Differential Titration of Acids and Very Weak Acids iln Petroleum with Tetrabutylammonium Hydroxide and Pyridine-Benzene Solvent B. E. Buell Union Research Center, Union Oil Co. of California, &ea, Calij:
Titration using tetrabutylammonium hydroxide and 1 to 1 pyridine-benzene as solvent is applied to petroleum samples. Compound classes known to be present in petroleum that titrate in this system are carboxylic acids, phenols, mercaptans, carbazoles, and amides such as 2-quinolone. Results are shown for crude oil and shale oil fractions. Routine applications for the determinations of additives are also cited.
TITRATION FOR ACIDS in petroleum and petroleum products routinely employs alcoholic potassium hydroxide as titrant and mixed 2-propanol-toluene-water as the sample solvent (1). Only acids with a pK, of about 10 or less (strong phenols, carboxylic acids, etc.) will titrate in this system. Weakly acidic compounds, such as hindered phenols and carbazoles, do not titrate. These very weak acids can be determined and differentiated from other acids by titration with tetrabutylammonium hydroxide (TBAH) in pyridine. The use of TBAH as titrant was first reported by Deal and Wyld (2) and was further studied in detail by Harlow and Wyld (3),Harlow and Bruss (4,and van der Heidje and Dahmen (5). Cundiff and Markunas (6) independently developed the same titration system. Although its use has since been reported extensively for pure compound studies and some practical determinations, applications to petroleum samples have been limited. The only reported example of the titration of acidic hydrogen in nitrogen compounds, such as carbazole, is that of Jewel and Hartung (7) employing sodium aminoethoxide in ethylamine as titrant. Little is known of the weakly acidic compounds in petroleum, other than that they are probably heterocylic compounds. A way of differentiating between weak and very weak acids in petroleum by chemical type would be valuable for classification of oxygen and nitrogen compounds and for evaluating treating and separating processes. This report discusses a titration technique using TBAH for such analyses and demonstrates its application to petroleum fractions. The determination of additives used in petroleum products is also described. EXPERIMENTAL
Reagents. All reagents conform to ACS reagent grade specifications unless specified otherwise. Tetrabutylammonium hydroxide (TBAH), 1M solution (1) Am. Soc. Testing Materials, Philadelphia, Pa., “ASTM Standards, 1964,” Part 17, p. 258. (2) V. Z. Deal and G. E. A. Wyld, ANAL.CHEM., 27,47 (1955). (3) G. A. Harlow and G . E. A. Wyld, Ibid.,30,73 (1958). (4) G. A. Harlow and D. B. Bruss, Ibid.,p. 1833 (1958). (.5,) H. B. van der Heidie and E. A. M. F. Dahrnen, Anal. Chim. Acfa, 16, 378, 392 (1957). (6) R. H. Cundiff and P. C. Markunas, ANAL.CHEM., 28,792 (1956). (7) D. hl. Jewel and G. H. Hartung, J. Chem. Eng. Data, 9,297 (1964).
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ANALYTICAL CHEMISTRY
in methanol. Keep refrigerated. Available from Southwestern Analytical Chemicals, Austin, Tex. Standard tetrabutylammonium hydroxide solution, 0.03N. Dilute 3 ml of 1M methanol solution of TBAH to 100 ml with reagent grade benzene. Prepare fresh every 2 months (the solution prepared as described is stable for at least 2 months). Apparatus. Beckman Zeromatic pH meter and Beckman combination electrode No. 39142 with methanolic electrolyte solution (6). Titration Cells, 7 cm tall X 3 cm in o.d., made from glass tubing and equipped with a polyethylene lid with holes for the electrode, a buret tip, and a tube for nitrogen purging. Procedure, described by Cundiff and Markunas (6), modified for titration of small volumes. Dissolve a sample containing about 0.01 meq of acids in 10 ml of benzene, add 10 ml of reagent grade pyridine, and titrate with 0.03N TBAH solution. Near the end point add the titrant in 0.02ml increments. For the determination of strong acid additives in the presence of phenols, calculate their concentration from the first break in the titration. Calculate total acids from the final break. For correlating half-neutralization potential (HNP), adjust the mv scale to 600 for the HNP of 0.01 meq of benzoic acid and relate all other HNP’s to that obtained for benzoic acid. Carbazole may be used as a secondary reference material for very weak acids. Pure Compound Titrations. For simplification, the term “acids” refers here to all compounds titrating in the TBAHpyridine system. “Very weak acids” are those weaker than phenol, with emphasis on acids with pK, greater than 10. “Weak acids” are compounds with a pKa range of about 3 to 10. In addition to carboxylic acids and phenols, other compound types which will titrate and are found in petroleum are mercaptans, carbazoles, and amides such as 2-hydroxyquinoline (2-quinolone) that are not N-substituted. As all amides titrate as very weak bases in acetic anhydride, a titration for very weak bases and a titration for very weak acids as described here provide a simple means of distinguishing N-substituted amides. In addition to compound types known to occur in petroleum, various additives employed in the petroleum industry and other types of compounds will titrate. Compounds such as benztriazole, anthrone, acetylacetone, nitroethane, dextrose, and highly hindered phenols such as di-tert-butyl-p-cresol (DBPC) will titrate. The titration system (as indicated in the cited references) has excellent differentiating characteristics, especially for multifunctional acids-for example, citric acid and phosphoric acid give three titration breaks each (6). A mixture of 2naphthoic acid and 2-naphthol is easily resolved. Above a pKa of about 10 the differentiating qualities become poorer. The system also enhances the strength of weaker acids coinpared to carboxylic acid-for example, thiophenol (PIC, = 6.5) apparently becomes stronger than benzoic acid (pKa = 4.2). Campoundc such as anthrone and benztriazole (pK, =
I400
r
1000I-
mv
/-
8ooE 600
400
0.2 I I 0.4 I ml TITRANT
'
1
Figure 1. Titration curves Carbazole b. Heavy gas oil c. Molecular distillate Mv scales displaced
a.
Table I. Extrapolated pK, Values for Weak Compounds L.it. pK, Extrapolated pK. Compound (water) (pyridine) Benzoic acid 4.2 a Thiophenol 6.5 6.9" QHydroxypyridine 11.1 8 Benzotriazole 8.6 8.2" Anthrone ... 8 Acetylacetone 8.2 9 2-Naphthol 9.9 10.2" 2-Hydroxyquinoline 11.7 11 Nitromethane 10.2 12 2-Nitro-1-butanol ... 12 1,2-Benzcarbazole ... 12 ... 12 2,3-Benzcarbazole Carbazole ... 12 4,4'-Methylenebis(2,&di-
rerr-butylphenol) Di-rert-butyl-pcresol Nonylphenol Dextrose a
8.6) approach benzoic acid in strength. The pK, for carbazole and benzcarbazoks appears to be about 12 in this system, equal in strength to very weak phenols such as DBPC. The compound 9-methylcsrrbazole does not titrate. Exceptions are nitro alkyls, which become weaker. Snyder and Buell (8) give a further discussion and tabulation of extrapolated pK, data obtained by the calibration technique of Streuli (9) for weak compounds (pK, range of 6 to 13). Typical extrapolated values and some literature pK, data for weak compounds are shown in Table I. When applied to a variety of compound types, serr iquantitative accuracy is expected for extrapolated values for certain compounds. A titration curve for carbazole is shown in Figure 1, a. If pyridine is replaced by methanol, carboxylic acids and thiophenol will titrate but acetylacetone will not. Despite the lack of resolution for compounds in the pK, range 10 to 13, this titration system can be very useful in conjunction with separation techniques and/or chemical reactions for differentiating types of very weak acidsfor example, techniq ies such as elution absorption chromatography and ion exchange can be used to separate carbazoles, weak phenols, and amides. Sample Titrations. The titration procedure was developed mainly for following acidic compound classes in various crude oil fractions bt:fore and after separation by ion exchange and elution adsorption chromatography. A Wilmington, Calif., crude oil was used for the separation studies. Three fractions were obtained from a distillation of the original crude and two from a molecular distillation of the residuum. Further analyses of the heavy gas oil (HGO), by ion exchange and chromatographic procedures, which will be discussed in subsequent papers, established that some of the acidic compound classes present were carboxylic acids, indoles, carbazoles, and benzocarbazoles (IO), and amides such as 2-hydroxyquinoline. Titration results fcsr ihe distilled fractions are listed in Table 11. A typical t!tir,tion for the HGO is given in Figure 1, b, showing two krcdks representing weak and very weak acids (pK, = 1 2 S;j. The concentration of weak acids is
...
12 12.0" 12 12.4"
12.2
...
12.2
Used for calibration to obtain extrapolated pK, values.
Table 11. Acidic Substances in Wilmington Crude Oil Fractions Total acids, B.P., "C Fraction meq/g Original crude ... 0.22 0.05 200-370 Light gas oil Heavy gas oil 370-540 0.20 Residuum 540 0.39 Molecular distillate 540-650 0.36 0.53 Molecular distillate residuum 650
+
+
0.05 meq per gram compared to 0.20 meq per gram of total acids. Titration breaks are small and become progressively smaller in heavier fractions of crude oil and shale oil. A typical titration curve for a difficult sample is shown in Figure 1, c for the crude oil molecular distillate with a boiling range of about 540" to 650" C. If the titmtion system is not optimized (electrode response becomes poor), the titration of heavy fractions may appear to consume titrant but gives no break. In this case, a practical procedure is to draw a tangent to the upper part of the titration curve where a break would normally be expected. As indicated by Katz and Glenn ( I I ) , another approach that seems to work in this case is the addition of a known, small amount of a weak phenol such as di-tert-butyl-pcresol (DBPC). This may be slightly stronger than the weakest substance present in the sample and seems to magnify the break. However, this technique appears to give slightly low results for certain samples. The addition of an acid slightly stronger than DBPC gives even lower results. Despite this problem, precision of the procedure was good even for a more difficult sample such as the molecular distillate. Over a period of 3 days, six titrations, adding DBPC to enhance end points, gave an average of 0.33 meq per gram of total acids with an average deviatiim from the mean of 0.01 meq per gram. Over a period of a month, six titrations without DBPC gave 0.37 meq per grain with an average deviation from the mean of 0.015 meq per g a m . For a shale oil rich ~ 0.46 meq per gram in phenols, six titrations in one & . ~gave ~-
..-_._^_^__.^___I._____
( I I ) M. Katz and K, A. Glenn, AXAL.. CHFM., 24, ! ? 5 7 ( i 9 5 2 j
0.06 0.19 0.11 0.14 0.08
