Molten Sulfur as a Solvent in Infrared Spectrometry
T. K. Wiewiorowski, R. F. Matson, wo
and C. T. Hodges, Freeport Sulphur Co., Belle Chase, La.
recent papers suggest that orthorhombic sulfur cryst'als can serve as window material for infrared equipment. We wish to report that molten elemental sulfur may be considered as a useful solvent in infrared spectrometry. INDEPENDENT
EXPERIMENTAL
The use of molten sulfur as a spectrometric solvent poses an experimental Problem which originates from the necessity of employing temperatures above 119' C. (the melting point of sulfur) during sample preparation and Rhile the spectrum is scanned. A simple cell was constructed to handle molten sulfur samples. The cell consists of two sodium chloride windows (25.2 mm. diameter X 5 mm. thick), a spacer, the thickness of which determines the light path of the cell, two silicone rubber 0 rings (1 inch 0.d. x '18 inch) and two machined shown in Figure '* Depending on the shape of the spacer, the cell may be used for single samples or in a continuous on-stream mode, Each of the blocks is drilled to accommoinch diameter date a 3 inch long x Watlow Firerod Heater. The sample temperature may be maintained a t any desired level between 120' and 300' c. The cell is equipped with a plate-which fits into most commercially spectrophotometers. hIolten sulfur purified by the method of Bacon and Fanelli (1) was employed as a solvent. All spectra were obtained on a Perkin Elmer 221 G Infrared Spectrophotometer. DISCUSSION
The infrared spectrum of molten sulfur purified by the method of Bacon and Fanelli (1) is shown in Figure 2, curve -1. The spectrum shows no absorption peaks over a wide range of wave numbers, and thus the primary requirement, of a spectrometric solvent is
satisfied. The absorption free range extends from 1400 to 4000 cm.-' I n the 950 to 1400 cm.-l range only one very weak overtone band apLears which can be easily compensated by using a sulfur blank in the reference beam. Molten sulfur may serve as a solvent for a large number of organic as well as inorganic compounds. T o illustrate this fact, the spectJra of tetrahydronaphthalene and hydrogen sulfide dissolved in molten sulfur are shown in Figure 2, curves B and C, respectively. There are several areas where the use of molten sulfur as a spectrometric solvent appears to be immediately applicable. T o those in sulfur chemistry, the technique described here provides a useful means of following numerous sulfur reactions. For example, currently under study in this laboratory are the kinetics and mechanism of dehydrogenation of tetrahydronaphthalene dissolved in molten sulfur, Many other sulfur reactions could be investigated by this thus allowing researchers to gain some insight into the various still unknown aspects of sulfur chemistry. For the analytical chemist, the technique described here will provide a new approach to rapid sulfur analysis. SO far, it has been utilized in this laboratory for the determination of trace amounts of various impurities dissolved in sulfur and also for on-stream monitoring of organic contaminants Present in Frasch sulfur ( 4 ) . Other possible applications may appear to those facing various problems connected with sulfur analysis. For example, research chemists requiring sulfur of extremely high purity may utilize this technique to determine trace amounts of hydrocarbons which are usually present in sulfur. Curve D of Figure 2 represents a spectrum obtained on a sample of laboratory grade high purity sulfur. The intensity of the peak at 2920 cm.-' indicates the
SODIUM CHLORIDE WlNDW
FREQUENCY (cu-') ,oo
SXK)
3ooo 2500 2000
1500
lo00
i
'
3500 3000 2500 2000 1500
1000
Figure 2. A. 8.
C.
D.
Spectrum of sulfur purifled by the method of Bacon and Fanelli ( 1 ) . light path-1.56 mm. Air reference Spectrum of 0.06 M tetrahydronaphthalene dissolved in molten sulfur. Light path-1.56 mm. Molten sulfur in reference beam Spectrum of hydrogen sulfide dissolved in mm. Molmolten sulfur. Light path-6.35 ten sulfur reference Spectrum of laboratory grade high purity sulfur. Light path-20 mm. Air reference
presence of 0.0016% carbon as hydrocarbons. We believe this technique to be the most sensitive and most reliable method known for detecting trace amounts of hydrocarbons, one of the commonest impurities of sulfur. With a light path of 10 cm., a limit of detection of 1 p.p.m. hydrocarbons in sulfur can be achieved. I n summary, molten sulfur may be considered as a useful solvent for infrared spectrometry. Compared to other solvents, it offers certain unique and potentially advantageous propIt is suitable for work a t erties. elevated temperatures ( 12O0-3OO0C.), free of absorption bands over a very wide spectral range (1400 to 4000 cm.-9 and sufficiently transparent in this range to be used in long path (10 cm.) cells. LITERATURE CITED
SUPPORTINQ
(1) Bacon, R. F., Fanelli, R., Ind. Eng. Chem. 34, 1043 (1942). CHLORIDE WINDOW
Figure 1 .
1080
ANALYTICAL CHEMISTRY
Infrared cell for molten sulfur
(2) Bdaban, A. T., Ileana Stanoin, Rev. Chim. Acad. Rep. Populaire. Roumaine 8(2), 197-8 (1963). C.A. 61, 5088e, (1964). (3) MacNeil, C., J . O p t . SOC. Am. 53, 398 (1963). (4) Matson, R. F., Wiewiorowski, T. K., Schof, D. E., Jr., Griffin, R. A., Chem. Eng. Progr, in press.
