Diamond-Windowed Cell for Spectrophotometry of Molten Fluoride Salts L. M. Toth, J. P. Young, and G . P. Smith Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 SPECTROPHOTOMETRY has proven a very useful tool for studying ionic species in molten fluoride salts. However, these media are corrosive toward common optical window materials and progress up to now has been based on the application of liquid holders that do not have windows ( I , 2). For example, the captive liquid cell (2) consists of a cylindrical container with holes below the liquid level through which the light beam passes. Surface tension prevents the liquid from running out through these holes. For solutions other than molten fluorides, a variety of liquid holders without windows have been used when conventional containers were not practical and which might be modified for molten fluoride spectroscopy. Despite their utility,windowless cells have several limitations among which are the following: the gas-liquid interface through which the light passes is curved and the curvature varies with temperature, melt composition, and other factors. Because of this curvature, it is difficult to obtain an accurate value for the path length, particularly when the path length is short. Furthermore, light transmission across this curved surface is not rectilinear and to minimize its effect on the spectra, the cells must be positioned at the focus of the spectrometer beam. Even under these conditions, the accurate reproduction of a given baseline cannot be guaranteed. Evaporative losses from windowless cells can cause difficulties at high temperatures when the system contains a volatile component. Finally, with most designs of windowless cells, homogenization of the melt by agitation is not practical. (The only exception with which we are familiar is the reflection cell (3) in which the light beam is transmitted vertically through the horizontal liquid surface and reflected back by a metal mirror immersed in the melt; but this design introduces a variety of new problems.) For molten fluoride spectroscopy, some of the limitations of windowless cells were circumvented by Cocks, Schroeder, and Schwartz who designed a cell with diamond windows ( 4 ) . Diamond is inert toward a wide variety of molten fluorides and has good to excellent optical transmission properties over the near infrared, visible, and ultraviolet regions. However, this particular cell design suffered from three disadvantages. First, it was difficult to fill, empty, and clean. Second, it was elaborately constructed. Third, the window mounting placed stresses on the diamonds that enhanced the possibilities for breakage. On the whole the cell was not suitable for routine measurements but it demonstrated the feasibility of using diamond windows in contact with fluoride melts. In an effort to use diamond windows on a more or less routine basis for molten fluoride spectroscopy, we designed a cell of simple construction that can be easily taken apart for cleaning. This cell is described here. It has a graphite body and is, of course, limited to use with systems that are inert toward carbon, but there is a wide variety of such systems. (1) J. P. Young and J. C. White, ANAL.CHEM., 32, 799 (1960). (2) J. P. Young, ibid.,36, 390 (1964). (3) J. Greenberg and L. J. Hallgren, Rev. Sci. Instrum., 31, 444
(1960). (4) G. G. Cocks, J. B. Schroeder and C. M. Schwartz, “The Spectroscopy of Fused Salts,” Battelle Memorial Institute Rept. No. BMI-1185, Columbus, Ohio, Feb-April 1957.
Figure 1. Two views of the diamond-windowed cell Exploded and partially cutaway view to the left, assembly cutaway view to the right
This design has been tested in practice and found to perform well. Design of the Cell. The construction of the diamond cell is displayed in Figure 1. All parts except the windows are machined from dense, reactor grade, Type ATJ graphite. In use, the cell is suspended in a heated inert-atmosphere chamber as described below. The fluid is contained in a cylindrical body with a 0.50inch outside diameter. The upper part of this body has a 0.375-inch inside diameter drilled to a depth of 0.688-inch. The lower part of the body has a 0.188-inch inside diameter drilled to an additional depth of 0.625-inch. A horizontal hole of 0.188-inch diameter, capped off by the diamond windows, allows passage of the light beam. The distance between windows is 0.25 inch. The windows are held in place firmly but without significant stresses by a graphite framing arrangement shown in Figure 1. The frame consists of two retainer plates (one for each window) a cylindrical sleeve which holds the retainer plates in place, and a graphite nut, which screws on to the bottom of the body and holds the sleeve in place. Positioning of a window is achieved by means of a horizontal groove in the body, to fix the vertical position, and a vertical groove in the retainer plate to fix the horizontal position. The depth of each groove is one-half the window thickness, which is nominally 1 mm, while the width of each groove is, of course, the same as the width of the window, nominally 5 mm square. With the above arrangement, the windows do not achieve a gas-tight seal with the cell body, but fluid leakage around the windows does not normally occur because most molten fluoride salts do not wet graphite. The possibility of such leakage constitutes a potential limitation in the use of this cell but in practice we have had no leakage problems. The advantages of this arrangement are simplicity of design, ease of assembly and disassembly, and no significant danger of window breakage due to loading stresses. We find that the procurement of diamond windows cut to precise dimensions is particularly expensive, and the above design permits appreciable variations in the window dimensions away from their nominal values. For example, one of VOL. 41, NO. 4, APRIL 1969
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Figure 2. Curve ( a ) apparent absorbance of diamond-windowed cell containing a transparent molten fluoride, LiF-BeF2 (66-34 mol %) at 550 "C and referenced against a neutral density screen with an absorbance of about one. Curve (b), absorption spectrum of a dilute solution of NiF2in the above LiF-BeF2 melt at 550 "C The absorbance of cell plus solvent has been subtracted from that of the solution to give the solute absorbance the plates we are now using is 5.0 X 4.3 X 1.3 mm as opposed to the design dimensions of 5.0 X 5.0 X 1.0 mm. A metal cap, not shown in the figures, welded co-axially to a %-inch diameter support rod, is slipped over the top of the graphite body and held in place by a metal pin. This pin passes through small holes in the cap and body. (The latter holes will be seen in Figure 1.) The support rod is used to suspend the cell in the inert-atmosphere chamber. It is necessary to enclose this cell in a nonoxidizing atmosphere to prevent atmospheric reaction with the cell and windows at high temperatures, and generally, it is also necessary to control the composition of the gas phase to prevent reaction with the melt. For this purpose we have used an inert-atmosphere furnace described previously (5)for use with captive liquid cells. This furnace fits the cell compartment of a Cary Model 14 spectrophotometer and can easily be modified to fit a Cary Model 14H spectrophotometer for service at high temperature. The diamonds used in our work are Type IIa because they have particularly good transmission characteristics in the ultraviolet. Type I diamonds, which are more common than Type IIa, are satisfactory in the near infrared and visible regions but absorb in the ultraviolet. Therefore it might be economically advantageous to use Type I diamonds for measurements restricted to these regions. Further information on the optical properties of diamonds is reported in the book edited by Berman (6). Although cost is a seriously limiting factor in the use of diamond windows, it is not as excessive as often supposed, and we recommend that costs be investigated before rejecting the use of diamonds on this basis. Particularly if routine work is planned, the reusability of diamond, compared with less corrosion resistant materials, will prove an economic saving. Procedure. The graphite pieces of the cell are cleaned in an ultrasonic bath to remove loose graphite dust and then heated in a vacuum at 800 "C to drive off adsorbed gaseous impuri(5) J. P. Young, Inorg. Chem., 6, 1486 (1967). (6) R. Berman, ed., Physical Properties of Diamonds, Clarendon Press, Oxford, 1965, p 295.
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Figure 3. Absorption spectrum of a dilute solution of UFI in molten LiF-BeF2 (66-34 mol %) at 550 "C The absorbance of the cell plus solvent has been subtracted from that of the solution to give the solute absorbance ties. The cell is assembled in a helium-filled drybox containing about 1 ppm by volume of water vapor, and loaded with approximately two grams of prepared salt in the form of a few large pieces. For this work it has been found convenient to purify and mix the solutions in metal containers. These are then cooled to room temperature and the solids kept in an inert atmosphere until needed. The loaded cell is fitted with the holder rod and placed in a metal transfer container where it is sealed in an inert atmosphere during transfer to the spectrometer furnace. The transfer and melting of the sample has been previously described (5). After a spectrum of the molten solution has been recorded, the furnace cell assembly is inverted to drain the salt from the window region into the top portion of the cell. The furnace is cooled to room temperature and the solid piece of salt easily picked out. Because of the nonwetting characteristics of most fluorides in contact with graphite, no residual salt remains in the window region. Upon removal of the nut and sleeve, the retainers and diamonds literally fall away from the cell body so that any piece can be cleaned separately in preparation for another spectrum. Performance. We follow a practice, common in molten salt spectrophotometry, of measuring the absorbances of the cell plus solvent and the cell plus solution separately as functions of wave length against an air reference. The absorbance of the solute is determined by subtracting these absorbance curves. The absorbance of a diamond-windowed cell containing a molten LiF-BeF2 (66-34 mol %) mixture (referred to here as L2B) was about one absorbance unit in the near-infrared region when measured against air with a Cary Model 14 spectrophotometer. A large fraction of this apparent absorbance is due to beam clipping and is not a serious problem because of the high absorbance measuring capabilities of the Model 14. In practice, most of this constant background absorption was compensated for by attenuating the reference beam with a neutral density screen. Figure 2 shows the complete absorbance curve for L2B in the cell at 550 "C as measured with an attenuated reference beam. The L2Bsolvent is transparent in the wavelength range investigated (7) so that the gradually rising absorption and eventual effective cut off near 240 mp is due to absorption by the diamond windows. (7) J. P. Young in "Characterization and Analysis in Molten Salts," Gleb Mamantov, Ed., Marcel Decker, New York, N.Y., 1969.
To illustrate the capabilities of the method, we show typical spectra in Figures 2 and 3. Figure 2 shows the spectrum of pure LeB plus cell (discussed above) and the solute spectrum of a dilute solution of NiF2 in L2B at 550 "C. This spectrum is similar to that observed in windowless containers (8). Figure 3 shows the solute spectrum of a dilute solution of UF4 in L2B at 550 "C. The weak band at 2.03 p in this spectrum of U(1V) was not observed in previous measurements with a windowless cell ( 5 ) because it could not be distinguished from a gradual variation in the baseline. The existence of such a band has been predicted (9, IO) and its observation with
the diamond-windowed cell helps confirm the value of this device.
(8) J. P. Young, Inorg. Chem., in press. (9) D. Cohen and W. T. Carnal], J . Phys. Chem., 64, 1933 (1960). (10) J. G. Conway, J. Chem. Phys., 31, 1002 (1959).
RECEIVED for review October 24, 1968. Accepted January 16, 1969. Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.
ACKNOWLEDGMENT
We gratefully acknowledge the assistance of F. Davis, W. Longaker, and D. Allen of the Field Engineering Staff in helping with cell design problems and S. M. Horszowski of the Diamond Research Laboratory, Johannesburg, South Africa who supplied the diamonds used here. We also wish to thank G. G. Cocks and C. M. Schwartz for their comments and advice.
Technique for Obtaining Absorption Spectra of lnsolu ble Metal Chelate Polymers P. S. Shew and Quintus Fernando Department of Chemistry, Uniaersity of Arizona, Tucson, Ariz. 85721
MANYMETAL CHELATES that are obtained from dithio-oxamide, (CSNH&, (rubeanic acid) and certain transition metal ions are polymeric and are potentially useful for a variety of purposes. One of the more important uses of rubeanic acid and its derivatives is the detection and determination of metal ions in low concentrations. The electronic spectra of these polymeric metal chelates provide analytically useful information, but cannot be readily obtained. Vaeck ( I ) measured the reflectance spectrum of nickel rubeanate adsorbed on paper, and Jacobs and Yoe ( 2 ) in their studies on the composition of nickel rubeanate used gum acacia as a protective colloid to prevent the coagulation of the polymeric nickel chelate. A simple method of obtaining the absorption spectra of polymeric metal chelates is proposed and its application to the study of the absorption spectrum of nickel rubeanate is described. The method consists of the electrodeposition of a film of the nickel chelate of rubeanic acid from an organic solvent on the surface of an electrode made of conducting glass which is transparent in the visible region of the spectrum. The conditions under which the film is electrodeposited can be used to control the thickness of the film. The absorption spectrum of the metal chelate film is recorded against air, and corrected for the absorbance of the conducting glass electrode. EXPERIMENTAL Reagents. Rubeanic acid was obtained from Eastman Organic Chemicals, Rochester, N.Y., and nickel perchlorate from G. F. Smith Chemical Company, Columbus, Ohio. All other compounds used in this work were of reagent grade purity. Apparatus. A dc power supply, capable of supplying a variable voltage up to 500 V at 100 mA was used for the electrodeposition. The electrodes consisted of a platinum anode and a conducting glass electrode, (No. 7740), manu( 1 ) s. V . Vaeck, Anal. Chim. Acta, 10, 48 (1954). (2) W. D. Jacobs and J. H. Yoe, ibid. 20, 332 (1959).
factured by Corning Glass Works, Corning, N.Y. A metal oxide film was permanently bonded by the manufacturer to one side of the glass, and it transmitted approximately 75% of the light in the visible region of the spectrum. All absorption spectra in the visible region were obtained with a Cary Model 14 recording spectrophotometer. The infrared spectra in KBr were recorded with a Perkin-Elmer 337B grating spectrophotometer. Method. The electrodeposition was carried out in a watercooled double-walled glass vessel containing an isopropanol solution of the metal chelate which was prepared by mixing 2 x lO-4M isopropanol solutions of nickel perchlorate and rubeanic acid in a 1 : 2.5 ratio. The total volume of the metal chelate solution was made up to 125 ml with isopropanol. The conducting glass electrode was rinsed with 2M nitric acid and washed thoroughly with ethanol. Before the electrodeposition was carried out, the absorption spectrum of the electrode OS. air was recorded. The platinum anode and the conducting glass cathode were placed in the metal chelate solution, the distance between the two electrodes was adjusted to obtain an appropriate current value, and electrodeposition was commenced. The solution was stirred by means of a magnetic stirrer throughout the course of the electrodeposition which usually lasted for about half an hour to obtain a sufficient quantity of an adherent film of a purple-blue nickel rubeanate. The absorption spectrum of the metal chelate film was recorded cs. air and corrected for the absorbance of the conducting glass. RESULTS AND DISCUSSION
The manner in which the following parameters affected the nature of the deposited metal chelate was investigated: voltage, current, time of deposition, concentration of the metal chelate, acidity of the medium, surface active agents, and water added to the isopropanol medium. In all the experiments, the initial current was 1 mA and the voltage was varied from 20 to 300 V. The rate of formation of the deposit increased with increasing voltage, but the current decreased with time as the thickness of the deposit increased. The presence of small amounts of water (up to VOL. 41, NO. 4, APRIL 1969
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