Thin Liquid Infrared Cell for Quantitative Studies Using Aqueous Solutions E. F. Rissmann General Technologies Corporation, 1821 Michael Faraday Drive, Reston, Va. 22070
OVERTHE PAST few decades, the field of infrared spectrometry has made significant advances. However, the problem of being able t o perform quantitative infrared studies of aqueous solutions has t o a large degree escaped solution, To date, water has been employed as a solvent in several infrared investigations, most of which, because of difficulties with the very thin cells of less than 10-micron thickness required, have been of essentially a qualitative nature (1-4). However, recent advances in cell technologies have made it possible to design single cells of a few microns in thickness which can be used for quantitative studies (5, 6). The methods of cell preparation, which consisted of deposition of a thin film of polymer on the outer edges of a flat salt plate by solution evaporation, have been useful only for preparation of single cells and are, essentially, of little use for construction of matched pairs of cells, as would be required, for some quantitative studies. More recently, work has been presented on a method of sulfate analysis in aqueous solution using cells of 3-micron thickness which had been prepared by vacuum evaporation of uniform thin silver films onto the outer edges of polished silver chloride plates (7). However, the approach taken in that study, while overcoming the difficulties involved in preparation of matched pairs of thin cells, possessed problems involving removal of trapped air bubbles formed on filling the cells. Removal of such air bubbles particularly in corners, in the past, has proved both troublesome and time consuming. Problems of erosion of the silver chloride windows with time were also encountered t o some degree. This paper presents some modifications in the above techniques, which are shown t o result in cells which are both usable for quantitative studies and free of several of the problems listed above. EXPERIMENTAL
Two designs of cells, one sealed and the other demountable, were used in these investigations. The demountable cells, as is shown in Figure 1, consisted of two optically flat Irtran 2 plates. On the outer portions of the bottom plate was deposited a 3-micron thick silver film which served as a cell spacer. The films were prepared by slow vacuum evaporation and the bottom plates used were always prepared in pairs, with film thicknesses being uniform and the same t o within +3%. Specifically for film preparation, the two windows to be coated and a test glass were mounted 120’ apart in a holder, which was positioned above the center of the evaporation boat. Previous tests with the type evaporation boat used and sample holding arrangement showed that when such a symmetrical holder was used, equal amounts, by weight, of material were (1) R. N. Jones and C. Sandorfy, “Infrared and Raman Spectra
Applications in Chemical Applications of Spectroscopy,” W. West, Ed., Interscience, New York, N. Y . , 1959, p 246. (2) F. C. Nachod and C. M. Martin, Appl. Spectrosc., 13, 45 (1959). (3) H. Sternglanz, ibid., 10, 77 (1956). (4) S . D. Kulbom and H. F. Smith, ANAL.CHEM., 35, 912 (1963). (5) W. K. Thompson, Tram. Faraday Soc., 61, 2635 (1965). (6) W. A. Senior and R. E. Vernall, J. Phys. Chem., 73,4242 (1969). (7) E. F. Rissmann and R. L. Larkin, ANAL.CHEM.,42, 1628 (1970).
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Top ZnS Plate
0.003rnm Film of Ag Deposited onto Outer Portions Only of Bottom
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deposited in all three positions. Calibrations, with respect t o the amounts of film deposited per unit time for the evaporation conditions used, were also conducted, Before evaporation, the center portions of the two windows were covered by a mask, as was the center portion of the test plate t o prevent deposition of material on these regions. The apparatus was then evacuated and evaporation was commenced. The evaporation was always carried out in a series of steps, because of a need for refilling the source boat. After each deposition, the test glass was removed from the equipment and the thickness of the film deposited during that portion of th? run was determined interferometrically using the 5460 A mercury line. For these measurements, eleven fringes corresponded t o a thickness of 3 microns and determinations, with the equipment available could be made t o about onefourth of a fringe (or an error of f3 %). Several of these evaporation steps were required to build up films of the desired 3-micron thickness. Between each of these steps, the two windows and test glass were alternated systematically in the three holder positions to reduce any errors due to possible evaporation asymmetries. Experiments using three test glasses and this general procedure, using both interferometric and weight change determination methods, showed that the amounts and thicknesses of film deposited of the 3 specimens was the same, for 3 microns of thickness, to within approximately 12z. It should be noted here that before film deposition, the windows were carefully polished to optical flatness and great care was taken to exclude any dust from surfaces of the materials to be coated. All of the above thin film preparation work for this study was performed by W. McMinn and Company, Beltsville, Md., according t o the specifications given above. The top plates of the cells used were also made of Irtran 2 material and were polished to optical flatness. These cells were filled by adding a drop of the solution of interest t o the bottom plate and then fitting the top plate over it. Here, the excess liquid was readily displaced with no formation of troublesome voids or bubbles, The Irtran windows used were sufficiently transparent under good room lighting to be able to see bubbles if such appeared. Other workers, using the thin polymer film technique, have found this approach gives both reproducible results and cells of a thickness essentially that of the film itself (5,6). Comparison of absorbance data for water obtained with our cells with previously published information further confirmed that the thickness of our assembled cells was close to 3 microns (5). It may be noted that the demountable cell described above does not appear t o be greatly different, except for the method of filling, from those used in earlier studies (7). The change to Irtran 2 (zinc sulfide) and n o filling ports does not seem novel, and others have pointed out that the use of metal leaf spacers or Irtran 2 or wedge shaped cells are impractical for thin cell use (5). The reasons for the impracticability of
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D
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. 4H~O/1oOcs
Figure 3. Calibration curves for aqueous solutions of calcium nitrate and nitrite 0.16
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Figure 2. Modified cell holder P = bottom plate C = heavy paper support F =foot WI = bottom cell window Wa = top cell window H = hole N = allenheadnut B = metal bar with tubing (0)attached by means of threaded seal (D) X = amalgamated lead washer
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metal foils or wedged cells are obvious. The problems with the use of Irtran 2 center around its greater hardness and the resulting difficulties with leakage due to a lack of compressibility. However, in our case, the thin silver film has been found to give good sealing without leakage problems and the use of Irtran 2 has made it possible to use the cells with aqueous EDTA solutions which would readily corrode other water resistant window materials such as the alkaline earth fluorides. The cell assembly was of the proper size that it could be used in conjunction with commercial cell holders. For analytical studies with these cells, one containing the solvent alone was used in the spectrometer reference beam and the other, containing the solution of interest, was placed in the sample beam. All spectra were recorded using a Beckman IR-10 infrared spectrometer. In some studies, sealed cells, filled by drawing liquid through the thin sample compartment by evacuation, were used. A diagram of one of these is shown in Figure 2. Here, the bottom cell plates containing the thin silver film were of the same type as those used for the demountable cells. The top plates for the sealed cells contained drilled holes for admission of sample. As can be seen in Figure 2, the base
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Figure 4. Calibration curves for calcium sulfite and sulfate in aqueous EDTA
plate has dimensions suitable for the cell holder in the spectrometer and a foot to enable it to stand upright when not in the holder. The window nearest the bottom plate contained the thin silver film. When in use, it rested on the plate but was cushioned by two pieces of heavy paper. The top window, which contained two 1/32-inchdiameter holes was posiANALYTICAL CHEMISTRY, VOL. 44, NO. 3, M A R C H 1972
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tioned on top of the first during assembly. The top window used was thinner than the bottom and was about 1.5 mm in thickness. This sandwich arrangement was held together tightly by two clamp bars in the center of which were 3/82-inchholes, into which were threaded metal tubes for attachment to external vacuum. Solvent tight seals were obtained by use of amalgamated lead washers between the top window and the bars and at the allen head screws. Even pressure on the windows could be achieved by adjustment of the four binding post nuts, with the pieces of heavy paper serving to relieve minor inequalities and strains. When in use, one of the tubes leading from the cell was immersed in the liquid to be drawn into the cell. The other was attached via a stopcock and glass joint to a vacuum system. Slow evacuation then enabled liquid to be drawn into and through the thin cell without leaving residual air bubbles in the cell. Closing of the stopcock followed by withdrawal of the tubing from the test solution then prevented drainage of liquid from the cell. Experiments performed with this device revealed it could readily be filled and emptied by evacuation and would remain leak free over an extended period. For quantitative studies, a matched pair of such cells were employed using the same approaches taken for the simpler demountable cells.
RESULTS AND DISCUSSION Both types of cells were used to obtain calibration curves of absorbance at specific frequencies us. concentration for solutions of calcium nitrate and nitrite in water and calcium sulfate, sulfite, nitrate, and nitrite in saturated aqueous tetrasodium EDTA solution. The curves for several aqueous solutions obtained with the demountable cells are shown in Figures 3 and 4. As can be seen, there is little scatter and good reproducibility. Similar quality data have been obtained with sealed cells. Data of this quality demonstrate the overall feasibility of using this method for conducting infrared studies for analyses of aqueous solutions. With these more resistant thin cells, it should be possible to develop more methods of analyses for several inorganic ions of the type which was recently given for sulfate in EDTA solutions (7). Such cells may also prove to be of use in applying the infrared method to studies of kinetics of reactions in aqueous solutions.
RECEIVED for review January 25, 1971. Accepted October 26, 1971. This work was performed pursuant to contract CPA 70-63 with the Air Pollution Control Office, Environmental Protection Agency. The mention of any tradenames in this paper does not imply endorsement by the Government of any specific product.
Improved Capillary Direct Current Cell Suitable for Conductometric Titrations Otto Hello School of Chemistry, Hobart Technical College, Hobart, Tasmania 7000, Australia
A STABLE AND SENSITIVE conductivity cell can be made by measuring current flow as limited by a small capillary tube. In such a cell, practically all the potential drop will be across the capillary and polarization effects will be reduced to a low level where it becomes practical to use direct current for conductivity measurements. As very small direct currents can be measured accurately, the sensitivity of the cell is well above that feasible with conventional alternating current methods. The generally accepted improved ac bridge method is accurate for conductivity measurements over 200 ohms ( I , 2 ) . Direct current methods using conventional conductivity cells are limited to resistances over 1 Mohm (3),or to halide solutions by using reversible Ag/Ag-halide electrodes ( 4 ) which has been improved by use of a special cell design (5). The cell described here provides a method more suitable for conductometric titrations than ac bridge techniques, but which provides stability and range comparable to that achieved by the more complex Bipolar Pulse Technique (6J A typical capillary-limited conductivity cell (Figure 1) was constructed from two 100-ml vessels (hard polyethylene centrifuge tubes) connected by 2 cm of 0.16-mm i.d. glass tubing, sealed into the vessels with epoxy resin. Each elec(1) F. Kohlrausch, W e d . An!?,69, 249 (1893). (2) G. Jones and R. Joseph, J . Amer. Clzem. SOC.,50, 1049 (1928). (3) R. M. Fuoss and C. A. Kraus, ibid.,55, 21 (1933). (4) A. R. Gordon et al., ihid., 75, 2855 (1953). ( 5 ) D. J. G. Ives and S. Swapoora, T r a m Faraday SOC.,49, 788
(1953). (6) D. E. Johnson and C. G. Enke, ANAL.CHEM., 42,329 (1970). 646
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
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Figure 1. Apparatus for dc capillary conductometry A 2-cm long 0.16-mm i.d. capillary between two 100-ml vessels. Electrodes 6 sq cm bright Pt mesh. Liquid levels over 5 cm apart
trode consisted of 6 sq cm of bright platinum mesh. By filling vessel B somewhat above the level of vessel A, the conductivity measurement is controlled primarily by the solution in vessel B, as only liquid from B will flow through the capillary. If the level difference is more than 5 cm and the voltage gradient across the capillary is more than 10 volts cm-l, polarity or level change has relatively little effect on the conductivity of the cell. Testing this cell with 0.1N KC1, the E us. Z graph (not shown) is a straight line from 5 to 100 volts (approximately 5 to 100 PA). Below 5 volts the current readings are unsteady, while above 100 volts the graph develops a positive deviation due to ohmic heating culminating in bubble formation at 700 volts (1.3 mA), where the current decreases rapidly to a low value.
