I AIDS FOR ANALYTICAL CHEMISTS
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Removal of Water from Weathered Petroleum Mark Ahmadjian, Carl D. Baer, and Chris W. Brown' Department of Chemistry, University of Rhode Island, Kingston, R.I. 0288 1
Virginia M. Westervelt, Douglas F. Grant, and Alan P. Bentz U S . Coast Guard RAD Center, Groton, Conn. 06340
One of the major difficulties in identifying weathered petroleum is the presence of occluded water in the samples. This is especially true for infrared analysis, since water distorts the fingerprint region. Previously ( I ) , we explored the possibility of removing water from oil by collecting spill samples on A1 foil; water is repelled by the foil, whereas oil adheres to it. Spectra can be measured without removing oil from the foil by placing the foil in a specular reflection attachment for an infrared spectrometer in such a manner that the light beam passes through the oil and is reflected back by the shiny surface of the foil. However, the usual transmission method of obtaining infrared spectra appears to give the most reliable spectra in the shortest period of time. Thus, we have explored other techniques for removing water from oil samples. Initially, we tried to remove oil from an oil-water emulsion by dissolving the oil in a suitable solvent such as CHC13, CS2, or pentane. After centrifuging, the bulk of the water was removed, and the remaining water was eliminated by adding anhydrous MgS04 and centrifuging. This treatment successfully removed the water; however, it was then necessary to remove the solvent, since CHC13, CS2, and pentane have bands in the fingerprint region (7001200 cm-I). We found that the only adequate means to remove the solvent was to heat a thin film of sample to -70 OC. However, many of the light components of the petroleum sample were lost by heating, thus affecting the spectrum. Recently, we found a very simple solution which not only solves the water problem, but also eliminates the use of solvent. We centrifuge samples of weathered oils a t 35-40 OC for 0.5 to 2 hr depending upon the viscosity of the oil; higher temperature and longer times are required for heavier oils. The bulk water is then removed with a syringe, a few grains of annhydrous MgS04 (-0.1 g/ml of oil) are thoroughly mixed with the oil (e.g., a S/P Delux Mixer Model S-8220 can be used), and the sample is centrifuged for another 0.5 to 2 hr. An example of this treatment is shown by the spectra in Figure 1. The bulk water was removed from sample b by centrifuging a t 35 "C; however, the remaining water distorts the fingerprint region between 700 and 900 cm-I. After treatment with MgS04 and centrifuging a t 35 "C, spectrum c was obtained; the close match with spectrum a , the unweathered oil, is obvious. The sample used in this demonstration is a very heavy crude oil. We chose this sample because the problem of removing water from weathered oil is most acute for heavy oils. T o heat samples while centrifuging, we have two centrifuges in a large cabinet, which is heated by nicrome wire (toaster elements) strung across the center. One centrifuge is used for light samples requiring less treatment and the other for heavy samples requiring long centrifuging times. 628
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
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Figure 1. Infrared spectra of crude oil (a) unweathered oil, (6)weathered oil after removing bulk water (note distortion of the 700-900 cm-' region), and (c)weathered oil after removing all water
The samples are centrifuged a t 3700 rpm. The problem of removing water from oil appears to be related to the viscosity (or pour point) of the oil, and to the conditions which caused the water to be occluded in the oil. An example of the latter problem is oil that is removed from wells by high presure steam. In this case, much higher temperatures must be used to remove the water during the centrifuging process. This can be accomplished by placing the tube containing the oil in warm water prior to centrifuging, and by also having warm water in the centrifuge receptacle. This approach permits the use of much higher temperatures, particularly for heavy oils, without endangering the centrifuge motor. Since the entire centrifuge does not have to be heated, this technique is more useful for field investigations. We have found that this method is effective in removing water from all but exceptional samples of the oils we have investigated (-300 samples), and it is certainly more desirable than solvent and high temperature treatments.
LITERATURE CITED (1) C. W. Brown, P. F. Lynch, and M. Ahmadjian, Anal. Chern., 46, 183 (1974).
RECEIVEDfor review July 21, 1975. Accepted October 13, 1975. This research was supported by a U S . Coast Guard Contract (DOT-CG-81-74-1099). The opinions or assertions contained herein are the private ones of the writers and are not t o be construed as official or reflecting the views of the Commandant or the Coast Guard a t large.
