Determination of Dissolved Oxygen in Hydrocarbon Solvents Using Gas-Liquid Chromatography with Electron Capture Detection P. T. Ford Shell Research, Ltd., Thornton Research Center, P. 0. Box I , Chester, CHI 3SH, England
WHENSTUDYING the stability of fuels for supersonic aircraft, we needed to determine rapidly the content of dissolved oxygen in flowing hydrocarbon streams. Although many different methods for determining oxygen have been reported, few are applicable t o the measurement of dissolved oxygen in hydrocarbons. The most convenient instruments available are those based on polarography, such as the Beckman Models 777 and 778 oxygen analyzers. However, these would not meet our requirements which were for absolute oxygen determination in conditions ranging from ambient up to temperatures of 200 “C and pressures of 150 lb/inc h *. A method based on gas-liquid chromatography (GLC) has been reported ( I ) but is relatively slow and suffers from the disadvantage that the fuel sample needs to be transferred by syringe. We report here a G L C method which gives absolute values of oxygen in liquid or gas streams in a few seconds. The possibility of contamination is excluded by the use of a sampling valve and in the system under study the electron capture detector responds specifically and with high sensitivity to oxygen. EXPERIMENTAL
Apparatus. The apparatus is shown diagramatically in Figure 1. A sampling valve feeds the sample to a precolumn where the dissolved gases are separated from the hydrocarbon liquid. Provision is made for backflushing the latter from the precolumn. The dissolved gases are further separated o n a second column (molecular sieve) and the oxygen determined by means of an electron capture detector. The time from sample injection to tracing of the oxygen peak on the recorder can be as little as ten seconds. SAMPLING VALVF. The sampling valve originally used was a Loenco L.S.V-220 slide valve injector of ten microliters capacity. Although the sampling valve proved suitable for monitoring flowing kerosine streams under pressure, it gave poor reproducibility at ambient pressure. This was particularly noticeable with pure liquid aromatics when the sampling error could be greater than =k 10 %. The valve was constructed of a stainless steel slider containing a space for the sample and moving between two washers made of Teflon (Du Pont) in which were drilled supply holes of smaller diameter than that of the sample space. The poor reproducibility arose from poor wetting of the Teflon, particularly with liquids of high surface tension, so that the sample space was not completely filled. In more recent work we have used a Teflonktainless steel slide valve designed by our associates at the Koninklijke/ Shell-Laboratorium, Amsterdam, in which the channels are all of the same diameter thereby giving plug flow. and which has provided reproducibility better than A 1 with all liquids and gases examined. GAS-LIQUID CHROMATOGRAPHIC COLUMNS.The precolumn is a 6-inch X 3/ls-inch stainless steel tube packed with squalane on Celite. The second column, also 6-inch X 3/ls-in~hstainless steel, is packed with molecular sieve 5A.
(1) J. A . Petrocelli and D. H. Lichtenfels, ANAL.CHEW,31, 2017
(1959).
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Figure 1. GLCjelectron capture apparatus for determining oxygen in hydrocarbons Both columns are normally operated at room temperature. A two-way Schrader valve is fitted to allow the precolumn to be backflushed between determinations. Backflushing is essential for the detector loses sensitivity when contaminated, but if rapid repeat determinations are required we have found it possible to delay backflushing for several hours after the initial injection for most hydrocarbon liquids. DETECTOR. The detector, which is identical to that used previously in a pipeline interface apparatus by Adlard (2), is of the electron capture type and its construction is shown in Figure 2. The cell contains a tritium-on-titanium source. The low energy P-rays ionize the gas and the ionization current is sampled by means of a pulsed voltage on the electrodes. ELECTRONIC EQUIPMENT.The power supply for the electron capture cell is provided by a generator giving an 80-V pulse of 0.5-psec duration at 10 kc’sec. The ionization current is detected by means of a transistorized amplifier with a six-range output which is fed to a I-mV recorder. The circuit diagram for the amplifier and the pulse generator is similar to that given by Lovelock (3). CARRIER GAS. The carrier gas is a 9 0 z argon/lO% methane mixture which is available from the British Oxygen Co. It is essential that this gas should be free from electro-
(2) E. R. Adlard. A. J. Davies, and A . Evans. 7th Symposium of Gas Chromatography, Copenhagen. June 1968. (3) J. E. Lovelock, ANAL.CHEW, 33, 162 (1961). VOL. 41, NO. 2, FEBRUARY 1969
393
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negative contaminants, such as oxygen, for their presence leads t o a marked reduction in sensitivity. Other contaminants, such as water, are removed by passing the carrier gas through a guard tube containing 5A molecular sieve before feeding it to the apparatus proper. Procedure. The apparatus is easy t o operate but requires fairly frequent calibration (see below). After the supplies have been switched on, including the carrier gas at a flow rate of about 0.5 liter/hour, about half a n hour should be allowed before the first sample is taken. The sampling valve may be placed in a by-pass line or, more usually, in a branch line from the flowing hydrocarbon stream with the outlet from the sampling valve leading to waste. After adequate time has been allowed for flushing these lines the sampling valve may be actuated every 15 seconds or so. The oxygen content is determined from the peak height o n the recorder. Calibration. We have found it desirable to calibrate at the beginning and end of an eight-hour shift when the apparatus is in use, owing t o the possibility of contamination affecting the sensitivity of the electron capture cell. The 0-radiation from tritium is of very low energy and the ionization is reduced by material adsorbed o n the surface of the tritium foil. 300
300
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Figure 3. Calibration curves for GLC/electron capture apparatus A . Five ranges of sensitivity B. Minimum sensitivity range 394
ANALYTICAL CHEMISTRY
0 British Oxygen Co. bottled gas mixture
0 Gas mixtures made using soap-film flowmeters
0
Solvent Benzene Toluene Iso-octane Heptane
Table I. Solubility of Oxygen in Hydrocarbons a t One Atmosphere Oxygen Pressure Oxvaen solubility Other workers Ostwald coefficient Bunsen coefficient Ostwald coefficient Bunsen coefficient Ostwald coefficient ( p ) at 15 "C (a)at 15 "C (p) at 20 "C (CY) at 20 "C (p) at 25 "C 0.194 i 0.003 0.184 0.2186 ( 4 ) ... ... 0.226 Z!Z 0.004 0.214 ... ... 0.369 SL 0.003 0.350 ... 0.351 ( 5 ) 0.3725 (6) 0.360 i 0.002 0.341 ... ... ...
Calibration is most conveniently carried out with oxygen/ nitrogen bottled gas mixtures of known composition; such mixtures are commercially available. We have also successfully used mixed gas streams monitored by soap-film flowmeters. Other types of flowmeter are not usually sufficiently accurate. Typical calibration curves are shown in Figure 3, from which it may be seen that the response is linear up to about 4 vol % oxygen. Most of our measurements on liquids have been made in the linear region but oxygen contents up to 40 vol have also been determined. Repeatability of the peak height in almost all cases is better than i1.O %. Accuracy. As a check on the absolute accuracy of the method we report several determinations of the saturation oxygen contents of some solvents at one atmosphere oxygen pressure (Table I). Sensitivity. The ultimate sensitivity of the instrument has not been determined but is certainly better than 0.01 vol % oxygen. The long-term drift is equivalent to about 0.02% oxygen/hour once steady conditions have been achieved. DISCUSSION
The main features of the GLC techniques described here are the use of a Teflon/stainless steel sampling valve and of an electron capture cell as detector. The sampling valve gives the apparatus great versatility in that all except the most highly corrosive liquids can be sampled over a wide range of temperatures and pressures. Use of a sampling valve is more convenient than transfer operations and avoids the possibility of contamination, but the valve requires careful design. The electron capture detector is selective and highly sensitive to electronegative materials and, therefore, in the system
that we have studied, responds only to oxygen. This makes it possible t o employ very rapid resolving times. In systems containing additional electronegative materials-e.g., halogenated compounds-greater resolution would be required for such materials would also give a detector output. The major disadvantage of the electron capture cell is its suceptibility to contamination, but this can be avoided with a reasonable amount of care-e.g., by backflushing and isolating the cell during periods when the apparatus is not in use. We have found the apparatus reliable, convenient to use, and easily transportable; we have used it successfully on open-air sites. The absolute accuracy of the method appears to be satisfactory up to oxygen contents of at least 40 vol % although it is most suitable for measurements in the range 0.01 t o 4 vol oxygen, which is the region of linear response. ACKNOWLEDGMENT
The author thanks E. R. Adlard for his many helpful suggestions and provision of equipment and R. R . Willcock who carried out much of the experimental work. RECEIVED for review July 26, 1968. Accepted October 28, 1968.
(4) J. Horiuti, Sci. Pup. Znst. Phys. Chem. Res. (Tokyo), 17, 125 (1931). (5) R. R. Baldwin and S. G. Daniel, J. Appl. Chem. (London), 2, 161 (1952). (6) C. B. Kretschrner, J. Nowaskowska, and R. Wiebe, Znd. Eng. Chem, 38, 506 (1946).
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