filtrate. Results are shown in Table 111. There is an appreciable difference in solubility between the two salts in the reaction solution, passing carbon dioxide through the solution does not greatly alter the solubility of the tetramethylammonium iodide, and a solubility correction will be necessary when the two salts are to be separated and determined individually. The correction could be relatively small when the concentration of the trimethylethyl salt is high in a solution of the two salts because the R,, of tetramethyl salt would be exceeded more readily. The possibility exists also of reducing the concentration of both salts in the reaction mixture by the addition of nheptane. The solubility relationships are somewhat more favorable for a good separation of the two salts when nitrobenzene rather than ethyl alcohol (11) is used as a solvent, because the tetramethyl salt in nitrobenzene has less than half of the solubility shown in ethyl alcohol, while the trimethylethyl salt is sufficiently soluble in either solvent.
Table 111. Solubility of Trimethylethylammonium and Tetramethylammonium Iodide a t 25.00’ C.
Compound (CHs)8I (CHa),NI (CHa)N (CHahNI
(CH3)rXI
C2H6(CH3)3NI C2Hs(CHB)aNI CIH,( CH3)BNI
CzHs(CH3)aNI
ACKNOWLEDGMENT
Solubility, Mg./Ml.
Solvent Xitrobenzene O.520N (CH3)aN in nitrobenzene 1.014N (CH3)aN in nitrobenzene 1.010N (CH3)3N in nitrobenzene satd. with COn 1.01ON (CH3)aN in nitrobenzene n-heptane, 2 : l Nitrobenzene 1.010N (CH3)sN in nitrobenzene 1.010N (CH3)3N in nitrobenzene n-heptane, 2: 1 1.010hr(CH3)3K in nitrobenzene n-heptane, 4: 1
0.170 0.155 0.140 0.176
0.045
9.541 7.120 0.970
2.083
The authors are indebted t o John L.
Engelke and Peter R. Gray for doing part of the kinetic determinations. LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, Washington, D. C., ‘[Official Methods of Analysis of Association of Official Agricultural Chemists,” 7th ed., p. 74.4, 1960. _._.
(2) Clark, E. P., J . Assoc. Oj%. Agr. Chemists 22, 622 (1930). (3) Cooke, L. M., Hibbert, Harold, IND. ENG. CHEM., ANAL. ED. 15, 24-5 (4)(%?:& Adalbert, Zbid., 11, 174 (1939). (5) Ffost, A. A., Pearson, R. G., “Kinetics and Mechanism,” pp. 28-9, Wiley, New York, 1953. (6) Kuster, W., Maag, W., 2. physiol. Chem. Hoppe-Seylers 127, 190-5 (1923). (7) Phillips, M., Goss, M., J . Assoc. Oflc.Agr. Chemists 20, 292-7 (1937). (8) Steyermark, AI, ANAL. CHEM. 20, 368-70 (1948). (9) Steyermark, Al, Alber, H. K., Aluise, V. A., Huffman, E. W. D., Jolley, E. L., Kuck, J. A., Moran, J. J., Ogg, C. L.,Zbid., 28, 112 (1956). (10) Rilstatter, R., Utzinger, M., Ann. 382, 148-50 (1911). (11) Wilson, J. B., J. Assoc. Ofic.Agr. Chemists 18, 477 (1935).
RECEIVED for review November 3, 1958. Accepted March 10, 1959. Presented in part before Division of Analytical Chemistry, 119th Meeting, ACS, Cleveland, Ohio, April 1951.
Microgas Cell for Infrared Spectroscopy c
J. U. WHITE and N. L. ALPERT’ The White Development Corp., Stomford, Conn.
W. M. WARD and W. S. GALLAWAY Beckman Instrumenfs, Inc., Fullerton, Calif.
b A microgas cell has been built for measuring the infrared spectra of very small gas samples. Its path length is 1 meter, its volume is 22 ml., and the transmittance through it and the beam condenser used with it is 3070. It has been used in conjunction with a Beckman IR-5 infrared spectrophotometer to observe the spectra of gas chromatograph fractions less than 0.05% of the total charge.
I
analysis of liquid samples by gas chromatography, it is often difficult to identify the different fractions as they emerge from the column. Numerous means have been used, such as collecting the cuts and measuring their absorption spectra. sampling the effluent continuously with an ultraN THE
Present address, Perkin-Elmer Corp., Norwalk, Conn.
violet or mass spectrometer, or making chemical tests on collected cuts. Unfortunately, most infrared spectra are too weak to be measured directly as they emerge from the column. Tmo milliliters of gas charged to a column just gives a strong enough infrared spectrum in a standard 10-cm. gas cell if it is a single component. To measure the spectra of minor components, the ratio of path length to cell volume must be increased in proportion to dilution of the components. A multiple traversal absorption cell, built for this purpose, uses a combination of Khite’s (3) mirror system with the three-lens beam condenser described recently (5). Its 100-cm. path in 22 nil. of volume gives it a length-volume ratio of 4.5 em. per ml., an increase of 50 to 100 times that of most standard 10-cm. infrared absorption cells. The optical arrangement of the system ifi shown in Figure 1 as it is used
in the Beckman IR-5 and IR-4 spectrophotometers. It may also be fitted in the IR-6 spectrometer by reversing it end for end. The two lenses on the left, L1 and L2, form a five-times-reduced image of the slit image, S. It is reflected into the cell through the window, W , by the first surface of the prism, P. After leaving the cell, the image of the slit is reflected back into its original path by the second surface of the prism and remagnified to its original size by the third lens, L3. The beam condenser has been modified from the one described previously to increase the free space around the reduced image and make room for the reflecting prism. Inside the cell the light passes close t o the edges of the single mirror, A l l . to the first of the double mirrors, M2. The figure shows the simplest arrangement of the mirrors, in which the light is reflected from M2 to M1, then to M 3 , and finally out past the other edge of VOL. 31, NO. 7,
JULY 1959
1267
Figure 1. condenser
Optical layout of microgar cell and beam 11,12,13. Lenses of beom condenser Ml,MZ.M3. Mirrors of gascell P. Diverting prim 5.
W.
Slit image Window of gas cell
.M1, giving four traversals of the space between the mirrors. By readjusting the double mirrors, the number of traversals may be increased to 24 without serious light losses. The cell is mounted in the beam condenser on three adjustable screw feet. Figure 2 shows how it fits between the two righehand lenses. The mounting screws are accessible through the three holes in the top. They extend through the cell housing to the base of the beam condenser, where two of them rest in an aligning groove and the third in a slow motion worm operated by the knob in the bottom right corner. L1 is a t the left, L2 in the knurled focusing mount in the middle, and L3 behind the cell. The sample inlet tube is the small one in the lower right, where it connects to the small end of the cell. The outlet is the large tube on the left. The other two tubes are the ends of a cooling loop soldered to the outer jacket for circulating air or water. Inside the jacket a removable heater winding surrounds the main body of the cell containing the sample and the mirrors. The double mirrors are at the left, just ahead of the exhaust tube, adjustably mounted on six screws in a removable end plat?. The single mirror and window are at the right near the middle lens. The cell and end plate are clamped together on Teflon gaskets by resiliently mounted screws. The cell is built to be heated to 250" C., where it is limited by the Teflon gaskets. The body is Monel, fastened with stainless steel screws. The glass mirrors should be satisfactory to 400" C. if gold coated; the limit of the aluminized mirrors in different atmospheres has not yet been ascertained. Thirtpwatt input heats it to a steady t,emperature of about 145" C. in an hour with 2.5 cubic feet per minute of air passing through the cooling tube. The temperature of the outer jacket is then around 45' C. The performance of the cell and beam condenser has been tested in an IR-6 spectrometer (4). At 13.5 microns the beam condenser had a transmittance
1268
ANALYTICAL CHEMISTRY
Figure 2.
o1 .I.
Microgas cell in beam condenser
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3r--
WAVELENGTH IN MICRONS
Figure 3.
Variation of transmittance with wave length
Io of instrument alone, lo with
beam condenser, and lo with beam condenser and 1-meter gar EOI Middle. Air and 0.1 6 ml. of propane at I-meter poth Bottom. 0.1 6 ml. methane at 1-meter path and lome after Rushing with two cell volumes of alr
Top.
...
IO0 80 60 40 I R SPECTRUM OF PEAK
A
20
--
0 2
5
10
WAVE
16
LENGTH IN MICRONS
Figure 4. Gas chromatogram of 0.025 ml. of absolute ethyl alcohol and infrared spectrum of peak A 90
80
10
60
50
Figure 5. Gas chromatogram of impure toluene with infrared spectra of first three peaks A. B. C.
Acetone Saturated Benzene
hydrocarbon
40
30
20
LOO
80
60 40
WAVE
LENGTH IN
MICRONS
of 60%. II-hen the cell was put in, the six different path length settings from 16.7 t o 100 cm. were found t o have the transmittances listed in Table I. No extraneous transmittance was observed between the different paths. The variation of transmittance u ith wave length is shown in the uppel curve5 of Figure 3. I n the lower two spectra the amplification of the spectrometer was increased by a factor of 3 to make up for the losses at the I-meter path length. The middle spcctruni is that of 0.16 ml. of propane blonn into the cell with air. The 3.4-micron C-H band is almost completely black, the 6.8-, 7.25-, and 13.34-micron bands are well defined, and other neaker bands are detectable. Air was used for the Io curve. The lower spectrum is that of 0.16 ml. of methane, where the bands around 3.3 and 7.7 microns are readily measurable. The comparison spectrum IS the amount of gas remaining after the cell was purged with twice its own volume of air. The residual &branch of the band a t 7.67 microns is still visible with a tenth of its former absorbance. The amount of methane present was then about 10 y. From these and other measurements it is calculated that flushing the cell with its own volume of gas removes approximately 70% of the original contents. The microgas cell and its beam condenser have been used in a Beckman IR-5 spectrophotometer (5) in conjunction with a Beckman model GC-2 gas chromatograph ( I ) by flowing the effluent from the column directly through the cell following the thermal conductivity gage. K h e n the gage indicated that a n unknon n peak had passed and should be in the cell, the flow to the column was stopped and the spectrum was measured. Figure 4 shows a n application of this procedure to a n unknown impurity that was detected in a previous chromatogram of absolute ethyl alcohol. The upper curve is the chromatogram of a 0.025ml. sample charged to the 6-foot column packed \f-ith Carbonax 1000 and operated a t 66" C. The flow to the column was stopped when the sinal1 peak before the main one was in the cell. The spectrometer was started and the spectrum shown in the bottorn half of the figure was recorded. The stiong P , &, and R branches of the 15-micron benzene band are readilj recognized, as are the other strong bands a t 3.3, 6.7, and 9.6 microns. In addition, there are extraneous band;. a t 5.7 and 8.4 microns. They have the wave lengths of strong bands in the spectrum of ethyl formate. As this should come out of the coluinn between benzene and alcohol, nhere the chromatogram does not quite drop to zero, it is believed to be present. The amount of benzene is calculated to be VOL. 31, NO. 7,JULY 1959
1269
Table 1. Optical Performance of Microgas Cell a t 13.5 Microns
Traversals
Path Length, Cm.
4 8 12 16 20 24
17 33 50 67 83 100
Transmittance, % Beam condenser Gas and gas cell cell 82 78 73 68 59 50
52 48 45 42 37 30
0.5%; that of ethyl formate, 0.1%. An extraneous band, at 7.2 microns, is
caused by a n impurity in one of the potassium bromide lenses. The microgas cell and chromatograph have also been used in a n IR-4 spectrophotometer (b), where the higher performance of the larger instrument was used to compensate for losses in the optical system. Figure 5 shows at t,he top the chromatogram of a 0.07m!. sample of impure toluene run on a 6-foot column packed with diethyl
phthalate, operated with a flow rate of 50 ml. per minute at 125' C. I n order below the chromatogram are the spectra of the first three peaks, A , B, and C, following the air peak, and identified as acetone, saturated hydrocarbon, and benzene. Their concentrations were measured from the gas chromatogram as 0.04, 0.26, and 1.3%. The major peak, representing a toluene concentration of 97.8%, was recorded at 1/100 of the sensitivity used for the other peaks. To record the spectra of all the fractions continuously as they emerge from the column, the flow rate would have t o be reduced to allow one scanning time per fraction. Then with a column that isolates 100 fractions and with the 3-minute scan and 30-inch chart of the IR-4, 5 hours and 250 feet of chart would be required. More practical procedures are to slow down the column and record spectra only for the slowest fractions where identifications are more difficult or to record only when an unknown peak appears on the chromatogram. Further
work is contemplated on this and on the use of delay lines to store the samples while the chromatograms are being examined. ACKNOWLEDGMENT
The authors thank James E. Stewart for making the measurements with the gas chromatograph. LITERATURE CITED
( 1 ) Carle, D. W., Donner, W., Pittsburgh
Conference on Analytical Chemistry and Applied Spectroscopy, March 1957. (2) Ward, W. M., Pittsburgh Conference on A4nalytical Chemistry and Applied Spectroscopy, March 1956. (3) White, J. U.,J. Opt. Soc. Am. 32, 285 (1942). (4) White, J. U., Alpert, N. L., Neiner, Seymour, Rev. Sci. Znstr. 29, 511 (1958). (5) White, J. U., Weiner, Seymour, Alpert, S . L., Ward, W. M., ANAL. CHEM.30, 1694 (1958).
RECEIVED for review November 28, 1958. Accepted March 13, 1959. Pittsburgh Conference on Analytical Chemistry and .4pplied Spectroscopy, March 1958.
Semimicrodetermination of Combined Tantalum and Niobium with Selenous Acid F. S. GRlMALDl and MARIAN SCHNEPFE U. S. Geological Survey, Washington 25, D. C.
b Tantalum and niobium are separated and determined gravimetrically by precipitation with selenous acid from highly acidic solutions in the absence of complexing agents. Hydrogen peroxide is used in the preparation of the solution and later catalytically destroyed during digestion of the precipitate. From 0.2 to 30 mg., separately or in mixtures, of niobium or tantalum pentoxide can b e separated from mixtures containing 100 mg. each of the oxides of scandium, yttrium, cerium, vanadium, molybdenum, iron, aluminum, tin, lead, and bismuth with a single precipitation; and from 30 mg. of titanium dioxide, and 50 mg. each of the oxides of antimony and thorium, when present separately, with three precipitations. At least 50 mg. of uranium(V1) oxide can b e separated with a single precipitation when present alone; otherwise, three precipitations may b e needed. Zirconium does not interfere when the tantalum and niobium contents of the sample are small, but in general, zirconium as well as tungsten interfere. The method is applied to the determination of the earth acids in tantaloniobate ores. 1270
ANALYTICAL CHEMISTRY
A
separation of tantalum from niobium, titanium, and some other elements by selenous acid from oxalate-tartrate medium has been described (3). This paper continues study on the use of selenous acid as a precipitant for niobium and tantalum on a semimicro scale and on the separation of these elements from titanium and other elements. Alimarin and Stepanyuk (1) originally proposed selenous acid as a reagent for the separation of tantalum and niobium from titanium. I n their procedure, precipitation is made with 20 ml. of 10% selenous acid from a total volume of 200 ml. containing also 2 grams of tartaric acid, 50 ml. of concentrated hydrochloric acid, and 4 grams of potassium pyrosulfate. The precipitate is filtered hot after 30 minutes' digestion and washed with hot 10% hydrochloric acid. I n this laboratory, this procedure gave low recoveries of niobium from pure niobium solutions. As examples, in the range from 0.6 to 2.5 mg. of niobium pentoxide, less than 0.5 mg. of niobium pentoxide was recovered; in the range from 6 to 30 mg. of niobium pentoxide, the results were low by from 1.2 to 1.5 SELECTIVE
mg. of niobium pentoxide. Several factors which contributed to low recoveries were the solubilizing effects of elevated temperatures, and excessive amounts of pyrosulfate, tartaric acid, and wash solution. The procedure of Alimarin and Stepanyuk was modified by making the precipitation from a volume of 150 ml. containing 35 ml. of hydrochloric acid, 2 grams of ammonium chloride, 1.5 of potassium pyrosulfate, and 1.5 of selenous acid. Ammonium nitrate mas substituted as a wash solution and hydrogen peroxide for tartaric acid in preparing the solution. Under these conditions, from 0.2 to 30 mg. of the oxides of niobium or tantalum or both can be quantitatively precipitated and separated from several elements including titanium. REAGENTS
Tantalum and niobium pentoxides were supplied by Fairmont Chemical Co., Inc. The purity of each dried product was better than 99.9% by spectrographic tests. Standard tantalum test solution, 1 ml. equal to 1.200 mg. of tantalum pentoxide: Fuse 0.6 gram of pure
