF7 and ClFa samples gave correct molecular weights within 0.5%. Several methods of incorporating t,he nuclide F18 into the halogen fluoride or fluorine were used. Table I summarizes a number of procedures which have hitherto not been reported. The simplest and most effective procedure so far employed is mcthod 5. I n thc preparation of labeled elemental fluorine, a high temperature exchange with a labeled halogen fluoride was used. The chemical purification procedures were adequate to ensure no radioactive contamination. I n one case the decay of CIF1* was observed to decay with the proper half-life over 8 half-lives. I n most cases initial specific activities greater than 100 counts/min. per millimole were readily obtained. Apparatus.-A vacuum system similar to that of Rogers and Katza was employed. The apparatus was constructed of copper and nickel throughout with silver-soldered and (1) Department of Chemistry, Illinois Institute of Technology. Work performed under Participating Institution Program. (2) H. W.Dodren and W.F. Libby, J . Chem. Phyr., 17, 951 (1949). (3) M. T. Rogen, and J. J. Kats, J . Am. Chsm. Soc., 74,1376 (1952). (4) W. C. Bchumb snd M. A, Lynch, I d . &no. Cham., 4S, 1888 (IOSa),
TABLE I PREPARATION OF F18LABELED HALOGEN FLUORIDES Material irradiated
Irradiation
Nuclear reaction
FIB(y,n)F1* Betatron (48 MeV. 7 ) Cyclotron (fast neutrons) FlQ(n,2n)Fl* Lie( d,n)Be7 3 LiNOJb Nuclear reactor Lie(n,a)HS Ol'(t,n)F" 4 LiAlO: Nuclear reactor Same as (3) 5 LiF AIz03' Nuclear reactor Same as (3) 0 HF* obtained by pyrolysis of KHFz*, and labeled halogen fluoride obtained by gas phase exchange with HF*. The halogen fluoride* is then purified by treatment with NaF (ref. 3) and subsequent distillation. b Treated with liquid hydrogen fluoride to produce HF* which is purified by absorption on NaF and subsequent pyrolysis of NaHFt*. Then proceed.as in (1). Approximately 1: 1 molar ratio of finely powdered and mixed material. Either radioactive halogen fluoride or hydrogen fluoride prepared by direct exchange of liquid with the irradiated solid, followed by distillation.
1 KHFZ" 2 KHFZ'
+
flared connections. Valves were Crane bellows or Hoke diaphragm type. Fluorethene traps and gages with monel Bourdon tubes were employed; a copper counting vessel into which a Victoreen 1B85 Aluminum Thyrode was sealed served as a suitable counting chamber.8 The reaction vessels of aluminum and nickel were electrically heated. A Booth-Cromer nickel diaphragm null-type pressure transmitter with an appropriate external pressure balancing system was used when precise pressure measurements were required. A fluorethene weighing ampoule equipped with a special small Crane bellows valve was useful for introducing weighed quantities of halogen fluorides. The pyrolysis chamber for KHF2 or NaHFz was constructed of nickel and was surrounded by a furnace. Method.-A description of an exchange experiment between ClFa and FZwill be given as a typical example of the method employed. A sample of LiF-AbOa, after irradiation in the nuclear pile for 1-2 hr., is introduced into a fluorethene trap and evacuated. Approximately one or two ml. of liquid ClFa obtained from the center third of a sin le-plate trap-to-trap distillation is frozen onto the LiF-Alnba mixture, and allowed to warm u to room temperature under its own vapor pressure. *he solid-liquid isotope exchange'is complete in about 15 min. The CIF** is then purificd by further distillation, and the final sample expanded int,o the counting vessel. Here the specific activity is determined over a period of approximately one quarter-life. Th.e ClFa* is transferred into the reaction vessel at its operating temperature. A suitable quantity of fluorine is then introduced and the exchange is allowed to proceed with the vessel cut off from the vacuum line. After the FrCIFI* exehenpe han prdaecded for the denired
R. B. BERNSTEIN AND J. J. KATZ
886
Vol,. 56
TABLE1.I EXCHANGE OF FLUORINE WITH HALOGEN FLUORIDES '
Halogen fluoride
Reaotion vessel
ClFs
cu cu cu A1 AI Ni A1 cu Ni Ni Ni Ni A1 A1 Ni
RrFs
IF,
Partial pressures (mm.) Halogen F2 fluoride
T, OC.
160 85 360 500 360 795 360 150 444 532 818 485 665 845 740
25 25 25 119 165 170 232 25 181 242 300 300 25 160 306
45 140 100 710 450 120 590 10 101 145 237 105 450 455 200
time, the mixture is passed slowly through two traps in series maintained at -195" to remove the ClFs*; the F2* is expanded into the counter. The measured activity of the Ft* is corrected by a P V / T factor to give the total activity of F2* present in the reaction vessel. Then the fluorine is removed and the total ClFa* introduced into the counter. The recovery of halogen fluoride is usually greater than 98%. The activity is then measured and the per cent. loss in specific activity due to exchange with Fa is measured. Due to the exchange with the fluoride coating of the reaction vessel the total inventory of FIE was never 100%. Depending on the temperature and the surface the loss due to exchange with the surface would range from 0-30y0 of the original activity. The gas phase exchange was calculated using the recoverable total activity, L e . , the sum of the Fp* and ClF3* activities after exchange. The loss of FlS inventory was shown to be due primarily to exchange with the metal fluoride coating by experiments using subsequent elution with tank ClF3. This elution could not be accomplished, however, by elemental fluorine in a reasonable time. The "fraction exchanged" waa calculated by dividing the observed percentage of the total activity in the Fa* by the atomic percentage of fluorine in the form of elemental fluorine in the mixture. Due to the many different conditions undcr which thc various experiments were carried out it is difficult to state an over-all probable error. One of the more serious errors may be the excess residual background in the counting chamber due to exchange with the labeled halogen fluoride, for which correction is necessary. By careful attention to detail a satisfactory F1*inventory can be obtained. Under the most favorable circumstances it appears that there is an uncertainty in the percentage exchanged of i 3% exchanged.
Results and Discussion The experiments carried out in the gas phase 0
II
P o
0
"
'
PO
I
"
'
40
I
'
60
80 TIME
0
20
Imtn~1esI.
.
1
40
.
1
bO
.
1
80
Fig, 1.-Exchange of CIFr* with &Fa: -----, calculated 1 8s in activity b sed on single layer surfane nxchange;
g,cl&uspar] e, Cl& liquid,
Contact time, min.
10 40 45 45
40 40 30 30 23 18 30 40 25 40 25
Exchanged. %
< <
98.0
