Nitryl Perchlorate, NOzC104. The sample was taken directly from an unpurified reaction product ; however, analysis for total acid content indicated high purity. KO further purification was attempted because of the extreme hygroscopicity of the material. The crystals are monoclinic, space group c2/c, a = 9.25 il., b = 6.99-i.)c = 7.34 A, , = 113.5" (4). Potassium Formate, HCOOK. Fisher Scientific Co. reagent grade material was used after recrystallization from methanol. Sodium Methyl Carbonate, KaCH3The sample was prepared a t Callery Chemical Co. and submitted in pure form. Intensities were measured with the diffractometer. Sodium Metaborate, KaBOz. The sample was prepared a t Callery Chemical Co. by dehydration of carefully purified sodium metaborate tetrahydrate. The crystals are rhombohedral, space group R3c, a = 11.93 A., c = 6.46 A. ( 3 , 5 ,6). Sodium Tetrafluoroborate (1-), NaBFI. The sample was prepared a t Callery Chemical Co. and was submitted in pure form. The crystals are orthorhombic with the following reported call dimensions: a = 6.25 A., b = 6.77 A., c = 6.82 A. (18); u = 6.20 A., b = 6.808 A., c = 6.825 A. ( 2 ) . Sodium Tetramethoxyborate (1-), NaB(OCH3)*. The sample was pre-
cos.
pared at Callery Chemical Co.; it was purified by repeated recrystallization from anhydrous methanol in a dry atmosphere. Sodium Tetraphenylborate (1-), KaB(CeH6),. J. T. Baker reagent grade chemical was used after recrystallization from a benzene-methanol solution. The crystals are tetragonal, space group I4m2 or I42m, a = 11.45 A,, c = 7.41 A. (1). Sodium Tridecahydrodecaborate (1-), KaBlaHi~. The material was prepared at Callery Chemical Co. in pure form by the method of Hough and Edwards (10). Trimethylamine-Borane, (CH3)3?S: BH3. The sample was prepared a t Callery Chemical Co. and purified by sublimation. The crystals are rhombohedral, space group R3m, a = 9.33 A., c = 5.90 A. ( 7 ) . The d-spacings calculated on the basis of these unit cell parameters agree well with those of the observed pattern. T r i m e t h y 1a m i n e - T r i b or a n e (7)) (CH3)3X:B3H7. The sample was prepared a t Callery Chemical Co. by the method of Hough and Edwards (9),and purified by recrystallization from a water-methanol solution. The crystals are rhombohedral, space group R3m, a = 9.535 A,, c = 7.45A. (14). The d-spacings calculated from these unit cell dimensions agree well with those of the observed pattern.
LITERATURE CITED
(1) Amott, S., Abrahams, S. C., Acta Cryst. 11, 449 (1959). (2) Rend. SOC. mineral. ital. ~, Bellanca. 3. 20 (1946). (3) 'Cole', S. S., Scholes, S. R., Amberg, C. R., J . Am. Ceram. SOC.20,215 (1937). (4)Cox, E. G., Jeffrey, G. A., Truter, M.R., Nature 162, 259 (1948). (5) Fang, Ssii-Mien, J . Am. Ceram. SOC. 20,214 (1937). (6) Fang, Ssii-Mien, 2.Krist 99, l(1938). (7) Geller, S., Hughes, R. E., Hoard, L. J., Acta Cryst. 4, 380 (1951). (8) Hannawalt, J. D., Rinn, H. W., Frevel, L. K., IND. EXQ.CHEW, ANAL. ED. 10, 457 (1938). (9) Hough, W. V., Edwards, L. J., Abstracts of Papers, 132nd Meeting, ACS, New York, Tu'. Y., September 1957. (10) Hough, W. V., Edwards, L. J., Symposium on Borax to Boranes, 133rd Meeting, ACS, San Francisco, Calif., April 1958. (11) Kasper, J. S., Lucht, C. M., Harker, D., Acta Cryst. 3, 436 (1950). (12) Klinkenbere. L. J.. Rec. trav. chim. ' 56,36-40 (1937). (13) Klug, H. P., Alexander, L. E., "XRay Diffraction Procedures for Polycrystalline and Amorphous Materials," K'ilev. New York. 1954. (14) NLrment, H. G., unpublished work. (15) van der Reddy, J. M., Lipscomb, W. N., J . Am. Chem. SOC.81,754 (1959). (16) Schaeffer, R., Ibid., 79, 1006 (1957). (17) Ibid., p . 2726. (18) Zachariasen, W. H., Acta Cryst. 7, 305 (1954). RECEIVED for review November 20, 1959. Accepted February 26, 1960.
Absorption Spectra of Molten Fluoride Salts Solutions of Several Metal Ions in Molten Lithium Fluoride-Sodium Fluoride-Potassium Fluoride J. P. YOUNG and J. C. WHITE Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b Spectra are presented for nickel fluoride, cobalt fluoride, chromic fluoride, praseodymium fluoride, uranium tetrafluoride, and uranyl fluoride dissolved in molten lithium fluoridesodium fluoride-potassium fluoride at temperatures ranging from 500" to approximately 650' C. These spectra are compared to similar spectra obtained in other molten salts and aqueous solutions. General procedures are described which were used in preparing the fluoride salt melt and in recording the spectra by means of a high-temperature cell assembly designed for use with a Cary recording spectrophotometer, Model 14M.
S
meaSUrementS of molten salt systems can be used to study the unique chemical phenomena which occur in the molten state. One outstanding advantage is the possible direct determination of unstable (with respect to aqueous systems) oxidation states or ionic species of a particular element-for example, divalent chromium or trivalent uranium. Another is the tremendous concentration effect of molten solutions as compared to aqueous solutions of the same salt. Consider a salt with a density, in the molten state, of 2.0 (grams per ml.) and an aqueous solution which contains 1.0 gram of this salt dissolved in 100 ml. of water; the melt contains 2000 mg. PECTROPHOTOMETRIC
of sample per ml. as compared to 10 mg. per ml. in the aqueous solution. Because of this concentration effect in a molten salt system, a molar absorbance index of 10, normally considered to be insensitive, is actually equivalent to 2000 in the determination of parts per million quantities of components in the salt. The papers in this series will deal primarily with the analytical usefulness of spectrophotometry of molten saltsspecifically molten fluoride salts; however, the data presented should also be of general value. APPARATUS
The high-temperature cell assembly (11) was used with a Cary recording VOL. 32, NO. 7, JUNE 1960
799
400 WAVE
I
I
500
600
LENGTH,mp
Figure 2. Sptctrum of cobalt fluoride in lithium fluoride-sodium-fluoride-potassium fluoride Cell. ~1 -cm. Pt tube Temperature. -500° C. CoFz concentration. 3% (w./w.J
04
1 I
Figure 1. Spectrum of nickel fluoride in lithium fluoride-sodium fluoride-potassium fluoride Cell. 0.5-cm. M g 0 Temperature. -500' C. NiFz concentration. 1% (w./w.)
spectrophotometer, Model 14M, in obtaining the spectra. For spectral study the molten fluoride samples were contained in a crystalline magnesium oxide cell or as a pendent drop in a platinum tube (11). REAGENTS
The eutectic lithium fluoride-sodium fluoride-potassium fluoride (46.5-1 1.542.0 mole %, melting point 454" C.) was prepared from reagent grade chemicals by Oak Ridge National Laboratory personnel. The oxide and water contents of this eutectic were reduced to a minimum by hydrofluorination of the molten salt. Routine analytical tests were made to ascertain the purity of the final product. The solutes-nickel fluoride, potassium hexafluochromate(III), cobaltous fluoride, uranium tetrafluoride, and praseodymium fluoride-were also prepared a t Oak Ridge National Laboratory and routinely tested as above. EXPERIMENTAL
The fluoride salt melts were prepared under an atmosphere of dry argon by heating weighed amounts of solvent and solute a t 600" to 650" C. in a platinum crucible until a clear melt was observed. The solutions were quickly cooled to room temperature, under a dry argon atmosphere, and the crucible was transferred to a dry box. The solid fluoride salt mixture was easily removed from the container by inverting the crucible. At this time 800
*
ANALYTICAL CHEMISTRY
any surface material which appeared foreign to the sample was removed by scraping, and the remainder of the sample pulverized. The powdered sample was then placed either in the magnesium oxide cell or in a small platinum cup, ca. 1.5-ml. volume, if the pendent drop technique was to be used. The filled sample containers were placed in a desiccator and removed from the dry box. Prior to sample introduction the high-temperature cell assembly was heated a t 150" C., with dry argon gas flowing through the system, for 1 hour to remove any water from the apparatus. The sample container was quickly transferred to the cell asseFbly, and heating continued a t 200 C., under the inert gas flow, for '/z to 1 hour. The temperature of the assembly was then raised to the desired level, 500" to 700" C., and the spectra of the molten sample were recorded. Until the sample was melted and prepared for spectral study, the high-temperature cell assembly was kept outside the sample compartment of the spectrophotometer; the melting process was followed by visual observation through the light ports of the apparatus. When the pendent drop technique was used, the platinum tube container was filled by immersing the tube in the melt and then withdrawing the molten salt pendent drop (11). The filled platinum tube was aligned, visually, so that the tube was located symmetrically in the center of the rectangular opening in the heated sample cavity of the
furnace and in line with the light ports. After spectral investigation of the alkali fluoride salt sample, the sample container was removed, and if possible the molten material poured out of the container; all platinum ware was cleaned by immersion in boiling sulfuric acid for several minutes. The magnesium oxide cell was cleaned with a cotton swab and flushed with water. RESULTS
The spectra presented can, a t present, be considered as analytically useful only for spectral identification and, in some cases, semiquantitative estimation of unknown components in lithium fluoride-sodium fluoride-potassium fluoride. Problems associated with the pendent drop technique do not permit quantitative interpretations of absorbance intensity as yet. Nickel Fluoride. The spectrum of nickel fluoride in the molten eutectic lithium fluoride-sodium fluoridepotassium fluoride is s h o w in Figure 1, as \$-ell as the spectrum of the solvent, lithium fluoride-sodium fluoride-potassium fluoride. The optical cutoff that is shown (Figure 1) a t 310 mp arises from the crystalline magnesium oxide cell; the solvent itself is transparent to ultraviolet light. Sickel fluoride exhibits a maximum absorbance a t 434 mp; there is also a much less intense, broad peak a t 850 mp. I n lithium nitrate-potassium nitrate (43 to 57 mole %), it has been reported that nickel nitrate exhibits peaks a t 425 and 775 m l (4). In 1M perchloric acid, absorbance maxima for nickel are found a t 395 and 720 mp (1). The spectrum of nickel chloride in molten chloride salts is different and has been discussed ( 2 , 5 ) . The molar absorbance index of nickel fluoride in lithium fluoride-
I
I
I
I
1
400
500
600
700
WAVE
LENGTH,rnp
-
Figure 3. Spectrum of chromium trifluoride fluoride-sodium fluoride-potassium fluoride
in lithium
~
600
WAVE L E N G T H rnp
Figure 4. Spectrum of praseodymium fluoride in lithium fluoride-sodium fluoride-potassium fluoride
Cell. -1 -cm. Pt tube C. Temperature. -650' CrFa concentration. 1.5% (w./w.)
sodium fluoride-potassium fluoride at 500" C. is approximately 7 , a t 434 mp. This value increases somewhat with increasing temperature of the solution. Cobalt Fluoride. In Figure 2 is shown the spectrum of cobaltous fluoride. The sample was confined as a pendent drop in obtaining these data; the path length was probably on the order of 0.5 cm. Cobaltous fluoride exhibits two peaks a t 510 and 580 mp. Cobaltous nitrate in lithium nitrate-potassium nitrate exhibits one peak a t 560 mp ( 4 ) ; the cobaltous ion in 1M perchloric acid exhibits one peak a t 510 mp (1). As was the case Kith nickel, the spectrum of cobaltous chloride in molten chloride melts is notably altered ( 4 ) . The molar absorbance index of the 580-mp peak of cobaltous fluoride is on the order of 10 to 20. I n obtaining spectra of pendent drops of liquid i t is necessary to mask a considerable portion of the available sample light beam of the Cnry spectrophotometer, because only the light that passes through the drop of sample is allowed to impinge on
5dO
4i3
Pt tube container Temperature. -550'
the detector of the instrument (11). To compensate partially for the decreased intensity of the sample light beam, light-absorbing screens, available from Applied Physics Corp., are used to attenuate the reference light beam of the instrument. It has not been found necessary to equalize the intensity of the two light beams; but rather, the inequality of the intensities is presented on the recorder of the Cary spectrophotometer as an absorbance which is essentially independent of wave length. I n all pendent drop spectra presented in this report, the ordinate is labeled "Absorbance (Arbitrary Zero)"; in these spectra an arbitrary absorbance was chosen as zero, which essentially eliminates the inequality of the light beams. The portions of the spectra where minimum absorbance occurs are assumed to represent zero absorbance of the particular solute involved. Chromium Trifluoride. The spectrum of chromium trifluoride has been determined (Figure 3). This spec-
Path length. -0.8 cm. Pr concentration.
