SuIfolanes s Solvents for Potentiometric Titrations D. H. Morman and G . A. Harlow Shell Development Co., Emeryville, Gal$
POTENTIOMETRIC TITRATION of acids and bases in nonaqueous solvents has proved to be a very useful technique both for analytical purposes and for theoretical studies. The choice of solvent for a given problem will depend on the characteristics of the solvent such as its acidity and basicity, capability for hydrogen bonding, dielectric constant, and other physical properties, Many solvents have been used successfully for the resolution of mixtures of acids or bases. Some of these solvents also permit the titration of very weak components. Only a few solvents have been reported, however, which have the broad capability of resolving both acid and base mixtures, and which are useful for both weak component and strong component determinations. Bruss and Wyld ( I ) demonstrated the versatility of methyl isobutyl ketone (MIBK) as a wide-range solvent for the titration of acid and base mixtures. MIBK permits the resolution of certain strong acid mixtures and gives useful inflections for very weak phenolic acidity. However, Cundiff and Markunas ( 2 ) reported that strong acids react with MIBK to give low results. Acetone and acetonitrile also provide a wide potential range (3) for the titration of acids and bases, but they are not highly satisfactory for phenolic acidity ( 4 ) and they also react somewhat with strong acids (2). We have recently become interested in several of the sulfolanes, which are aprotic solvents of moderately high dielectric constant (5), and have studied the titration characteristics of sulfolane, 3-methylsulfolane (3-MS), and 2,4-dimethylsulfolane (2,4-DMS). They provide an extremely wide potential range for the differentiation of both mixtures of acids and bases. The very weakly acidic and basic character of these solvents makes them suitable for both very weak acid and very weak base titrations while at the same time they are nonleveling for strong acids and bases. The low hydrogen bonding capability of the sulfolanes makes them useful in the study of homoconjugated AHA- and BHBf complexes. This same property, however, leads to a steep slope in the plateau region of the titration curve of those acids with a high tendency to associate. This can have a detrimental effect on the resolution of some mixtures. EXPERIMENTAL
The titrations were performed manually using a ShellPrecision Scientific Co. dual titrometer. Titrations were carried out in 20 ml of solvent under a blanket of nitrogen. The electrode holder served as a beaker cover. A general purpose glass and calomel electrode pair (Beckman 41263 and 40463, respectively) were used. The calomel electrode was connected to the titration solution by a nonaqueous bridge of 3-MS saturated with tetraethylammonium perchlorate in order to avoid introducing water and potassium ion, both of which would depress the potential span. The (I) D. B. Bruss and G. E. A. Wyld, ANAL.CHEM., 29,232 (1957). (2) R. H. Cundiff and P. C. Markunas, Zbid.,30,1447 (1958). (3) . I H. B. van der Heiide and E. A. M. F. Dahmen. Anal. Chim. Acta, 16, 378 (1957): (4) N. T. Crabb and F. E. Critchfield. Talunta. 10.271 (1963). ( 5 ) E. M. Arnett and 6.F. Douty,'J. Am.'Ch& Soc., 86, 409
(1964).
Table I.
Properties of Sulfolanes
Sulfolane
3-MS
cH3T7 LJ 6 o" "o
2,4-DMS
*
c . 3 ~ C H '
od
MW b.p., 'C m.p., "C Dendty, glml Viscosity (3OoC),c.p. Dielectric Constant
120 285 28 1.26 10.3
I34 276 0 1.19
0 148 281 -3.3 1.14 7.9
44
saturated KCl solution in the electrode was replaced with 0.1M tetramethylammonium chloride to avoid precipitation of potassium perchlorate in the ground glass junction, The titrants were delivered with either a 5-ml buret graduated in 0.01-ml divisions or a microsyringe buret (Micro-Metric Instrument Co., Cleveland, Ohio) with 0.001-ml divisions. A 0.2N solution of perchloric acid in dioxane was used for the titration of bases. A 0.2N solution of tetra-n-butylammonium hydroxide (TBAH) in isopropyl alcohol, prepared by the ion exchange procedure as described by Harlow, Noble, and Wyld (6), was normally used for the titration of acids. For titrations in which it was desired to obtain the maximum potential span or to accentuate the effects of ion association, a concentrated 1.2N solution of TBAH was used to reduce the amount of alcohol introduced. It was prepared by a reduced temperature vacuum evaporation of the 0.2N titrant and was stored in a freezer under nitrogen to avoid decomposition and COzpick-up. All compounds titrated were reagent grade materials used as received with the exception of guanidine. This base was prepared from the hydrochloride salt by adding potassium hydroxide in isopropyl alcohol, filtering off the precipitated potassium chloride, and evaporating the solvent. Sample sizes were chosen to give titrations of approximately 1.0 ml of 0.2N titrant or 0.10 ml of the 1.2Ntitrant. The sulfolane was obtained from Shell Chemical Corp. and the 2,4-DMS from Shell Development Co.; 3-MS was obtained from Phillips Petroleum Co. All three of the solvents contain acidic impurities and require purification before use. Although they can be fractionally distilled under vacuum, they can be more easily purified for titration purposes by passing them through a column of freshly activated alumina (Alcoa F-20 chromatographic alumina). 2,4-DMS also contains a basic impurity which can be removed with an additional section of an anhydrous, strong acid ion exchange resin, Amberlyst 14, added on top of the alumina column. The structures and some of the properties of the sulfolanes are shown in Table I. Both 3-MS and 2,4-DMS are liquids at room temperature. Pure sulfolane itself is a solid at about 28" C. It can be easily used, however, by storing it in a slightly warm location such as the top of an oven. The addition of the sample and the first increments of titrant lower the melting point sufficiently to avoid freezing. The viscosity of the solvents is roughly twice that of tert-butyl alcohol. The dielectric constant of sulfolane itself is a (6) G. A. Harlow, C. M. Noble, and G. E. A. Wyld, ANAL.CHEM., 28, 787 (1956). VQL. 39, NO. 14, DECEMBER 1967
o
1869
1200 Solvent: 3-MS T i t r a n t : 1.2N TBAH
I-
/---
800
6 00
400
-600k
HClO,
-800 0
0.02
0.04
0.06 0.08 0.10 Volume of T i t r a n t , ml
0.12
0.14
C
5
Solvent: 3-MS Titrant: 0.2N TBAH
Figure 1. Titration of a mixture of acids 0
moderately high 44 (5). Although data are not available for the other two, it is expected that their dielectric constants are close to that of sulfolane. The solubility of the common acids and bases which were titrated in these solvents was good with the exception of some dicarboxylic acids, such as phthalic and succinic acids, which required heating to achieve solution. On the basis of physical properties, 3-MS is the preferred solvent for titrations. It has the advantage over sulfolane of being a liquid at room temperature, and it is more easily purified than 2,4-DMS. No significant differences were found in the titration characteristics of the three solvents; therefore, 3-MS was used for most of the investigation. There is a considerable difference in the price of the three sulfolanes. It is possible to combine the advantages of the low cost of sulfolane and the lower freezing point of 3-MS by using a mixture of the two solvents. Sulfolane containing about 5 % of 3-MS has a freezing point near 13" C. RESULTS AND DISCUSSION
The sulfolanes are very weakly acidic as well as very weakly basic and therefore have extremely low autoprotolysis constants (5). This is readily apparent from the extremely wide potential range which is available between solutions containing excess strong acid and excess strong base (- 800 to 1200 mV). The upper potential is limited by the amount of isopropyl alcohol introduced with the titrant and may reach only 1000 to 1100 mV in an actual titration. This potential span of 1800 to 2000 mV is one of the largest ranges available in any titration solvent and makes the sulfolanes excellent differentiating solvents both for mixtures of acids and for mixtures of bases. Titration of Acids. Several of the significant characteristics of the sulfolanes as titration solvents are demonstrated in Figure 1 with the titration of a mixture of acids in 3-MS using concentrated 1.2N TBAH as the titrant. The large potential span of 1800 mV easily permits the resolution of
+
1870
a
ANALYTICAL CHEMISTRY
0.2
0.4
0.6 0.8 Volume of Titrant, ml
1.0
1.2
4
Figure 2. Titration of dicarboxylic acids. Curves shifted vertically for clarity
five acids. The acids selected for this curve have low hydrogen bonding capability because resolution depends on the slopes of the plateaus, which in turn depends on the tendency toward homoconjugation. Thus, acid-anion hydrogen bonding not only reduces resolving power but it can also provide information on the nature of the acid as will be discussed later. Because the sulfolanes are so weakly basic, strong acids are not leveled, permitting the resolution of certain strong acid mixtures such as perchloric and nitric acid. Perchloric and hydrochloric acids can also be quite well resolved, while perchloric and sulfuric acids are only partially resolved. Unlike most other solvents capable of resolving strong acids, there is no apparent reaction between strong acids and the sulfolanes except for possible discoloration due to impurities. The very weakly acidic character of the sulfolanes permits the titration of very weak acids as demonstrated here by the good inflection obtained for a very weak hindered phenol (2,6-di-tert-butyl-4-methylphenol). The titration of several dibasic acids is shown in Figure 2. These titrations were performed using the 0.2N TBAH titrant and a calomel reference electrode placed directly in the titration solution without the use of the nonaqueous bridge. The curve for succinic acid has been shifted up 200 mV and the curve for oxalic acid has been shifted down 200 mV in relation to the curve for phthalic acid for clarity. The actual half neutralization potentials for the first equivalent of each acid were oxalic acid (- 157 mV), phthalic acid (- 180 mV), and succinic acid (+30 mV). Titration of Bases. The titration of a mixture of bases in 3-MS with perchloric acid is shown in Figure 3(a). This figure again shows the wide potential range available and demonstrates the resolution of five basic components. The
1200
C4
I
\
Titrant: O.2N HClO,
Volume of Titrant, ml
Figure 3. Titration of (a) a mixture of bases, and (b) benzyldimethylamine oxide -800
2
0
weakly basic character of 3-MS is shown by the ability to titrate a very weak base such as o-chloroaniline which has a pK,(H2Q) of 2.6. It is also possible to titrate caffeine which has a pK, of less than 1 . The titration of several other bases is shown in Figure 4. Hydrogen Bonding and Ion Association. The sulfolanes have moderately high dielectric constants but they have very little tendency to solvate ions or to form hydrogen bonds with other compounds (5). This makes them interesting solvents for theoretical studies involving homoconjugation, Le., the formation of acid-anion and base-cation complexes as indicated by the general equations:
+ A-+ AHAB + BH+ -+ BHB+
HA
0.2
0.4
0.6 0.8 1.0 Volume of Titrant, m l
1.2
4
Figure 4. Titration of bases
(1)
(2)
Figures 5(a) compares the titration of phenol and 2,6-ditert-butyl-4-methylphenol. The latter compound, in which the phenolic hydroxyl group is highly hindered by the tertiary butyl groups, shows a normal titration curve for a monobasic phenolic acid. Phenol, on the other hand, with no shielding of the hydroxyl group, shows two inflections although it is a monofunctional acid also. This type of behavior has been reported previously in certain solvents by van der Heijde (7), and by Harlow and Bruss (8). It is explained on the basis of the formation of an acid-anion complex (AHA-) during the titration as indicated by Equaton 3. OH
0-
0
Volume of Titrant, m l
(7) H. B. van der Heijde, Anal. Chim. Acta, 16, 392 (1957). (8) G. A. Harlow and D. B. Bruss, ANAL.CHEM., 30, 1833 (1958).
Figure 5. Titration of (a) phenol and 2,6-di-tert-butyl4-methyl phenol with 1.2N TBAH and (b) formic acid with 0.2N TBAH VOL. 39,
NQ. 14,
DECEMBER 1967
e
1871
At the beginning of the titration, the phenol is converted to the phenolate ion. Because the 3-MS solvent has little tendency to form hydrogen bonds, it does not compete with the association of the phenolate anion with the remaining phenol by hydrogen bonding. This increases the apparent acid strength of the free phenol while at the same time forming the more weakly acidic phenol-phenolate complex. Thus at the midpoint of the titration, there is an inflection for the completion of the free phenol titration and the beginning of the acid-anion complex titration. Behavior of this type is useful in studying the degree of hindering of certain acids. The titration of formic acid also shows definite evidence of complex formation as can be seen in Figure 5(b). The curve for the titration of an amine oxide in 3-MS with perchloric acid is shown in Figure 3(b). Again two inflections are obtained for a monofunctional compound, with the first inflection corresponding to one-half theory and the final inflection occurring at the theoretical end point. The titration curve indicates a remarkable difference in the basicities of the two species being titrated. The half neutralization potential (HNP) of the first specie indicates a base almost as strong as guanidine (pK, 13.6). The HNP of the second
base is close to that of pyridine whose pK, of 5.2 is approximately that expected for the amine oxide. A full explanation has not yet been developed for this behavior, but it may involve the formation of a base-cation complex (BHBf) in a manner analogous to the acid-anion complex formed in the titration of phenols. The double inflection for the amine oxides is a function of the hydrogen bonding capability of the titration solvent and also the ion association tendency of the anion of the acid titrant. Further studies are being made and will be the subject of another report. At their present cost, moderate purity, and limited availability, it is not expected that the sulfolanes will become widely used as solvents for routine titrations. However, because of the very large potential span available, their moderately high dielectric constant, and low hydrogen bonding capability, the sulfolanes will be useful for special applications and theoretical studies. RECEIVED for review July 27, 1967. Accepted September 14, 1967. Presented in part at the 20th Annual Summer Symposium on Analytical Chemistry, Claremont, Calif., June 1967.
High Resolution Proton Magnetic Resonance of Liquids Adsorbed QTP a Pyrogenic Silica J. H. Pickett and L. B. Rogers Department of Chemistry, Purdue University, Lafayette, Ind. 47907
NUCLEARMAGNETIC RESONANCE STUDIES of molecules adsorbed on solid surfaces have usually necessitated the use of broad-line or spin-echo techniques because of short spinspin relaxation times. For example, water adsorbed on silica has been studied by Zimmerman et al. ( I ) , by Woessner ( 2 , 3 ) , and by Kvlividze (4, and benzene adsorbed on silica has been studied by Woessner (5). Relaxation times reported by these authors would imply line widths in the range of 500 Hz. The only study of adsorbates on silica in which high resolution NMR spectra were observed is that reported by Karagounis (6), who found that mesitylene adsorbed on silica gave sharp resonance lines (twice the line width of the liquid) at a two-layer surface coverage and considerably broader lines as the coverage was increased to thirty layers. However, Karagounis gave no details about the type of silica used. Recent work in this laboratory has shown that high resolution spectra can be obtained for a variety of organic molecules adsorbed on a pyrogenic silica, but that other types of silica do not give similar results. EXPERIMENTAL
Apparatus. Spectra were taken at ambient temperature with a Varian A-60A NMR spectrometer. Reagents. The following silica adsorbents were studied : Cab-0-Si1 M-5 (7) and Aerosil 2491-380 (8), which are extremely fine, nonporous, pyrogenic (fumed) silicas; Quso F 22 (P), a low porosity, microfine, precipitated silica; and Davison Grade 81 (IO), a high porosity, precipitated silica gel, N o drying or other pretreatment was necessary. Procedure. To prepare the samples which had low values of 0 (0 = calculated number of adsorbed layers), the silica 1872
e
ANALYTICAL CHEMISTRY
was packed in a standard 5-mm 0.d. NMR sample tube and adsorbates were added as vapors or liquids, the same results being obtained in either case. However, for higher 6 (6 > 10) preparations, it was necessary to add liquid to a known weight of silica in a larger container, mix thoroughly, and allow several hours for equilibration before the mixture was added to a sample tube. Chemical shifts were measured relative to tetramethylsilane, which was added directly to the sample to give an internal reference. Line widths are reported as the full width at half-maximum height (FWHM). RESULTS
Preliminary studies with mesitylene adsorbed on two different types of silica at low surface coverage (0 = 2) showed broad lines (FWHM = 100 Hz) for Davison 81 silica, but narrow lines (FWHM = 4 Hz) for Cab-0-Si1 M-5. As more mesitylene was added, the lines got much broader (100 Hz) on Cab-0-Si1 and only slightly narrower (90 Hz) on Davison 81. Similar behavior was observed for benzene, acetone, and acetaldehyde adsorbed on these two silicas. (1) J. R. Zirnmerman, B. G. Holmes, and J. A. Lasater, J. Phys. Chem., 60, 1157, (1956); 62, 1157 (1958). (2) D. E. Woessner and J. R. Zimmerman,J . Phys. Chem., 67,1590 (1963). (3) D.E.Woessner, J. Chem. Phys., 39, 2783 (1963). (4) V. 1. Kvlhidze, Dokl. Akad. Nazik SSSR, 157, 158 (1964). (5) D. E. Woessner, J. Phys. Chem., 70, 1217 (1966). (6) G. Karagounis, Nature, 201, 604 (1964). (7) Cabot Gorp., Oxides Division, Boston, Mass. (8) Degussa, Inc.,Pigments Div., Kearney, N. J. (9) Philadelphia Quartz Co., Philadelphia, Pa. (10)Davison Chemical Co., Baltimore, Md.
