E out,
Concn.a
end point 760 732 725 768 815 a
that the sulfuric acid-sodium hydroxide system had a toosharp color change at the end point for use with the rapid addition option of the program. These could, however, be done with the single addition option of the program. This is a significant improvement over the synchronous system in terms of the smaller sample size which may be delivered and the increased precision of the addition of the titrant (the error in this case is noncumulative). An acetic acid-sodium hydroxide titration was therefore used for evaluation of the rapid addition system since it has a slower color change at the end point. The results of some of these titrations are shown in Table 11. The titrations were run with the rapid addition option of the program where 5 additions were made per reading until the end point was near. Figure 4 shows a typical titration curve obtained with this system. The average sample concentration calculated from the end point was found to be 4.89 X lO-3M with a standard deviation of 1.17 X 10-5M. This rapid addition method provides a great improvement (about a factor of 1/4 difference) in the amount of time needed to do a titration.
Titrations with Stepper Motor
Table 11.
x
Addn No. 339 339 340 338 338
103~
4.89 4.89 4.91 4.88 4.88
Calculated concentration was 4.92 X 103M.
CONCLUSIONS
0.1
1 0
I
IO
20
I
I
30
PO
50
60
70
80
It has been demonstrated that a great aid to the automation of analytical experimentation is the computer controlled sampling system. The use of the computer to make decisions, and on the basis of this to modify the progress of the experiment, has also been demonstrated. The obvious application to this is in auto-analysis such as for amino acid analysis. This type of sampling system would also be of use in kinetic studies as was suggested by Hicks, Eggert, and Toren (8).
TiTRAHT AOOTllON NUMBIR
Figure 4. Typical titration curve of an acid with a base using Thymol Blue indicator with the spectrophotometerset at 615 nm
RECEIVED for review August 3, 1970. Accepted February 8, 1971.
Gel Permeation Chromatographic Separation of Petroleum Acids T. E. Cogswell, J. F. McKay, and D . R. Latham Laramie Petroleum Research Center, Bureau of Mines, U. S . Department of Interior, Laramie, Wyo. 82070
Gel permeation chromatography (GPC) was used to separate the components of an acid concentrate. This concentrate was prepared from the 400-500 O C distillate of Wilmington (Calif.) petroleum. The separation was made with a cross-linked polystyrene gel, using methylene chloride as a solvent; and the fractions obtained were characterized by infrared spectra and by molecular weight data. A carboxylic acid fraction obtained was essentially free of phenolic and nitrogen-containing compounds. Results indicate that molecular association of some compound types is responsible for the separation.
RECENTLY, GEL PERMEATION CHROMATOGRAPHY (GPC) has been used as a highly effective tool for separating high-molecular-weight materials. The early progress in this area has been summarized by Moore (I), and extensive model-compound studies have been carried out to clarify the basic separation mechanism (2). The first polymer gels available for (1) J, C. Moore, J. Polymer Sci., C21, 1 (1968). (2) T. Edstrom and B. A. Petro, ibid., p 171.
GPC were limited in use to aqueous systems, but the development of rigidly cross-linked polymer gels has extended the uses of GPC into systems requiring organic solvents ( 3 , 4 ) . Many of the important applications of GPC using organic solvents are in the field of petroleum where GPC has been used to characterize total crude oils (5, 6) and crude oil fractions (6-11). Structural investigations of petroleum fractions,
(3) J. C. Moore, J . Polymer Sci., A2, 835 (1964). (4) K. H. Altgelt and J. C. Moore in “Polymer Fractionation,” M. J. K. Cantow, Ed., Academic Press, New York, N. Y . , 1967. ( 5 ) H. H. Oelert, D. R. Latham, and W. E. Haines, Preprints, Dic. Petrol. Chem., ACS, 15 (2), A204, Feb. 1970. (6) J. N. Done and W . K. Reid, ibid., A242. (7) J . G. Bergmann and L. J . Duffy, ibid., A217. (8) H . J. Coleman, D. E. Hirsch, and J. E. Dooley, ANAL.CHEM., 41, 8 (1969). (9) K. H. Altgelt, Mukromol. Chem., 88, 75 (1965). (10) K. H. Altgelt, J. Appl. Polymer Sci., 9, 3389 (1965). (11) R. J. Rosscup and H. P. Pohlmann, Preprints, Diu. of Petrol. Chem., ACS, 12 (2), A103 (1967). ANALYTICAL CHEMISTRY, VOL. 43,
NO. 6, MAY 1971
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400-500°C Wilmington
Anion exchange chromatography
1 I
I2
Gel permeation chromatography
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16
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l
l
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20 24 28 32 36 4 0 44 GPC SUBFRACTIONS ( 3 . 4 m l e a c h )
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48
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52
Figure 2. GPC chromatogram GPC rubfractions 13.4 ml each)
T T T T T T T T T T T T ~ T T T r T r r T
GPC Separation of Wilmington Acid Concentrate Per cent by weight of recovered acid concentrate Fraction 3, Fraction 1, Fraction 2, nitrogen Fraction 4, Acid high mol carboxylic compds, fluorescent phenols materials sample wt material acids 34.3 0.7 1 2.8 62.2 2 3.1 64.1 31.7 1.1
Table I.
I
-1st
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o
n
I
1 3 r d Fraction
1
I
4th Fraction
1
Figure 1. Separation scheme
including hydrocarbons and polar concentrates (12-15), have provided valuable information about petroleum and, in addition, have stimulated ideas about the GPC tzchnique itself. This paper describes the successful application of GPC to the separation of components of a petroleum acid concentrate. Specific compound types have been isolated using this technique. Of primary importance is the separation of carboxylic acids from phenolic and nitrogen-containing compounds. Evidence has been obtained which suggests that the separation results from intermolecular association of certain compound types. EXPERIMENTAL Apparatus. A pressurized solvent reservoir containing flash-distilled reagent-grade methylene chloride under 15 psig pressure of N Pprovided the continuous flow of eluting solvent. The injector block was a simple three-way valve. The separation column was a water-jacketed glass tube, 1.3-cm i.d. by 150 cm, packed with 80 grams of cross-linked polystyrene gel (Waters Associates Poragel, A-1) preswollen in methylene chloride. The column had a total volume of about 200 ml and a void volume, V, (mobile phase volume), of about 60 ml. A continuous-flow refractometer (Waters Associates, Model 34 H) was used to monitor the output of the column, with the signal recorded by a chart recorder (Honeywell, Electronik 19). A constant-volume fraction collector (Model 270, Instrumentation Specialities Co., Inc., ISCO) was adjusted to collect 3.4-ml subfractions. The solvent flow rate was adjusted to 0.8 ml/min flow rate. Each subfraction was automatically marked on the chart paper by a signal from the frac-
(12) J. H. Weber andH. H. Oelert, Preprints, Diu.ofpetrol Chem., A C S , 15 (2), A212, February 1970. (13) B. J. Mair, P. T. R. Hwang, and R. G . Ruberto, ANAL.CHEM., 39, 838 (1967). (14) H. H. Oelert, Minutes of Advisory Committee Meeting, American Petroleum Research Institute Research Project 60, July 1969. (15) W. K. Seifert, R. M. Teeter, W. G . Howells, and M. J. R. 41, 1638 (1969). Cantow, ANAL.CHEM., 646
ANALYTICAL CHEMISTRY, VOL. 43,
NO. 6, MAY 1971
tion collector. After the GPC separation, the subfractions were analyzed using a Perkin-Elmer Model 621 infrared spectrometer. All molecular weights were determined using a Mechrolab Model 301A vapor-phase osmometer. Separation Procedure. Figure 1 illustrates the separation scheme used to obtain the four GPC fractions from the petroleum distillate cut. The acid concentrate was obtained by passing the total 400-500 "C distillate cut of Wilmington (Calif.) crude oil over Amberlyst A-29 anion-exchange resin. Recovery of the acid concentrate (about 5 of the distillate cut) from the resin provided the starting material for the GPC separations. Samples (100-200 mg) of the acid concentrate were diluted to 1 ml in methylene chloride and injected into the GPC system with a hypodermic syringe. An individual run, from sample injection to collection of subfractions 1-50 (170 ml), required about 3.5 hours. The temperature of the column water jacket was checked periodically during various runs and found to be 12 f 2 "C. The GPC subfractions were collected and analyzed by infrared spectrometry. On the basis of the infrared data, the subfractions were combined as shown in Figure 1 to produce the four fractions. The chromatogram in Figure 2 shows refractometer response as a function of GPC subfraction. The cutpoints for the four fractions, as determined by infrared analysis, are indicated on the chromatogram. The ordinate of the chromatogram shows refractometer response, and because of the complexity of the sample, no quantitative interpretation can be made from Figure 2. RESULTS AND DISCUSSION GPC Separation of the Acid Concentrate. GPC separated the acid concentrate into four spectroscopically definable fractions. The weight per cent of each fraction based on recovered material is shown in Table I. The data for two runs are recorded to indicate the reproducibility of the technique. Selected portions of the infrared spectra of the acid concentrate and of the GPC fractions 1, 2, 3, and 4 are shown in Figure 3. The spectrum of the acid concentrate in methylene chloride indicated that phenolic compounds (3590 cm-', phenolic 0 - H stretching) were present together with nitrogencontaining compounds (3460 cm-', N-H stretching), carboxylic acids (1710 cm-I, carbonyl stretching), and aromatic compounds (1600 cm-l). The infrared spectra of GPC frac-
Table 11. Per Cent of Acid Concentrate Recovered from GPC Column Acid Weight Weight sample injected, mg recovered, mg Recovery, 1 2
131.5 141.5
125.1 139.8
95.5 98.8
Table 111. Vapor-Phase Osmometry Molecular Weights Fraction 2 Fraction 3 Solvent Fraction 1 CHiClz 1862 619 319 THF 185 474 36 1
tions 1-4 show that a separation of compound types has been obtained. The separation of carboxylic acids from other components of the acid concentrate is especially important because carboxylic acids represent approximately 60 per cent of the acid concentrate. The infrared spectrum of fraction 1 recorded in methylene chloride and in tetrahydrofuran did not show any of the absorption bands cited above. The only absorption bands observed were those of the solvent, although the sample concentration was very high (0.02M). Fraction 2 was found to be composed of compounds having strong carboxylic acid carbonyl absorption. Carbonyl absorption of the acid dimer is observed at 1710 cm-I, together with the free acid carbonyl band at 1730 cm-l. Other than an absorption band at 1600 cm-1, which may be due to the presence of aromatic carboxylic acids, compounds having other functional groups could not be detected in fraction 2. Fraction 3 had strong absorption at 3460 cm-I (NH stretching) and 3590 cm-1 (phenolic OH stretching), suggesting the presence of carbazoles and phenols. Despite the strength of the acid carbonyl absorption ( E up to 1500) (16), these bands could not be detected in fraction 3, indicating that the GPC separation of carboxylic acids from phenols and nitrogen-containing compounds was excellent. Fraction 4 was a highly fluorescent material having an infrared spectrum suggestive of phenolic or nitrogen-containing compounds. The infrared spectrum of fraction 4, recorded as a smear on KBr, shows aromatic absorption at 1600 cm-1 and broad OH hydrogen-bonding bands in the 3300 cm-l region. Carboxylic acid carbonyl absorption bands were not observed in this fraction. Recovery of Acid Concentrate from GPC Column. In separate experiments, designed to determine recovery of the acid concentrate, samples of the concentrate were weighed and injected into the column. The material recovered from the column in each run was collected as a single fraction in order to obtain a quantitative weight measurement. The volume of methylene chloride collected was 170 ml, and compounds were not detected by the refractometer beyond this volume. The solvent was removed by passing nitrogen over the sample until continued application of nitrogen brought the sample to a constant weight. Table I1 shows the percentage of material recovered from the column for each of two runs. Less than 5 % of the injected sample was lost. Most of this loss occurred during sample handling, and only a very small portion of this loss was due to column sorption. These runs are not the same as those shown in Table I. (16) Koji Nakanishi, “Infrared Absorption Spectroscopy,” Holden-
Day Inc., San Francisco, Calif., and Nankodo Co., Ltd., Tokyo, 1962.
90 W V
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5 z
GPC froction 2 (Carboxylic acids)
70 c
z
60
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GPC fraction 3 (Phenols and nitrogen compounds 1
”
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pq
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3200
1
GPC fraction 4 fluorescent material
1800 1700 vFREQUENCY, cm-1 l
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1
1600 1
1
1500 1
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1400 I
Figure 3. Infrared spectrum Evidence for Molecular Association. Theoretically, GPC separates molecules according to their size. Evidence has been obtained which indicates that molecular association of certain acid compound types is responsible for the observed separation of the acid concentrate. If, for example, carboxylic acids exist as dimers in methylene chloride solvent, these molecules would be expected to have larger molecular sizes than monomeric materials and would be separated from the other compounds by GPC. The molecular weights of fractions 1, 2, and 3 determined by vapor-phase osmometry in both methylene chloride and tetrahydrofuran are shown in Table 111. Fraction 4 contained insufficient material for molecular-weight determinaANALYTICAL CHEMISTRY, VOL. 43, NO. 6, M A Y 1971
647
tion. The data show that fractions 1 and 2 are more associated in methylene chloride than in tetrahydrofuran. The molecular weight of fraction 3 appears to be unaffected by these two solvents. That is, the degree of association among the components of fraction 3 is not significantly altered by one solvent relative to the other solvent. In general, the molecular-weight data of Table I11 suggest that molecular association is responsible for the GPC separation of certain compound types. The carboxylic acids of fraction 2 illustrate the ability of a specific compound type to undergo molecular association. Fraction 2 had a molecular weight of 619 in methylene chloride and 474 in tetrahydrofuran. The molecular weights of 619 and 474 represent equilibrium mixtures of monomers and dimers. Furthermore, these two weights suggest that methylene chloride shifts the equilibrium toward dimers, while tetrahydrofuran shifts the equilibrium toward monomers. Model compound studies reported by Helm and Petersen (17) have shown that carboxylic acids are present as an equilibrium mixture of monomers and dimers in methylene chloride. The equilibrium ratio of monomers and dimers as determined by infrared spectrometry was found to be concentration dependent as well as solvent dependent. The osmometric molecular weight of the carboxylic acid fraction determined in tetrahydrofuran solvent, i.e., 474, indicated that some association is still present because mass spectrometric data show the average molecular weight to be approximately 370. Additional evidence suggests that dimerization of carboxylic acids in methylene chloride is responsible for the observed GPC separation. First, infrared analysis indicated that both dimeric and monomeric acids were present when fraction 2 was dissolved in methylene chloride. The carbonyl absorption band of a nonassociated acid was observed at 1730 cm-’, while the carbonyl absorption band of a dimeric acid was observed at 1710 cm-1. Second, methyl ester derivatives of fraction 2 carboxylic acids were prepared using diazomethane, and the elution volumes of the esters compared with those of (17) R. V. Helm and J. C . Petersen, ANAL.CHEM., 40, 1100 (1968).
the carboxylic acids. The esters had elution volumes greater than those of the carboxylic acids, indicating that the molecular volumes of the ester derivatives were smaller than those of the carboxylic acids. Although carboxylic acid monomers and dimers are known to be present as an equilibrium mixture in methylene chloride, the monomer-dimer ratio present in the GPC column is difficult to determine. Nevertheless, because the monomerdimer equilibrium is a dynamic process, a separation of carboxylic acids from other materials can take place. The average size of associated molecules in a monomer-dimer mixture would be larger than that of molecules which do not associate. CONCLUSIONS
Gel permeation chromatography has been found to be a useful tool for quickly separating the components of a petroleum acid concentrate. Molecular weight, infrared, and elution volume data indicate that molecular association of compound types is responsible for the separation. Carboxylic acids obtained using gel permeation chromatography are essentially free of phenolic and nitrogen-containing compounds. GPC may find general application as a method to be used for isolating compounds which selectively associate. ACKNOWLEDGMENT
The authors thank H. H. Oelert of the University of Clausthal, Clausthal-Zellerfeld, Germany, for his inspiration and ideas. RECEIVED for review October 5, 1970. Accepted February 3, 1971. Work presented in this report was done under a cooperative agreement between the Bureau of Mines, U. S. Department of the Interior, and the American Petroleum Institute as part of Research Project 60. Presented at the 160th National Meeting, American Chemical Society, Chicago, Ill., September 1970. Mention of specific brand names or models of equipment is made for identification only and does not imply endorsement by the Bureau of Mines.
Wet Weights of Ion Exchange Resin Beads by Centrifugation Gordon H. Fricke and Donald Rosenthal Department of Chemistry, and Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam, N . Y . 13676
George A. Welford Health and Safety Laboratory, U . S . Atomic Energy Commission, New York, N . Y . 10014
A centrifugation method of determining the wet weight of ion exchange resin beads without surface liquid i s proposed. The volume per cent of liquid remaining on the surface of the centrifuged beads at the plateau centrifugal force is found of volume per cent liquid to depend on the relative disDersion of the size distribution Of the beads. Surface sulfonated Copolymer beads were preparedm These beads and glass beads were employed to test this method.
beads if care is not taken when the beads are returned to their orieinal condition Therefore, it is desirable to have a method of determining the wet weight of ion exchange resin beads under the conditions in which they are used exDerimentallv. There are advantages to determining the wet rather &n just the dry weight. -The wet weight can be used to calculate the molality or molarity in the resin phase. In this study. surface sulfonated copolymer beads and - glass beads, materials with different surface characteristics, _
THEDRY WEIGHT of ion exchange resins is frequently used to express the exchange capacity or the concentration of species in the resin phase. The method of obtaining dry weights is time consuming and may lead to cracking of the 648
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
I
(I) Discussion with D. H. Freeman, National Bureau of Standards, Washington, D. C.
