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Stability of BF3OH− ion in molten and solid NaBF4 and NaFNaBF4 eutectics
J. inorg,nucl.Chem., 1972,Vol.34, pp. 2721-2727. PergamonPress. Printedin Great Britain
STABILITY OF BF3OH- ION IN MOLTEN A N D SOLID NaBF4 A N D NaF-NaBF4 EUTECTICS* J. B. BATES, J. P. Y O U N G , M. M. M U R R A Y , H. W. K O H N t and G. E. BOYD Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (First received I November 1971 ; in revised form 17 December 1971)
Abstract--I.R. measurements on solutions of NaBF3OH in crystalline and molten NaBF4 were employed to determine the stability of the BF3OH- ion with respect to its unimolecular decomposition. Isotopic substitution experiments with D~O, D F and D2 have shown that the species BFzOD- is formed in molten NaBF4 either by reaction with BF3OH-, with BF20-, or with BzF602- mediated by a transition metal oxide. These isotope reactions give encouragement to the view that tritium released during the operation of a molten salt breeder reactor might be trapped by reaction with BF3OH- or with oxyfluoroborates present as impurities in the molten sodium fluoroborate coolant salt. INTRODUCTION
THE PROPOSAL t o employ the eutectic composition N a B F 4 - N a F (8% NaF) as a
coolant for the molten salt breeder reactor (MSBR)[1] has prompted many investigations on the chemical and physical properties of pure NaBF4 as well as on the N a F - N a B F 4 eutectic. For example, the i.r. and Raman spectra of crystalline NaBF4 and the Raman spectra of molten NaBF4 and N a B F 4 - N a F have been thoroughly studied in this Laboratory [2-4]. The identities and properties of the impurities in the NaBF4 employed, however, were not investigated. It now appears that most of the samples of this compound must have contained at least a small amount 0 0 0 0 - 2 0 0 0 ppm) of NaBF3OH which was extremely difficult to remove from NaBF4 melts. Tritium formed and released in comparatively large amounts during the operation of molten salt breeder reactors may constitute a serious environmental hazard. A possible way to minimize the hazard would be to trap the tritium by isotopic exchange reactions with protonic species in the N a B F 4 - N a F coolant salt. If the BF3OH- ion were present in the melt, the reaction would be described by the equation: 2BF3OH- + T 2 ~ 2BF3OT- + H2.
(1)
The identification of NaBF3OH in molten and solid NaBF4 provided the first *Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. ?Present address: Chemistry Department, Dickinson College, Carlisle, PP. 17013. 1. R. C. Robertson, O. L. Smith, R. B. Briggs and E. S. Bettis, Two-Fluid Molten-Salt Reactor Design Study, ORNL-4528, Oak Ridge National Laboratory (! 970). 2. J. B. Bates, A. S. Quist and G. E. Boyd, J. chem. Phys. 54, 124 (1971). 3. J. B. Bates, ibid. 55, 489 (1971). 4. A. S. Quist, J. B. Bates and G. E. Boyd, ibid. $4, 4896 (1971). 2721
2722
J . B . BATES et al.
evidence that such a proton-containing species could be stable in these media. This paper will report results from recent experiments directed toward confirming the identity of BFaOH- ion in NaBF4, determining its stability in the molten salt, and investigating its reaction with deuterium in molten NaBF4. EXPERIMENTAL Evidence for the presence of hydrogen-containing impurities in NaBF4 was obtained in preliminary measurements of the near-ix, spectra of the molten salt and from mid-i.r, spectra of pressed pellets of the crystalline material. Additionally, the polarized i.r. transmission spectrum of a single crystal of NaBF4 (1/4-1/2ram thick) revealed a single sharp band at 3641 cm -1. This band was assigned tentatively to the O - H stretching mode of the BFsOH- ion in solid solution with NaBF4. Samples of NaBF4 or NaBF4-NaF which contained protonic impurities were prepared in several ways, viz. by the addition of H3BO3 to the molten solvents; by physically mixing solid NaBFaOH; with the powdered salts; or by recrystallizing NaBF4 from water or from aqueous 0.1 M H F solution. Identical mid-ix, spectra were obtained from pellets prepared from these mixtures. The spectrum shown in Fig. 1 was obtained with a pellet containing - 0.5% by weight of NaBF3OH in NaBF4. The bands at 3620 and 767 cm -x have been observed in the i.r. and Raman spectra of crystalline NaBF3OH [5], and the band at 779 cm -x can be assigned to the vl mode of BF4- ion [2, 3]. On heating the pellet at 120°C for several minutes, the bands at 3620 and 767 cm -1 disappear, while the bands at 3641, 779 and 773 cm -~ remain unchanged as shown in Fig. 1. In addition, the 3620 cm -1 band was not found in the vibrational spectra of samples of (pure) crystalline NaBF3OH which had been heated above 100°C [5, 6].
A
I 73
3X
779
Z 0
3641
(13
:E
(/3 z n. F-
B
f
3X
-773 364t 779 FREQUENCY (cm- t )
Fig. I. I.R. spectrum of a pellet of NaBF4 containing 0.5 wt% of NaBF3OH. A = spectrum of pellet before heating; B = spectrum of pellet after heating above 100°C. 5. J. B. Bates. Work in progress. 6. L. Kolditz and C. S. Lung, Z. Chem. 12, 469 (1967).
Stability of BF3OH- ion
2723
Initial studies of the reaction of the BF3OH- ion with deuterium were made by slowly bubbling D20 vapor into a molten N a F - N a B F 4 eutectic mixture at 400°C. The amount of deuterium introduced into the melt corresponded to about 200 ppm D~O as determined by a material balance calculation. The i.r. spectrum of pellets pressed from a quenched sample exhibited a sharp band at 3641 cm -~ and a new band at 2688 cm -1. The extent of the reaction of D F with NaBF3OH in NaBF4 was investigated by equilibrating molten NaBF4 at 450°C with a DF-argon mixture. I.R. measurements on pressed pellets of the solid material obtained from quenching this melt also revealed the two sharp bands, 3641 and 2688 cm-L The spectrum of a 100 mg pellet of D F treated molten NaBF4 in the 3600 and 2600 cm -1 regions is shown in Fig. 2.
! 0 O3 (./3 CO
Z or" I--
2688 5641
FREQUENCY (cm-1) Fig. 2. I.R. spectrum of 1O0 mg pellet of a quenched melt of D F treated NaBF4. The reaction of BF3OH- ion in NaBF4 with atomic deuterium was investigated by heating a sealed nickel capsule containing NaBF4 at 600°C in an atmosphere of about 25 mm of D2 gas for three days. The i.r. spectrum of a 15 mg pellet pressed from a quenched sample of this melt is shown in Fig. 3. Contrary to expectations, no measurable change in the absorbance of the 3641 cm -1 band was observed after treatment with D~. The possibility that the 3641 and 2688 cm -1 absorptions were due, respectively, to H F and D F dissolved in solid NaBF4 was examined by treating samples of solid NaBF4 with H F and D F at 350°C. No bands were observed above 2500 cm -1 in i.r. spectra of pellets pressed from this sample, although the NaBF4 contained 0.097 meq/g of acid as was shown by titration with standard alkali.
1'
Z
o (,r) 03 CO z < n.I-
-
T
I 3641
2688
W
,,..) z <[ .,,n n-" 0 03 m ,< FREQUENCY
(cm -I]
Fig. 3. I.R. spectrum of a 15 mg pellet of a quenched melt of D2 treated NaBF4.
2724
J . B . BATES et al.
The pellets used in the foregoing experiments were prepared by grinding 15-100 mg of sample in a "Wiggle-Bug" micro-ball mill and pressing with a 0.5 in. dia. die using a total load of about 10,000 lb as read from the gauge on a hydraulic press. For thin pellets (15-25 mg samples), the sample was pressed in a paper collar (inserted into the die) to give the pellet extra support. Measurements of the OH stretching mode frequency of BF3OH- ion in molten NaBF4 could not be made with the equipment available. Although fluoroborate melts will dissolve most of the common i.r. cell materials, they can be contained in cells made of silica. Silica exhibits a strong absorption in the region near 3600 cm -1 which would obscure the BF3OH- band, but it is quite transparent in the region near 2600-2700 cm-L Therefore, i.r. spectral measurements were carried out on solutions of BF3OD- ion in molten NaBF4. A sample prepared by adding D3BO3 to NaBF4 was contained in a 2 mm thick silica cell and placed in a high temperature furnace described previously [7]. The spectrum at 425°(2 observed in the 2700 cm -~ region is shown in Fig. 4. The absorption band centered at 2690
T z o ¢j) 00 z tw b-
FREQUENCY (cm -t) Fig. 4. I.R. spectrum of a solution of D3BO3 in molten NaBF4 at 425°C. cm -1 was not observed in samples of pure molten NaBF4 and is assigned to the O D stretching mode of the BF3OD- ion produced by reaction of NaBF4 with D3BO3: N a F + D3BO3 + 2NaBF4 ~ 3NaBF3OD.
(2)
The absorbance at 2690 cm -t was checked over a period of 3 days, during which time the melt temperature was maintained at - 425°C. No measurable change in the concentration of BF3OD- was detected during this period. Also, no change was observed when small pieces of Hastelloy N [8] were added to the melt. The absorbance of the 2690 cm -~ decreased with time, however, following the addition of small chips of chromium metal to the NaBF4 melt. This decrease in BF3OD- concentration occurred possibly because of the following reaction: 3NaF + Cr ° + 3NaBF3OD ~ 3/2D2 + Na3CrF~ + 3NaBFzO.
(3)
The above i.r. spectral measurements with pellets and with molten salt samples were made with a Perkin-Elmer model 621 spectrophotometer. Although radiation is chopped after passing through.the sample in the Perkin-Elmer 621, i.r. emission from the molten fluoroborate did not appear to interfere with the measurement of the absorption of the 2690 cm -1 band. The relative emissivity from this melt was found to be about 2% above the background level at 2690 cm -1. 7. J. P. Young, lnorg. Chem. 6, 1486 (1967). 8. A nickel base alloy (Ni, 16Mo, 7Cr, 4Fe, 0.05C wt%).
Stability of BF3OH- ion
2725
DISCUSSION
A. Band assignments The assignment of the bands at 3641 and 2688 cm-' (Figs. 2 and 3) to the O - H and O - D stretching modes, respectively, of BF:~OH- and BF3OD- in solid solution with NaBF4 is consistent with the spectra observed for pure crystalline NaBF.~OH [5]. As stated earlier, the O - H stretching mode of BF:~OH- in NaBF:~OH was observed at 3620 cm-L A complete explanation of why this mode shifts to 3641 cm-' when BF:~OH- goes into solid solution with NaBF4 cannot be given; however, the difference in the static field experienced by BF3OH- in NaBF4 compared with the field it encounters in pure NaBF3OH may account for this frequency shift. Furthermore, if any hydrogen bonding existed between neighboring BF3OH- ions in NaBF3OH, this condition would not exist for BF3OH- ions isolated in NaBF~, and the absence of hydrogen bonding would result in a positive increase in the O H stretching mode frequency. The apparent ease with which NaBF3OH goes into solid solution with NaBF4 is not surprising because of the close similarities between the structures of the BF4- and BF3OH- anions [9]. Evidently, the conditions for pellet preparation were sufficient to form solid solutions of NaBF3OH in NaBF4. The spectrum shown in Fig. 1, however, indicates that not all of the NaBF3OH formed a solid solution with NaBF4 as a result of pressing a pellet of the mixed crystals. The bands at 3620 and 767 cm-' shown in Fig. 1 are believed to be caused by NaBFzO H molecules which did not go into solid solution as these two bands are observed in the i.r. spectra of (pure) crystalline NaBF3OH [5, 6]. A preliminary analysis of the BF:~OH- vibrational spectrum [5] indicates that the 767 cm ' band can be attributed to the symmetric B - F stretching mode. The 779 cm-' band shown in Fig. 1 is the B,u component of the v,(A,) mode of BF4- ion as determined[3] from single crystal vibrational spectra of NaBF4. The 773 cm -1 band was observed before and after heating the pellet (Fig. 1). It is not due to NaBF4 [2, 3], hence the assignment of the band to the symmetric B - F stretching mode of BF3OH- ion dissolved in NaBF4 seems logical. Vibrational spectra of NaBF:~OD have not been recorded so that a definite assignment of the 2688 cm -1 band to the O D stretching mode of BF3OD- cannot be made at this time. However, using the force constants determined from a vibrational calculation [5] on the BF3OH- ion, the frequency of the O D stretch of BF3OD- was calculated as 2660 cm-' in good agreement with the frequency observed at 2688 cm '. B. Thermal stability of sodium hydroxyfluoroborate The stability of the hydroxyfluoroborate anion (BF3OH-, BF3OD- or BF3OT-) with respect to unimolecular decomposition in a quiescent melt of N a B F 4 - N a F at 425°C was established from the experimental results described above in which no change in concentration of BF3OD- was observed at this temperature over a period of several days. A key to the understanding of the apparent thermal instability of pure NaBF3OH was provided by the spectroscopic results shown in Fig. 1. Thus, when the NaBF4 pellet containing NaBF3OH was heated above 9. M . J . R . Clark and H. Lynton, Can. J. Chem. 48, 405 (1970).
2726
J.B. BATES et al.
100°(2, the BF3OH- ions which were not in solid solution decomposed while the dissolved (matrix isolated) BF3OH- ions remain unchanged. These observations support the conclusions of Kolditz and Lung [6] that NaBF3OH decomposes by way of a condensation reaction at about 100°C: 2NaBF3OH --* Na~BzF60 + H20.
(4)
Therefore, if the decomposition of BF3OH- involves at least a bimolecular process, this species can be stabilized when it is isolated in a matrix or when it is present in low concentrations in molten NaBF4 as demonstrated by the experiments discussed above. The observation that the BF3OH- ion was thermally stable when pieces of Hastelloy N were added to the melt indicates that this species may also be stable under the operating conditions of the MSBR[1]. However, since the NaBF4N a F is circulated at a relatively high flow rate and at operating temperatures of about 700°C, this conclusion must be tested with additional experiments. C. Reactions of NaBF3OH in molten N a B F 4 - N a F with D20, DF, and D2 The experimental results obtained from the treatment of NaBF4 with D20 do not show to what extent the BF3OD- was formed by hydrolysis of NaBF4,
D20 + NaBF4 ~-~ NaBF3OD + D F
(5)
or by isotopic exchange with BF3OH-, D20 + BF3OH- ~ BF3OD- + H D O .
(6)
Since the intensity of the BF3OH- band was not diminished by D20 treatment, hydrolysis appears to be the predominant reaction. Furthermore, if isotopic exchange, rather than hydrolysis, were favored, one would expect the reverse reaction to be promoted by treatment with D F , e.g. NaBF3OH + D F ~,~ NaBF4 + H D O .
(7)
However, since we find the 2688 cm -1 band in D F treated molten NaBF4, it must be formed either by exchange with NaBF3OH, NaBF3OH + D F ~ NaBF3OD + H F
(8)
or by reaction with residual oxide N a B F 2 0 + D F --~ NaBF3OD.
(9)
Our data at present tell us only that reaction (7) is not favored. The increase in the intensity of the band at 3641 cm -1 and the appearance of a single band at 2688 cm -1 in the i.r. spectra of H 2 0 and D20 treated NaBF4, respectively, strongly suggests that a single type of hydroxyfluoroborate is formed during the hydrolysis. It was observed in several experiments, however,
Stability of BF3OH- ion
2727
that pellets pressed from quenched samples of NaBF4 exhibited a second, weaker band at 3580 cm -1 in addition to that at 3641 cm -1. The origin of this weaker band is unknown. Conceivably it may be caused by a cyclic hydroxyfluoroborate anion such as
I HO\/F -13. ~ o/B'o /B"0/B~
F
F/ |
Fj
produced by the hydrolysis of (BF20)aa-[6]. The 3580 cm -1 band might also arise from BF2(OH)2- or BF(OH)3- ions produced by hydrolysis reactions of BFaOH-. However, on considering the thermal instability of pure NaBFaOH, this explanation may not be valid. Strong absorption by NaBF4 bands in the region between 800 and 1200 cm -1 precluded the possibility of identifying the cyclic hydroxyfluoroborate from its characteristic absorptions in this spectral region [10]. Additional work is needed to identify hydroxyfluoroborates other than the BF3OH- monomer in molten NaF-NaBF4. The experiment in which a sample of molten NaBF4 sealed in a nickel capsule was equilibrated with Dz at 600°C was intended to demonstrate isotopic exchange of D2, outside the capsule, with BF3OH-, inside the capsule. Although BFaODwas found in the melt after the experiment (Fig. 3), a proportionate decrease in the intensity of the BFaOH- band did not occur. This result strongly suggests that rather than isotopic exchange with atomic deuterium, the D2, which diffused through the nickel capsule, reduced a (surface) metal oxide impurity and formed BF:~OD-, i.e. D2 + B2F602- + MO ~ 2BF3OD- + M °. (10) Possibly, the exchange reaction is initiated by deuterium atoms and is catalyzed by the nickel wall. The extent to which D2 will undergo an isotopic exchange reaction with BFaOH- ions will require a melt of low oxide content. With regard to the tritium release problem, however, it makes no difference whether or not the reaction to form BF3OT- proceeds via isotopic exchange or by reaction ofT2 with oxide impurities in NaBF4. CONCLUSIONS
The experimental results presented in this paper show that the hydroxyfluoroborate anion, BFaOH-, is stable when it is dissolved in a quiescent melt of the NaBF4-NaF eutectic mixture. Furthermore, deuterium undergoes reaction with metal oxides and/or with BF3OH- in molten NaBF4 to form BFaOD-. These two observations suggest that tritium released during the operation of a molten salt breeder reactor can be contained in the N a B F I - N a F coolant by formation of the stable BF3OT- ion. Acknowledgements- We acknowledge the contribution of S. Cantor who designed and carried out the experimental work on the D2 exchange with molten NaBF4 in a sealed nickel ampul. We also acknowledge the help of G. M. Herbert who prepared the NaBF3OH used in this study. 10. M. V. Aknmanova and G. E. Kurilchikova, Opt. Spectrosc. 8, 264 (1960).
STABILITY OF BF3OH- ION IN MOLTEN A N D SOLID NaBF4 A N D NaF-NaBF4 EUTECTICS* J. B. BATES, J. P. Y O U N G , M. M. M U R R A Y , H. W. K O H N t and G. E. BOYD Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (First received I November 1971 ; in revised form 17 December 1971)
Abstract--I.R. measurements on solutions of NaBF3OH in crystalline and molten NaBF4 were employed to determine the stability of the BF3OH- ion with respect to its unimolecular decomposition. Isotopic substitution experiments with D~O, D F and D2 have shown that the species BFzOD- is formed in molten NaBF4 either by reaction with BF3OH-, with BF20-, or with BzF602- mediated by a transition metal oxide. These isotope reactions give encouragement to the view that tritium released during the operation of a molten salt breeder reactor might be trapped by reaction with BF3OH- or with oxyfluoroborates present as impurities in the molten sodium fluoroborate coolant salt. INTRODUCTION
THE PROPOSAL t o employ the eutectic composition N a B F 4 - N a F (8% NaF) as a
coolant for the molten salt breeder reactor (MSBR)[1] has prompted many investigations on the chemical and physical properties of pure NaBF4 as well as on the N a F - N a B F 4 eutectic. For example, the i.r. and Raman spectra of crystalline NaBF4 and the Raman spectra of molten NaBF4 and N a B F 4 - N a F have been thoroughly studied in this Laboratory [2-4]. The identities and properties of the impurities in the NaBF4 employed, however, were not investigated. It now appears that most of the samples of this compound must have contained at least a small amount 0 0 0 0 - 2 0 0 0 ppm) of NaBF3OH which was extremely difficult to remove from NaBF4 melts. Tritium formed and released in comparatively large amounts during the operation of molten salt breeder reactors may constitute a serious environmental hazard. A possible way to minimize the hazard would be to trap the tritium by isotopic exchange reactions with protonic species in the N a B F 4 - N a F coolant salt. If the BF3OH- ion were present in the melt, the reaction would be described by the equation: 2BF3OH- + T 2 ~ 2BF3OT- + H2.
(1)
The identification of NaBF3OH in molten and solid NaBF4 provided the first *Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. ?Present address: Chemistry Department, Dickinson College, Carlisle, PP. 17013. 1. R. C. Robertson, O. L. Smith, R. B. Briggs and E. S. Bettis, Two-Fluid Molten-Salt Reactor Design Study, ORNL-4528, Oak Ridge National Laboratory (! 970). 2. J. B. Bates, A. S. Quist and G. E. Boyd, J. chem. Phys. 54, 124 (1971). 3. J. B. Bates, ibid. 55, 489 (1971). 4. A. S. Quist, J. B. Bates and G. E. Boyd, ibid. $4, 4896 (1971). 2721
2722
J . B . BATES et al.
evidence that such a proton-containing species could be stable in these media. This paper will report results from recent experiments directed toward confirming the identity of BFaOH- ion in NaBF4, determining its stability in the molten salt, and investigating its reaction with deuterium in molten NaBF4. EXPERIMENTAL Evidence for the presence of hydrogen-containing impurities in NaBF4 was obtained in preliminary measurements of the near-ix, spectra of the molten salt and from mid-i.r, spectra of pressed pellets of the crystalline material. Additionally, the polarized i.r. transmission spectrum of a single crystal of NaBF4 (1/4-1/2ram thick) revealed a single sharp band at 3641 cm -1. This band was assigned tentatively to the O - H stretching mode of the BFsOH- ion in solid solution with NaBF4. Samples of NaBF4 or NaBF4-NaF which contained protonic impurities were prepared in several ways, viz. by the addition of H3BO3 to the molten solvents; by physically mixing solid NaBFaOH; with the powdered salts; or by recrystallizing NaBF4 from water or from aqueous 0.1 M H F solution. Identical mid-ix, spectra were obtained from pellets prepared from these mixtures. The spectrum shown in Fig. 1 was obtained with a pellet containing - 0.5% by weight of NaBF3OH in NaBF4. The bands at 3620 and 767 cm -x have been observed in the i.r. and Raman spectra of crystalline NaBF3OH [5], and the band at 779 cm -x can be assigned to the vl mode of BF4- ion [2, 3]. On heating the pellet at 120°C for several minutes, the bands at 3620 and 767 cm -1 disappear, while the bands at 3641, 779 and 773 cm -~ remain unchanged as shown in Fig. 1. In addition, the 3620 cm -1 band was not found in the vibrational spectra of samples of (pure) crystalline NaBF3OH which had been heated above 100°C [5, 6].
A
I 73
3X
779
Z 0
3641
(13
:E
(/3 z n. F-
B
f
3X
-773 364t 779 FREQUENCY (cm- t )
Fig. I. I.R. spectrum of a pellet of NaBF4 containing 0.5 wt% of NaBF3OH. A = spectrum of pellet before heating; B = spectrum of pellet after heating above 100°C. 5. J. B. Bates. Work in progress. 6. L. Kolditz and C. S. Lung, Z. Chem. 12, 469 (1967).
Stability of BF3OH- ion
2723
Initial studies of the reaction of the BF3OH- ion with deuterium were made by slowly bubbling D20 vapor into a molten N a F - N a B F 4 eutectic mixture at 400°C. The amount of deuterium introduced into the melt corresponded to about 200 ppm D~O as determined by a material balance calculation. The i.r. spectrum of pellets pressed from a quenched sample exhibited a sharp band at 3641 cm -~ and a new band at 2688 cm -1. The extent of the reaction of D F with NaBF3OH in NaBF4 was investigated by equilibrating molten NaBF4 at 450°C with a DF-argon mixture. I.R. measurements on pressed pellets of the solid material obtained from quenching this melt also revealed the two sharp bands, 3641 and 2688 cm-L The spectrum of a 100 mg pellet of D F treated molten NaBF4 in the 3600 and 2600 cm -1 regions is shown in Fig. 2.
! 0 O3 (./3 CO
Z or" I--
2688 5641
FREQUENCY (cm-1) Fig. 2. I.R. spectrum of 1O0 mg pellet of a quenched melt of D F treated NaBF4. The reaction of BF3OH- ion in NaBF4 with atomic deuterium was investigated by heating a sealed nickel capsule containing NaBF4 at 600°C in an atmosphere of about 25 mm of D2 gas for three days. The i.r. spectrum of a 15 mg pellet pressed from a quenched sample of this melt is shown in Fig. 3. Contrary to expectations, no measurable change in the absorbance of the 3641 cm -1 band was observed after treatment with D~. The possibility that the 3641 and 2688 cm -1 absorptions were due, respectively, to H F and D F dissolved in solid NaBF4 was examined by treating samples of solid NaBF4 with H F and D F at 350°C. No bands were observed above 2500 cm -1 in i.r. spectra of pellets pressed from this sample, although the NaBF4 contained 0.097 meq/g of acid as was shown by titration with standard alkali.
1'
Z
o (,r) 03 CO z < n.I-
-
T
I 3641
2688
W
,,..) z <[ .,,n n-" 0 03 m ,< FREQUENCY
(cm -I]
Fig. 3. I.R. spectrum of a 15 mg pellet of a quenched melt of D2 treated NaBF4.
2724
J . B . BATES et al.
The pellets used in the foregoing experiments were prepared by grinding 15-100 mg of sample in a "Wiggle-Bug" micro-ball mill and pressing with a 0.5 in. dia. die using a total load of about 10,000 lb as read from the gauge on a hydraulic press. For thin pellets (15-25 mg samples), the sample was pressed in a paper collar (inserted into the die) to give the pellet extra support. Measurements of the OH stretching mode frequency of BF3OH- ion in molten NaBF4 could not be made with the equipment available. Although fluoroborate melts will dissolve most of the common i.r. cell materials, they can be contained in cells made of silica. Silica exhibits a strong absorption in the region near 3600 cm -1 which would obscure the BF3OH- band, but it is quite transparent in the region near 2600-2700 cm-L Therefore, i.r. spectral measurements were carried out on solutions of BF3OD- ion in molten NaBF4. A sample prepared by adding D3BO3 to NaBF4 was contained in a 2 mm thick silica cell and placed in a high temperature furnace described previously [7]. The spectrum at 425°(2 observed in the 2700 cm -~ region is shown in Fig. 4. The absorption band centered at 2690
T z o ¢j) 00 z tw b-
FREQUENCY (cm -t) Fig. 4. I.R. spectrum of a solution of D3BO3 in molten NaBF4 at 425°C. cm -1 was not observed in samples of pure molten NaBF4 and is assigned to the O D stretching mode of the BF3OD- ion produced by reaction of NaBF4 with D3BO3: N a F + D3BO3 + 2NaBF4 ~ 3NaBF3OD.
(2)
The absorbance at 2690 cm -t was checked over a period of 3 days, during which time the melt temperature was maintained at - 425°C. No measurable change in the concentration of BF3OD- was detected during this period. Also, no change was observed when small pieces of Hastelloy N [8] were added to the melt. The absorbance of the 2690 cm -~ decreased with time, however, following the addition of small chips of chromium metal to the NaBF4 melt. This decrease in BF3OD- concentration occurred possibly because of the following reaction: 3NaF + Cr ° + 3NaBF3OD ~ 3/2D2 + Na3CrF~ + 3NaBFzO.
(3)
The above i.r. spectral measurements with pellets and with molten salt samples were made with a Perkin-Elmer model 621 spectrophotometer. Although radiation is chopped after passing through.the sample in the Perkin-Elmer 621, i.r. emission from the molten fluoroborate did not appear to interfere with the measurement of the absorption of the 2690 cm -1 band. The relative emissivity from this melt was found to be about 2% above the background level at 2690 cm -1. 7. J. P. Young, lnorg. Chem. 6, 1486 (1967). 8. A nickel base alloy (Ni, 16Mo, 7Cr, 4Fe, 0.05C wt%).
Stability of BF3OH- ion
2725
DISCUSSION
A. Band assignments The assignment of the bands at 3641 and 2688 cm-' (Figs. 2 and 3) to the O - H and O - D stretching modes, respectively, of BF:~OH- and BF3OD- in solid solution with NaBF4 is consistent with the spectra observed for pure crystalline NaBF.~OH [5]. As stated earlier, the O - H stretching mode of BF:~OH- in NaBF:~OH was observed at 3620 cm-L A complete explanation of why this mode shifts to 3641 cm-' when BF:~OH- goes into solid solution with NaBF4 cannot be given; however, the difference in the static field experienced by BF3OH- in NaBF4 compared with the field it encounters in pure NaBF3OH may account for this frequency shift. Furthermore, if any hydrogen bonding existed between neighboring BF3OH- ions in NaBF3OH, this condition would not exist for BF3OH- ions isolated in NaBF~, and the absence of hydrogen bonding would result in a positive increase in the O H stretching mode frequency. The apparent ease with which NaBF3OH goes into solid solution with NaBF4 is not surprising because of the close similarities between the structures of the BF4- and BF3OH- anions [9]. Evidently, the conditions for pellet preparation were sufficient to form solid solutions of NaBF3OH in NaBF4. The spectrum shown in Fig. 1, however, indicates that not all of the NaBF3OH formed a solid solution with NaBF4 as a result of pressing a pellet of the mixed crystals. The bands at 3620 and 767 cm-' shown in Fig. 1 are believed to be caused by NaBFzO H molecules which did not go into solid solution as these two bands are observed in the i.r. spectra of (pure) crystalline NaBF3OH [5, 6]. A preliminary analysis of the BF:~OH- vibrational spectrum [5] indicates that the 767 cm ' band can be attributed to the symmetric B - F stretching mode. The 779 cm-' band shown in Fig. 1 is the B,u component of the v,(A,) mode of BF4- ion as determined[3] from single crystal vibrational spectra of NaBF4. The 773 cm -1 band was observed before and after heating the pellet (Fig. 1). It is not due to NaBF4 [2, 3], hence the assignment of the band to the symmetric B - F stretching mode of BF3OH- ion dissolved in NaBF4 seems logical. Vibrational spectra of NaBF:~OD have not been recorded so that a definite assignment of the 2688 cm -1 band to the O D stretching mode of BF3OD- cannot be made at this time. However, using the force constants determined from a vibrational calculation [5] on the BF3OH- ion, the frequency of the O D stretch of BF3OD- was calculated as 2660 cm-' in good agreement with the frequency observed at 2688 cm '. B. Thermal stability of sodium hydroxyfluoroborate The stability of the hydroxyfluoroborate anion (BF3OH-, BF3OD- or BF3OT-) with respect to unimolecular decomposition in a quiescent melt of N a B F 4 - N a F at 425°C was established from the experimental results described above in which no change in concentration of BF3OD- was observed at this temperature over a period of several days. A key to the understanding of the apparent thermal instability of pure NaBF3OH was provided by the spectroscopic results shown in Fig. 1. Thus, when the NaBF4 pellet containing NaBF3OH was heated above 9. M . J . R . Clark and H. Lynton, Can. J. Chem. 48, 405 (1970).
2726
J.B. BATES et al.
100°(2, the BF3OH- ions which were not in solid solution decomposed while the dissolved (matrix isolated) BF3OH- ions remain unchanged. These observations support the conclusions of Kolditz and Lung [6] that NaBF3OH decomposes by way of a condensation reaction at about 100°C: 2NaBF3OH --* Na~BzF60 + H20.
(4)
Therefore, if the decomposition of BF3OH- involves at least a bimolecular process, this species can be stabilized when it is isolated in a matrix or when it is present in low concentrations in molten NaBF4 as demonstrated by the experiments discussed above. The observation that the BF3OH- ion was thermally stable when pieces of Hastelloy N were added to the melt indicates that this species may also be stable under the operating conditions of the MSBR[1]. However, since the NaBF4N a F is circulated at a relatively high flow rate and at operating temperatures of about 700°C, this conclusion must be tested with additional experiments. C. Reactions of NaBF3OH in molten N a B F 4 - N a F with D20, DF, and D2 The experimental results obtained from the treatment of NaBF4 with D20 do not show to what extent the BF3OD- was formed by hydrolysis of NaBF4,
D20 + NaBF4 ~-~ NaBF3OD + D F
(5)
or by isotopic exchange with BF3OH-, D20 + BF3OH- ~ BF3OD- + H D O .
(6)
Since the intensity of the BF3OH- band was not diminished by D20 treatment, hydrolysis appears to be the predominant reaction. Furthermore, if isotopic exchange, rather than hydrolysis, were favored, one would expect the reverse reaction to be promoted by treatment with D F , e.g. NaBF3OH + D F ~,~ NaBF4 + H D O .
(7)
However, since we find the 2688 cm -1 band in D F treated molten NaBF4, it must be formed either by exchange with NaBF3OH, NaBF3OH + D F ~ NaBF3OD + H F
(8)
or by reaction with residual oxide N a B F 2 0 + D F --~ NaBF3OD.
(9)
Our data at present tell us only that reaction (7) is not favored. The increase in the intensity of the band at 3641 cm -1 and the appearance of a single band at 2688 cm -1 in the i.r. spectra of H 2 0 and D20 treated NaBF4, respectively, strongly suggests that a single type of hydroxyfluoroborate is formed during the hydrolysis. It was observed in several experiments, however,
Stability of BF3OH- ion
2727
that pellets pressed from quenched samples of NaBF4 exhibited a second, weaker band at 3580 cm -1 in addition to that at 3641 cm -1. The origin of this weaker band is unknown. Conceivably it may be caused by a cyclic hydroxyfluoroborate anion such as
I HO\/F -13. ~ o/B'o /B"0/B~
F
F/ |
Fj
produced by the hydrolysis of (BF20)aa-[6]. The 3580 cm -1 band might also arise from BF2(OH)2- or BF(OH)3- ions produced by hydrolysis reactions of BFaOH-. However, on considering the thermal instability of pure NaBFaOH, this explanation may not be valid. Strong absorption by NaBF4 bands in the region between 800 and 1200 cm -1 precluded the possibility of identifying the cyclic hydroxyfluoroborate from its characteristic absorptions in this spectral region [10]. Additional work is needed to identify hydroxyfluoroborates other than the BF3OH- monomer in molten NaF-NaBF4. The experiment in which a sample of molten NaBF4 sealed in a nickel capsule was equilibrated with Dz at 600°C was intended to demonstrate isotopic exchange of D2, outside the capsule, with BF3OH-, inside the capsule. Although BFaODwas found in the melt after the experiment (Fig. 3), a proportionate decrease in the intensity of the BFaOH- band did not occur. This result strongly suggests that rather than isotopic exchange with atomic deuterium, the D2, which diffused through the nickel capsule, reduced a (surface) metal oxide impurity and formed BF:~OD-, i.e. D2 + B2F602- + MO ~ 2BF3OD- + M °. (10) Possibly, the exchange reaction is initiated by deuterium atoms and is catalyzed by the nickel wall. The extent to which D2 will undergo an isotopic exchange reaction with BFaOH- ions will require a melt of low oxide content. With regard to the tritium release problem, however, it makes no difference whether or not the reaction to form BF3OT- proceeds via isotopic exchange or by reaction ofT2 with oxide impurities in NaBF4. CONCLUSIONS
The experimental results presented in this paper show that the hydroxyfluoroborate anion, BFaOH-, is stable when it is dissolved in a quiescent melt of the NaBF4-NaF eutectic mixture. Furthermore, deuterium undergoes reaction with metal oxides and/or with BF3OH- in molten NaBF4 to form BFaOD-. These two observations suggest that tritium released during the operation of a molten salt breeder reactor can be contained in the N a B F I - N a F coolant by formation of the stable BF3OT- ion. Acknowledgements- We acknowledge the contribution of S. Cantor who designed and carried out the experimental work on the D2 exchange with molten NaBF4 in a sealed nickel ampul. We also acknowledge the help of G. M. Herbert who prepared the NaBF3OH used in this study. 10. M. V. Aknmanova and G. E. Kurilchikova, Opt. Spectrosc. 8, 264 (1960).