Anal. Chem. 1988, 58,3017-3021
3017
Determination of Sulfur Compound Distribution in Petroleum by Gas Chromatography with a Flame Photometric Detector C h e r l y n l a v a u g h n Bradley* a n d Douglas J. Schiller Amoco Corporation, Amoco Research Center, Naperville, Illinois 60566
A slmulated distillation using a flame photometric detector (FPD)is reported for determinlng the bolling range dWrlbutlon of sulfur-contalnlngcompounds In middle-to-heavy petroleum distillates. Its success Is based on a new callbratlon method for the FPD involving convertlng all the sulfur-contalnlng compounds to SO, before detection and llnearlzing the data. A slngle callbratlon equation Is formulated expressing a unlform and linear relationship between detector response and sulfur concentration for the varlous types of sulfur compounds in petroleum. Consequently, the problems of handllng the detector's nonllnearity and varlance of response to dlffereni sulfur compounds are eliminated. The method Is quantitative and appllcable to petroleum dWHlates with a final bolllng point of 1184 O F (640 OC) and 1 8 5 0 ppm by welght total sulfur content.
Knowledge of the types and quantities of sulfur-containing compounds in petroleum is of great importance to the petroleum industry because they poison catalysts, cause corrosion, and contribute to pollution. Therefore, sensitive and accurate analytical methods .are needed to determine the distribution of the sulfur compounds. Over a decade ago, Martin and Grant developed a method for the determination of sulfur compound distribution by gas chromatography ( I ) . They also developed a method for separating sulfur compounds contained in gas oils into thiophenic/non-thiophenic fractions. In the latter method, the thiophenic fractions were separated into individual single-, double-, triple-, and >triple-ring structures (2). In both of these methods, a microcoulometer was used to detect the sulfur compounds. The thiophenic method was later improved using a flame photometric detector (FPD) ( 3 ) . This detector is more sensitive and stable and gives more reproducible results than a microcoulometer. This latter work with a FPD gave promise of a sulfur-simulated distillation method. The F P D is both highly sensitive and selective to sulfur. As such, it is one of the most suitable GC detectors available for determining sulfur compounds in complex mixtures such as petroleum distillates. There are two types of flame photometric detectors commercially available: a single-flame FPD design described by Brody and Chaney ( 4 ) and a dual-flame F P D described by Patterson et al. ( 5 ) . A single-flame F P D was employed in the present work. Two other characteristics of the FPD, especially true of the single-flame FPD, are that its response varies with sulfur compound type and is nonlinear for a given type of sulfur compound. This means that quantitating the different sulfur compounds in a sample requires knowing the identity of each sulfur component and calibrating for each one. For complex mixtures such as petroleum samples, calibrating for each sulfur compound is tedious and time-consuming. Moreover, it is virtually impossible to do a complete calibration this way, since standards for many of the sulfur-containing components in petroleum are not available.
This paper reports a method by which the single-flame FPD's nonlinearity and response variance is eliminated by conversion of all sulfur in the sample to sulfur dioxide. As a result of this conversion, the complications in calibrating this detector for different types of sulfur compounds are bypassed. In the method, the detector only responds to one type of sulfur compound. Therefore, only one calibration is needed for all types of sulfur compounds detected. A single calibration equation is formulated that expresses a linear relationship between the detector response and the sulfur concentration. We have used the method to develop a sulfur-simulated distillation method to determine the boiling range distribution of sulfur compounds in middle-to-heavy petroleum distillates. It is used to report weight percent sulfur vs. boiling point data of petroleum distillates. Results using the simulated distillation method for sulfur-containing compounds (usually referred to as the "sulfur by boiling point (SBP) m e t h o d ) are reported herein. They show that quantitative sulfur distributions by boiling point of a variety of petroleum distillates can be successfully obtained. EXPERIMENTAL SECTION Apparatus. The gas chromatograph used was a HewlettPackard Model 5710A equipped with a Melpar lOOAT FPD and a Hewlett-Packard 7650A electrometer. The temperature of the FPD was 150 "C with H2flow rate of 120 mL/min, air at 70 mL/min, and O2at 10 mL/min. A 4-ft X l/&x-o.d. stainless-steel tubing (Type 316, chromatographic grade) was packed with 3% SP-2100 on 80/100 mesh Supelcoport. The tubing was purchased from Alltech Associates, Inc., and the packing from Supelco, Inc. The injection port temperature was 350 "C, and the nitrogen carrier gas flow rate was 30 mL/min. The amount of petroleum distillate charged to the column was 0.2-0.4 pL at 0 "C, and the column temperature was programmed to 350 "C at 8 "C/min. A @-1.0-mV full-scale recorder with a chart speed of 0.5 cm/min was used. A furnace (Tracor Instruments) containing an empty 2-mm (i.d.) x 19-mm quartz pyrolysis tube having a total volume of 0.6 mL was connected to the chromatograph's column exit. The furnace was operated at 800 "C in the oxidative mode to burn the sulfur effluent to sulfur dioxide and any hydrocarbons to carbon dioxide and water. Three thermocouples (one each at the inlet, center, and outlet of the furnace) were used to maintain the furnace temperature. Variable transformers and a thermocouple switch were used, respectively, to control and monitor the temperatures of the thermocouples. Temperature readings were displayed by a digital pyrometer. Oxygen, used as the combustion gas, was plumbed into the furnace with a gas line different from the detector's O2supply line and maintained at a flow rate of 10 mL/min. The combustion products exited the furnace through heated (350 "C) stainless-steel transfer lines and flowed into the FPD that detects sulfur effluent as SOz. Output from the detector's photomultiplier tube was fed into the electrometer that interfaced simultaneously with a strip chart recorder and a computer. A schematic diagram of the chromatographic system is given in Figure la. Materials. Reference sulfur compounds were obtained from commercial sources. Other reagents were ACS reagent grade.
0003-2700/86/0358-3017$0 1.5010 0 1986 American Chemical Society
3018
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 Table 11. Percent Sulfur Recovery of Sulfur Compound Blends" ng sulfur injected calcd measd
sampleb I
Chrornatooraoh
b
NZ -+
One-Component Solutions 1
r SO21
di-n-propyl sulfide N p cylinder
69.6 32.4 59.8 34.7 57.4 29.6 58.5 33.3
di-n-pentyl disulfide
Sampling valve/loop
dibenzothiophene Recorder
n-decyl mercaptan IChrornatograph]
70 S recovery
Figure 1. Schematic of apparatus: (a) sample analysis mode and (b) SO, calibration mode.
Table I. Petroleum Distillates Used in Study boiling range, O
LCCO LVGO CGO HVGO (feed)
400-650 400-800 450-950 650-1200
97 99 97 98 99 98 86 85
Three-Component Solutions
woc
type material'
67.8 32.1 57.9 34.0 56.9 28.9 50.2 28.3
F
total wt % S 1.82 1.96 1.30 2.99
"LCCO, light catalytic cycle oil; LVGO, light virgin gas oil; CGO, coker gas oil; HVGO, heavy virgin gas oil. Solutions were prepared with toluene or isooctane solvent. The petroleum fractions studied included a light catalytic cycle oil, a light virgin gas oil, a heavy virgin gas oil, and a coker gas oil. The boiling range and total sulfur content of the distillates are shown in Table I. The calibration standard was SO, in nitrogen at a concentration of 200 ppm SO2 by weight from Airco in a specially treated aluminum cylinder. Determination of Sulfur Compound Recovery as SO2. For the sulfur by boiling point method to be applicable for all types of sulfur compounds in middle-to-heavy petroleum distillates, the percent recovery of sulfur compounds as SOz should be independent of the type and concentration of sulfur compounds. Solutions containing selected sulfur compounds in reagent grade isooctane were prepared in concentrations ranging from 20 to 90 ng of sulfur/pL of solution. Known volumes of each solution were injected by syringe and the expected weight of sulfur compared with the amount measured. Duplicate measurements were made on each solution. Table I1 shows results for one-, three-, and four-sulfur-component solutions of sulfides, disulfides, thiophenes, and mercaptans that were analyzed in order to measure their percent sulfur recovery in the oxidative furnace. Percent recovery of sulfides, disulfides, and thiophenes in the single-sulfur-component solutions was better than 9 8 % . In the three- and four-sulfur-component solutions, recovery ranged from 89% in the mercaptan blends to 97% in the thiophene and disulfide blends. Thus, in this method, the recovery of sulfur as SO2appeared to be independent of the type of sulfur compound. Moreover, within the concentration range tested, sulfur recovery appeared to be independent of the concentration of sulfur. Percent recovery of mercaptans appeared lower (83-92% ) than the other sulfur compounds. This probably does not represent poor conversion to SO2 or a detector effect but is probably due to greater adsorption of the highly reactive mercaptans on the stainless-steel column wall and/or the column packing. What supports this idea is our experience of obtaining more quantitative mercaptan recoveries as SO2(94-98%) when using a fused-silica capillary column. The latter type of column has a more inert surface than stainless steel. Calibration Apparatus and Procedure. A. SOzCalibration. Prior to calibration, the SO2concentration in the calibration blend was assayed in our laboratory by GC/FPD using SO2standards prepared in isooctane.
2-ethylthiophene benzothiophene dibenzothiophene n-pentyl mercaptan n-hexyl mercaptan n-heptyl mercaptan diethyl sulfide diisopropyl sulfide di-n-butyl sulfide dimethyl disulfide di-n-butyl disulfide di-n-pentyl disulfide
{:E
84.8 40.9
97 97
82.7 27.5
75.2 23.1
91 84
63.8 47.3
60.0 45.1
94 97
79.1 Lo.9
77.8 29.2
98 95
{ {
Four-Component Solutions 2-ethylthiophene n-propyl disulfide n-decyl mercaptan diphenyl sulfide
67.2 141.1
62.3 36.9
93 90
a Average of duplicate measurements obtained. *Isooctane solvent.
An exponential dilution flask (EDF) is used extensively to calibrate gas chromatographic detectors (6-12). However, several users report a lack of accuracy and precision in obtaining an initial sample concentration in the EDF when gas-tight syringes are used to inject the test gas into the EDF. Our calibration method, however, uses a gas sampling valve system, instead of a gas syringe, to inject SO2 into the EDF. It is well-known that improved precision and accuracy of sampling are achieved with a gas sampling valve/sample loop system compared with a syringe (13). Therefore, use of a sampling valve results in precise amounts of SOz introduced into the EDF for calibrating a FPD. The combination of a sampling valve and a EDF produces a calibration in a dynamic fashion over a wide concentration range that is accurate and reproducible. Reproducibility at a given initial concentration is &2% relative. While the initial concentration of SO2 in the dilution flask can be as low as 1.0 ng of sulfur/mL, the maximum concentration is ca. 10 ng of sulfur/mL. Initial concentrations higher than this result in a steady decrease in the slope and intercept of the response vs. concentration calibration curve. This is probably due to exceeding the upper limit of detector response (1.4 V) and, thus, overloading the detector. A 254-mL Teflon EDF was plumbed into the internal GC carrier gas flow system. The carrier gas (N2) to the EDF was maintained a t a known flow rate, and a known quantity of SO2 was introduced into the EDF using a Rheodyne Teflon sampling valve and sample loop system. In this way the initial concentration of SO2 (as nanograms of sulfur per milliliter) in the EDF was precisely known. The detector responded almost immediately to the initial concentration. The SO2 in the EDF was then continuously diluted with N2 carrier gas-the concentration of the SO2 decreasing exponentially with time as it flowed out of the dilution flask. Since a chromatographic column causes excessive delay of SO2 elution when calibrating, the column was replaced with a short piece of empty Teflon tubing connected to
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
3019
Table 111. Boiling Point Calibration File
20
0
Time (rnin.1 Figure 2. Exponential decay curve.
-
50
the EDF outlet and the furnace inlet so that the SO2 from the flask was routed directly into the furnace and detector (Figure lb). The SO2 concentration-time profile followed an exponential decay curve from the initial concentration of SOz down to a detection limit a t which the response leveled off (Figure 2). In this way a wide range of sulfur concentrations was provided to the detector in a single run and in a dynamic fashion. The curve was completed in ca. 50 min. Any desired concentration can be obtained by use of the decay curve equation C, = Coe-(tU/V) where C, is the concentration is the concentration of SO2 a t any given time, t , Co is the initial concentration of SO2 a t t = 0, U is the flow rate of the gas through the EDF, and Vis the volume of the EDF. To indicate how quickly the SO2 concentration decayed, an initial concentration of 6 ng of S/mL (using a 5.30-mL sample loop) decreased to 10% of its original value after only 2 min (Figure 2). The initial concentration in the exponential dilution flask was varied by using different size sample loops in the sample valve. The sample loops should be purged with the carrier gas before they are used. Otherwise, the sample loops adsorb SOz, which results in broadening of the exponential decay curve and throwing off the entire calibration. Three calibration runs were made at different initial SO2 concentrations to obtain the best calibration curve to fit the data. Runs a t a given initial concentration were duplicated to ensure accuracy. From a 200 ppm SOz in N2 standard, initial concentrations of SO2in the EDF were 1.13,1.69, and 2.25 ng of S/mL. These concentrations were obtained by using 1.0-, 1.5, and 2.0-mL sample loops, respectively. Data pairs (detector response (pV) vs. sulfur concentration) were taken from each response vs. time calibration run and fitted into a cubic equation of the general form C = sulfur concn = e(A + B) (In (R - base line)) + C (In ( R - base line))2 + D (In ( R - base line))3 where R = detector response. The result was a logarithmic, linear plot of detector response vs. sulfur concentration. B. Boiling Point Calibration. A second type of calibration needed was a retention time vs. boiling point calibration of reference sulfur compounds. This calibration was needed to determine the boiling points of the various sulfur compounds in petroleum distillates. The retention times of 23 sulfur compounds (sulfides, disulfides, mercaptans, and thiophenes) in toluene were obtained on the same packed column and under the same chromatographic conditions used to analyze samples. The sulfur compounds used in the boiling point calibration have boiling points ranging from 115 up to 905 O F (Table 111). The linear boiling point calibration plot of the sulfur compounds was extrapolated from 905 OF,which corresponds to the highest boiling, pure sulfur compound available, to 1184 "F (retention time equal to 2640 s). The purpose of this extrapolation was to extend the sulfur boiling point calibration curve to the corresponding retention time of the highest boiling normal paraffin, Cd4 (bp = 1018 O F ) , used in hydrocarbon-simulated distillation (15). The retention time of n-Cd4was obtained on the same column and at the same conditions as the sulfur compounds but was detected with a FID. Analyses of Petroleum Samples. Petroleum samples were assayed for total sulfur by X-ray fluorescence. The percent total
sulfur compd
retention time, s
deg F
deg C
carbon disulfide thiophene ethyl sulfide 3-methylthiophene 2-ethylthiophene n-propyl sulfide ethyl disulfide n-heptyl mercaptan isopropyl disulfide n-butyl sulfide n-propyl disulfide benzothiophene n-butyl disulfide n-decyl mercaptan n-pentyl disulfide diphenyl sulfide dibenzothiophene n-octyl sulfide phenyl disulfide hexadecyl mercaptan octadecyl mercaptan n-decyl sulfide n-dodecyl sulfide extrapolated to 640 "C
69 170 233 309 415 456 486 606 620 692 706 785 875 954 1108 1161 1291 1350 1430 1470 1580
115 183 198 239 273 289 307 325 345 372 388 428 448 466 527 565 640 651 657 693 702 752 905 1184
46 84 92
1700
1933 2640
115
134 143 153 163 174 189 198 220 231 241
275 296 338 344 347 367 375 400 485 640
sulfur value was used to determine how much sample to inject into the chromatograph and to calculate the percent of sulfur converted to SO2in the samples. Samples containing up to 0.7% sulfur could be analyzed without dilution if they were not viscous. Viscous samples, and those samples having more than ca. 1% total sulfur, were diluted in toluene. An analysis began by conditioning the column by programming the chromatographic oven through the temperature range used for sample analysis. This conditioning was followed by a blank run to account for any base-line deviation. The area accumulated from the blank run was subsequently subtracted from the sample run. A sample (0.2-0.4 pL) was then injected into the chromatograph. This small sample size was necessary to keep the amount of sulfur within the calibrated range of the detector. The sample was separated by the column and flowed into the oxidative furnace. Sulfur effluent was then converted to SOz without disturbing the elution order achieved in the column separation. To determine the sulfur by boiling point distribution of the sample, the computer determined cumulative peak areas at regular intervals during the elution of the sample as SO,. Each peak area was divided by the total area of the chromatogram to get the fraction of material eluting a t each point. The computer then applied the calibration equation to convert the detector's response vs. time data of the sample to sulfur concentration vs. time data, which were integrated to get sulfur mass (ng) vs. time data for each fraction. The boiling point calibration was then applied to convert the sulfur mass vs. time data into weight percent sulfur vs. boiling point data a t regular intervals from initial to final boiling point. Samples having a final boiling point beyond the limit of the temperature program of the column (i.e., not all sulfur components eluted) were analyzed by an internal standard method. Since the two main types of sulfur compounds in petroleum distillates are sulfides and thiophenes, these two types of sulfur compounds were used as internal standards. Moreover, using only one internal standard sometimes resulted in detector saturation. Therefore, as many as four sulfur compounds, which elute before the sample, were weighed into the sample as internal standards. The combined concentrations of the internal standards resulted in a concentration similar to the sulfur content in the sample without saturating the detector. The theoretical total sample area is calculated from the weight ratio of the sample to internal standard multiplied by the internal standard area. The accumulated weight percent S is then related to the boiling point via the boiling point calibration data, and the weight percent sulfur vs. boiling point report is obtained.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table IV. Sulfur-Simulated Distillations of a LCCO and HVGO temp, OF
I
1
I
I
w t 70sulfur compd eluted
LCCO
HVGO
IBP 10 20 50 70 80 90 95 FBP
520 660 667 695 700 704 727 772 931
679 772 874 1003 1094 1159
I
200 400 600 800 lo00
O F
Figure 3. Sulfur chromatogram of a LCCO.
Table V. Precision of Sulfur-Simulated Distillation”
2-ethyl thiophene
1
I
200
!-propyl sulfide /n-butyl
sulfide
&-HVGO
400 600 800 loo0 1200 O F
Figure 4. Sulfur chromatograms of a HVGO: (a) HVGO only and (b) HVGO with
internal standards.
RESULTS AND DISCUSSION S a m p l e Analyses. A chromatogram of the sulfw distribution of a light catalytic cycle oil (LCCO) distillate is shown in Figure 3. Figure 4a is a chromatogram of a heavy virgin gas oil (HVGO) feed. There is a striking difference between this chromatogram and that of the LCCO. First of all, base-line resolution is not obtained in the HVGO distillate because of the complexity of the sulfur compounds in this type of petroleum distillate. In the case of the LCCO distillate, base-line resolution is significantly enhanced because of the more discrete compound nature of this type of distillate. Another difference between the HVGO and the LCCO chromatograms is that all the sulfur compounds in the HVGO do not elute as indicated by the arrow on the far right-hand side marking the elution endpoint. Table IV is a partial report of weight percent sulfur as a function of boiling point for a LCCO and a HVGO distillate. The initial boiling point of the LCCO was 520 OF. Fifty percent of the sulfur compounds in the distillate eluted by 695 OF,and all of the sample eluted with a final boiling point of 931 OF. In the case of the HVGO, the initial boiling point was 679 O F , but only 80% of the sulfur compounds eluted by the end of the run. Therefore, four internal standards (sulfides and thiophenes) were added and the weight percent sulfur vs.
w t 9’0 sulfur compd eluted
av of 10 runs, O F
std dev
control limits
IBP 5.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 95.0 FBP
524 64 1 660 668 685 689 695 698 701 ’704 727 766 911
2.4 2.2 1.1 2.8 1.2 1.6 0.6 0.7 0.9 0.7 4.3 7.6 14.6
517-531 634-648 657-663 660-676 681-689 684-694 683-697 696-700 698-704 702-706 714-740 743-789 867-955
“LCCO.
boiling point data calculated for this distillate (Table IV). Fifty percent of the sulfur compounds eluted by 1003 OF, and by end of the run, at 1159 O F , 80% of the sulfur compounds had eluted. Figure 4b shows a chromatogram of the HVGO with four internal standards. Sample quantitation of the light cat cycle oil, light virgin gas oil, and coker gas oil distillates tested was monitored by comparing the measured and calculated amounts of sulfurcontaining compounds injected. Percent sulfur recoveries were >95%. Quenching. It is well-known that the response of the flame photometric detector is affected not only by the presence of hydrocarbons (16-21) but also by the presence of hydrocarbon combustion products, COz and H,O, as well (22,23). In the sulfur by boiling point analysis, this quenching phenomenon of the detector’s response by COz and H,O is pronounced with gas oils and light distillates (particularly naphthas) containing less than 850 ppm total sulfur and is detrimental to the accuracy of analyzing such samples. The sulfur by boiling point method, therefore, is limited to samples containing 2850 ppm sulfur. Precision. A . Boiling Point Calibration. Repeatability of running the boiling point calibration solution over a 4-week period is good to within *lo s and shows column and system reproducibility. B . Simulated Distillation. Precision data are given in Table V of a LCCO used as a quality control sample. The boiling point quality control limits for each weight percent fraction in Table V are taken a t 3 standard deviations from the average boiling point values and establishes a temperature range in which the quality control sample is limited from initial to final boiling point. The best precision occurs from the initial boiling point up to 90% sample elution. At the 90% point, precision is f 4 O F . The least Drecise data are obtained at the samde’s final boiling
Anal. Chem. 1986, 58, 3021-3027
point where the deviation is h15
OF.
CONCLUSIONS In conclusion, this work shows that (1)problems of handling the FPD's nonlinearity and variance of response to different sulfur compounds can be eliminated by converting the sulfur-containing compounds to SOz and linearizing the data, (2) a single calibration equation can be formulated for the various sulfur compounds in petroleum distillates, and (3) a sulfursimulated distillation method using a FPD is achievable. Further development of this technique is being done in our laboratory to extend the method to higher boiling petroleum distillates and to synthetic fuels. Copies of the calibration plots may be obtained on request from the authors.
ACKNOWLEDGMENT We are grateful to N. F. Schiller and E. R. Ziegel of Amoco Corp. for developing and implementing the calibration equation and to G. R. Hoekstra of Amoco Oil Co. for supplying the petroleum samples and several helpful discussions.
LITERATURE CITED (1) Martin, R. L.; Grant, J. A. Anal. Chem. 1965, 37,644. (2) Martin, R. L.; Grant, J. A. Anal. Chem. 1965, 37,649.
3021
Wenzel, B. E.; Aiken, R. L. J . Chromatogr. Sci. 1979, 17, 503. Brody, S. S.;Chaney, J. E. J . Gas Chromatogr. 1966, 4 , 42. Patterson, P. L.; Howe, R. L.; Abu-Shumays, A. Anal. Chem. 1978, 50, 339. Bruner, F.; Canulli, C.; Possanzani, M. Anal. Chem. 1973, 4 5 , 1790. Lovelock, J. E. Anal. Chem. 1961, 33, 162. Hartmann, C. H.;Dimick, K. P. J . Gas Chromatogr. 1966, 163. Williams, li. P.: Winefordner, J. D. J . Gas Chromatogr. 1966, 271. Bruner, F.; Liberti, A.; Possanzini, M.; Allegrini, I. Anal. Chem. 1972, 4 4 , 2070. Ritter, J. J.; Adams, N. K. Anal. Chem. 1976, 4 8 , 612. Bruner, F.; Ciccioli, P.; DiNardo, F. Anal. Chem. 1975, 4 7 , 141. Thompson, B. Fundamentals of Gas Analysis by Gas Chromatography; Varian Associates: Palo Alto, CA, 1977; Chapter 7 . Williams, M. P.; Winefordner, J. D. J , Gas Chromatogr. 1966, 4 , 271. 1985 Annual Book of ASTM Standards : ASTM: Philadelphia, PA: Part 24, Section 5, pp 798-804, method D2887-84. Maruyama, M.; Kakemoto, M. J . Chromatogr. Sci. 1979, 76, 1. Rupprecht, W. E.; Phillips, T. R. Anal. Chim. Acta 1969, 47,439. Sugiyama, T.; Suzuki, Y.; Takeuchi, T. J . Chromatogr. 1973, 80, 67. Pearson. C. D.: Hines, W. J. Anal. Chem. 1977, 4 9 , 123. CLay, D. A.; Rogers, C. H.;Jungers, R. H.Anal. Chem. 1977, 4 9 , 126. Patterson, P. L. Anal. Chem. 1978, 50,345. Olsen, K. B.; Ludwich, J. D. Talanta 1980, 27,665. Sevick, J. Detectors in Gas Chromatography; Elsevier: New York, 1976; Chapter 9.
RECEIVEDfor review May 12,1986. Accepted August 18,1986. This work was presented in part a t the 36th Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA, 1985 (paper no. 626).
Gas-Liquid Chromatographic Method for Determination of Partial Molar Free Energy at Infinite Dilution in Volatile Solvent and Its Application to Acetonitrile/Benzene Systems Seiji Terasawa,' Hidenori Itsuki,' and Hiroshi Yamaki Department of Chemical Engineering, Faculty of Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, J a p a n
A gas-liquid chromatographlc apparatus for volatlie stationary-phase liquid was constructed and applied to the systems of acetonttrlle in benzene and benzene In acetonttrlle at 20.0 O C . Variatlons of the retentlon tlme were measured, and then the solvent weights In the cdumn at the end of the runs were evaluated by welghlng. The retentlon times wlthin each run were extrapolated to the end tlme. The retentlon volumes were derlved by taklng Into account the coexistence of the solvent vapor in the carrier gas. A plot of retentlon volumes at the end times agalnst the experlmental solvent weights Is found to give a straight line. The specific retention volume was evaluated from the gradient of the line. The values of the partial molar Glbbs free energy at Infinite dllutlon, taklng the ideal gas of the solute under 1 atm as the standard state, are determined to be -0.666 kcal mol-' ( 1 kcal = 4.184 kJ) for acetonttrlle in benzene and -0.67, kcal mol-' for benzene In acetonttrlle. These values are satisfactorily conslstent with those from the vapor pressure data and also from the other gas-liquld chromatographic retention data in the literature.
Gas-liquid chromatography has been used in the determination of the partial molar Gibbs free energy of certain Present address: T o k y o Kaseigakuin College, Sanbancho, C h i yoda-ku, T o k y o 102, Japan.
solutes a t infinite dilution in many nonvolatile solvents. In order to extend the principle of gas-liquid chromatography to systems including volatile solvents, Kwantes and Rijinders ( I ) proposed use of a presaturator. The carrier gas flowing into the column was presaturated with the vapor of the volatile solvent in order to suppress the amount of bleeding of the solvent in the column. The presaturator method has frequently has been adopted in the pertinent measurements (2). When a presaturator is applied, the volatile solvent liquid in the column is vaporized. This is unavoidable because the carrier gas expands during its flow through the column. Thus, it was necessary to correct the retention time by the decreased quantity of the solvent (2). In order to avoid such a correction, Kwantes and Rijinders ( I ) proposed the simultaneous determination of the retention time of a solute whose partition coefficient is already known. Barker and Hilmi (3)applied this method to systems for several solutes in n-propyl alcohol, in toluene, and in 1,Zdichloroethane. In n-propyl alcohol solvent, the activity coefficient at infinite dilution, y-, for the standard solute of benzene was obtained by extrapolating the vapor pressure data t o infinite dilution and interpolating to the relevant temperature by an Arrhenius plot. In toluene and 1,2-dichloroethane, benzene was assumed to behave ideally and hence ym= 1. Eckert and his collaborators ( 4 , 5 ) adopted this method to determine the ym values for the systems of 35 solutes in 34 solvents. Naturally, this method is under the strict limitation that the activity coefficient or vapor-liquid equilibrium constant a t infinite dilution must
0003-2700/86/0358-3021$01.50/0 0 1986 American Chemical Society
