orientation of the hydroxyl and nitro groups. 0
0
Similar differences should exist in the tendency of these compounds to form acid-anion complexes and these can be noted from the titration curves of Figure 3. Only a single inflection is obtained with o-nitrophenol, while V I nitrophenol gives an additional inflection a t the titration mid-point. The substitution of a chlorine atom in the ortho position in phenol should not greatly decrease its tendency to form the complex, because the internal hydrogen bond between oxygen and chlorine is relatively weak. The t n o inflections in the titration curve for o-chlorophenol (Figure 3) bear out this view. The tendency of phenol t o form hydrogen bonds is known to be reduced by alkyl substituents on the carbon atoms adjacent to the hydroxyl group. The o-alkyl phenols should thus form m-eaker acid-anion complexes. The titration curves for phenol and three alkvl phenols (Figure 4) shom that the mid-
point inflection disappears as the degree of shielding is increased. The sulfhydryl group has less tendency to form hydrogen bonds than the hydroxyl group. The thiophenol-thiophenolate complex should thus be leqs stable than the phenol-phenolate complex. The single inflection in the titration curve for thiophenol (not shoiin) supports this vien-, The explanation given for the shape of the titration curves for phenols also applies to rarboxylic acids. As s h o m in Figure 5, t n o inflections are obtained for three common carboxylic acids, acetic, formic, and trichloroacetic, in the solvent toluene. The range of potentials spanned by these titrations is noten-orthy-over 800 mi-. for acetic acid, 1100 mv. for formic acid, and 1200 mv. for trichloroacetic acid. Dibasic phenols exhibit similar titration behavior in toluene as in other nonaqueous solvents ( 1 1 ) . The titration curves for the three dibasic phenols shown in Figure 6 can be explained n ithout the assumption of an acid-anion complex. K h e n the two hydroxyl groups are favorably oriented as in the structure shown, internal hydrogen honding causes one of the hydrogens to OH
/
0-H \
become more acidic, while the other decreases in acid strength to the point
where it cannot be titrated a t all. The titration curves for t n o compounds of this general structure are shown in curves A and B of Figure 6. T h e n the hydroxyl groups of a dibasic phenol are oriented so as to preclude the popsibility of internal hydrogen bond formation, the titration curve shows inflections for both hydrogens (Figure 6, C). LITERATURE CITED
(1) Barrow, G. M.>J . d m . C'henl. S i c . 78, 5802 (1956). ( 2 ) Bruss, D. B., Harlow, G. .I.,. ~ A L CHEV 30, 1836 (1958).' (1958). CHEX (3) Cundiff, R. H., LIarliunas, P. C C.,, IIbzrl., bzrl,
28,792 (1956).
C. M.,11-JId, (4) Harlow, G. A., Xohle, C G. E. --I.,IIbid., b i d , 28, 787 (1956). ( 5 ) Harlon, G. A, 1 Wyld. 1-Id. G. E. .I.,Zbid 30. 69 11958). 11058). (6) Kaufman, S.,Singleterry, C. R.,J . Phus. Chem. 56, 604 (1952). (T) La Mer, V. K., Don-nes, H. C., -1. -4m. Cheni. Soc. 53, 888 (19311. (8) Marvott, A. A,, J . Research *Vat/.Bur. Standards 38, 527 (1947). (9) Palevsky, H., Sn-ank, R. IC., Grenchik, R., ReL. Scz. Instr. 18, 298 (1947). (10) Pauling, L., "The Sature of the Chemical Bond," 2nd ed., Cornel1 Cniversity Press. Ithaca, S. T., 1940. (11) Sprengling, G. R., J . A m . Chet,) Scc. 76. 1190 11954). (1%)van 'der Heijde, H. G., A n d . Chzni. Acta 16, 392 (1957). (13) Yerger, A , , Barron-, G. R-., J . A m . Chem. Soc. 77, 44T5 (1955). ~
RECEIVED for review February 25, 1958. Accepted June 23, 1958. Division of A4nalytical Chemistry, 133rd Meeting, AiCS,San Francisco, Calif., April 1958.
Titration of Weak Acids in Nonaqueous Solvents Conductometric Studies DOUGLAS 8. BRUSS and GERALD A. HARLOW Shell Development Co., Emeryville, Calif.
b Conductometric titrations of phenols were investigated in media of low dielectric constant using both alternating and direct current methods. Phenols which are not sterically hindered exhibit conductance mid-point maxima which indicate association of exceptional strength in benzene, xylene, toluene, carbon tetrachloride, pyridine, acetone, and methyl isobutyl ketone. ' Sterically hindered phenols do not exhibit maxima in the conductance curve. Formation of an ion pair between the titrant cation and the hydrogen-bonded acid-anion complex is postulated to account for the observed effects. 1836
ANALYTICAL CHEMISTRY
I
acid-base behavior in nonaqueous solution, potentiometric ( 5 )and conductometric titrations of weak acids were studied. Conductometric methods, particularly those measurements a t low frequencies, have found little application in the determination of weak acidity when compared n-ith potentiometric methods. Several investigators, hon ever, h a r e studied the conductometric behavior of \Teak and strong acids in nonaqueous solution. Lippmaa (12),using a solvent mixture of dimethylformamide, diethylamine, and triethylamine, titrated several phenols a t high frequency using potassium methoxide as the titrant. Similar N IKvE8TIG.4rING
titrations were performed by Lane (11) in ethylenediamine using sodium methoxide as the titrant. Phenols and phenolic mixtures were titrated by Karrnian and Johansson (9) a t high frequencies. They obtained resolution of certain mixtures using potassium methoxide as the titrant in benzenemethanol solution. Other authors have investigated acids of stronger nature in nonaqueous media. Higuchi and Rehm ( 7 ) titrated mixtures of sulfuric and hydrochloric acid in glacial acetic acid using acetate ion as the base. Masui (14) and Ishidate and Nasui (8) applied high frequency methods to the titration of dicarboxylic
acids. They also reported the titration of phenols with strong negative groups attached. Aliphatic amines were titrated in benzene IT ith trichloroacetic acid by La Mer and Downes (10) and later by lIar\-ott (13), while Hall and Spengeninn (4) used glacial acetic acid as the solvent for the conductometric determination of organic acids. Recently Bryant and Wardrop ( 2 ) studied the interaction> bctwcen organic acids and tertiary bases in acetone and acetonitrile. I n this nork, a number of hindered and unhindered phenols n ere titrated in a variety of materials ranging from such inert solvents as benzene and carbon t(>tra(hloride to thP niore polar solx-ents such a tcmpeiat,ure n as held a t 25" + 0. I o C. The water bath \vas mounted on a magnetic stirrer. I n use, t h r coll was sealed with a heavy rubber stopper provided with openings for the infertion of a buret aiid for a nitrogc~n flushing linc. All titrat>ions Jvere c:irried out in 50 nil. of solvent using :i 5-ml. buret calibrated in 0.02-1111. irwrenients. The cell was calibratcd \vitli 0.0100011~aqucous pot'assium chlorid[, prepared from boiled distilled water. Tlic cell m i s t a n t obt&etl using this solution was 0.2T8. The titration curves in the inert solvt.nts (bcnzcnc, toluene, xylcne, and r:irlion tetrachloride) Tvere obtained by tlw apparatus illustrat'ed in Figure 1. The rcsistances of these inert solvents ivere so much greater than those of the iiiic:rmediate dielrctric constant that siwcial twhniques were necessary to iiicasure the extremely small currents flon.ing through t h r cell. A small \-oltage of 1.5 volts was impressed nc~ose tlic cc.11. Thc small direct ciirrents flon-ing in the circuit u-ere mcaeured by obtaining the voltage drop wroes a serirs of high resistances placed in series n-it'h the cell. The voltage drop and the current were measured with :t 1-ibrating reed electrometer, Applied Physics Corp. Model 31. Resistances of IOy, 1O1O! and 1012 ohms could be selected by using the turret sc.lector of the elcctromrter, giving currents ia the range of 10-8 to 10-12 ampere. The vibrating reed electrom1.tc.r has also been used for potentiorwtric titrations in inert solvents ( 5 ) . Because the current across a standard rcsistance n-as measured, the conduct:tiice curves in the inert solvents are shoivii with this ordinate instea,d of conduct,ance. The cell and working battery were shielded in a metal case mounted on a
MICRO-SYRINGE BURET
'
BATTERY
I I
CONDUCTIVITY CELL SHIELDING
I
IO"
I I
I I
1
1 I L
I
I
ELECTROMETER
_________-__
Figure 1 . apparatus
Direct
I
J
current titration
I
I
I
\
i I
A-CHLOROPHENOL\
I
I
1
!
I
1 J
0.8 1.2 1.6 VOLUME O F T I T R A N T . mi. 0.4
Figure 2. Conductance curves of unhindered phenols in pyridine
magnetic stirrer. Connections t,o the turret selector of the vibrating reed electrometer were made wit'h shielded leads. A microsyringe buret, calibrated in 0.001 ml., was used to add the titrant. The same cell was used for both the alternating and direct current measurements. The cell had a radius of 5 em. aiid a height of 10 cm. Two polished platinum electrodes of 1-sq. em. area were mounted in t,he sides of the cell and spaced about 2 mni. apart. The electrodes were placed a t a sufficient height above the hottom of the re11 to allow for the rotation of a magnetic stirring bar. Solvents. The solvents used viere all of reagent grade. The benzene, xylene, toluene, and carbon tetrachloride were used as obt'ained (about 0.02% by weight of water) and also dried over calcium hydride and distilled (about 0.002 % by weight of water). Pyridine was dried over potassium hydroside for several days and fractionated, and the center c u t taken. The alcohols and M o n s were used as obtained. SODILX AMIKOETHOXIDE, O.L\-, prepared by reaction of 2.3 grams of clean sodium metal with 50 ml. of ethanolamine under nitrogen, and dilut'ing t o 1 liter x i t h pyridine. SODIUMETHOXIDE, 0.1S, prepared by reaction of 2.3 grams of clean sodium metal with 500 ml. of ethyl alcohol under nitrogen and diluting to 1 liter with pyridine. SoDiuar ISOPROPOXIDE, O . l S , prepared by reaction of 2.3 g r a m of clean sodium metal n-ith 500 ml. of isopropyl alcohol iindcr nitrogcn and diluting to 1 liter with pyridine. PoTASSIChI HYDROXIDE, 0,1*y, z&LCOHOLIC, prepared a' dcscribd ( 1 ) .
T E T R A B U T T L ~ J I J IHYDROXIDE, OSI~~I O.lAYjALCOHOLIC, prepared as drscribed (6). TETR.~BUT~LSRI~IOSIURI HYDROXIDE, l.O-Y, ALCOHOLIC, prepared by 10-fold 1-acuum cvaporation of 0.1-Y tetrabutylammonium hydroxide ( 5 ) . RESULTS AND DISCUSSION
0.4 0.8 1.2 1.6 VOLUME O F TITRANT, ml.
Figure 3. Conductance curves hindered phenols in pyridine
of
Conductance Curves in Solvents of Intermediate Dielectric Constant. 111solvents of intermediate dielectric constant (12 t o 30), in nhich nppreciable electrical conductivity may exist, titiations n ere performed using t h e alternating curient bridge. Frequently t h e initial bridge reading was unstable because of t h e high cell resistance, but this condition usually stopped after the addition of the first fen- increments of titrant. Titrations of some unhindered phenols in pyridine are slionn in Figure 2. I n each case a conductance maximum n a s obtained a t about the mid-point of the titration curve similar to that noticed by Maryott (13) in conductometric titrations in benzene and dioxane. Naryott attributed the enhanced conductivity of picric, trichloroacetic, and camphorwlfonic acids to the VOL. 30, NO. 1 1 , NOVEMBER 1958
1837
formation of a complrx anion of the type
//o
ClSCC \OH
(-)
0
\c-c
i
h
0. 8
I. 2
’/I
CI?
O/
The larger the size of the anion, the better the shielding of the charge and the greater the dissociation of the complex ion pair into free ions. It is apparent from Figure 2, comparing phenol with p-chlorophenol, m-nitrophenol, and o-ethylphenol, that the magnitude of the conductance midpoint maximum increases with increasing acid strength. m-Nitrophenol has a pK, of 8.38 in water; p-chlorophenol, 9.42; phenol, 9.98; and o-ethylphenol, greater than 10. This maximum is not apparent when both ortho positions of the phenol are occupied. Figure 3 shows the coni-entional conductance curves obtainrd with the sterically hindered 2.6-di-tert-butyl4-methylphenol, 2,4,6-trimethylphenol, and 2,6-dimethylphenol. \\-hen a t least one ortho position is unoccupied, as with o-chlorophenol, 2,4-dimethylphenol, oc4hylpheno1, and the compounds shown in Figure 2, some indication of the conductance maximum in the titration curre is present. Only by substitution of the oxygen atom of the phenol TI ith a sulfur atom (thiophenol) does a conventional conductance curve result for a weak acid with one or more ortho positions open. Figure 4 shows the titration of phenol nith several different titrants. Titration with tetrabutylammonium hydroxide, n hich may be used for the potentiometric determination of phenolic acidity ( 6 ) ,did not show a n end point in any of the solvents of intermediate dielectric constant. Similar titration curves for phenols were obtained in methyl isobutyl ketone and acetone; no end points n ere distinguished in ethanol, methanol, or isopropanol. (In this paper the quaternary ammonium titrants are referred to as hydroxides, though they are probably mixtures of hydroxides and alcoholates.) From these results, it was postulated that ortho groups prevent the formation of some highly conductive species by steric hindrance of the phenolic group. Because the conductance maximum occurs approximately a t the mid-point of the titration curve regardless of the phenol concentration, it was postulated that the conductive species, which is more conductive than phenol or the phenolate ion, is a one to one acid-anion complex which is ion paired to the titrant cation. Thus n-e might have
1838
I
ANALYTICAL CHEMISTRY
0.4
1. 6
VOLUME OF T I T R A N T , ml.
Figure 4. Effect of titrant on conductance curve of phenol in pyridine A. 0.1 N sodium ethoxide 6. 0.1 N potassium hydroxide 0.1 N sodium isopropoxide 0.1 N tetrabutylammonium hydroxide 0.1 N sodium aminoethoxide
C. D. E.
3.
a
3 2.5 W
a
*z
-
2.c
0
X
k. 1.:
2 W 0:
5U
I.(
0.:
I
I
I
I
0.08 0. 10 0. 12 VOLUME OF T I T R A N T . mL
0.04
Figure 5.
Phenol titrations in toluene
A. 2,3,5-TrimethylphenoI 6. 2,4-Dimethylphenol C. D. E.
3,5-Diethylphenol Phenol m-Nitrophenol
Up to the mid-point of the titration, the phenolate ions form a hydrogenbonded acid-anion complex with the untitrated phenol molecules. These acid-anions arc associated \I ith the titrant cation (ion pair) due to the low dielectric constants of the solvents employed. The ion pair of thc acidanion rompleu and tho titrant cation is more conductive than either the phenol or the phenolate ion-titrant cation ion pair, because the ncxgative charge of the acid-anion complex is more spread out in the ring system and holds the titrant
cation more loosely, thus giving :t greater dissociation constant than the other species. Bfter the mid-point of the titration, increasing amounts of simple phenolate ion pairs are present in the solution and the conductance decrcascs until the end point is reached. Ortho substitution in both positions prevents formation of the acid-anion complex and a normal conductance curve rcsults. Tetrabutylanimonium ion s h o w less tendency for ion pair formation than do either sodium or potassium ions ( 3 ) . Because the ion pairs formed between the phenolate ion and the tetrabutylammonium ion, and the acid-anion complex and the tetrabutylaninioniuni ion are both dissociated to about the same eutcnt, there is little relative difference in conductivity and no coliductance mid-point maximum is obtained. A more stable acid-anion coniples should give a greater dissociation constant and a larger conductance maximum. Stronger acids give more stable hydrogen boiids and thus more stable acid-anion complexes. This probably accounts for the rclatiw difference in coiiductance maxima shown in Figure 2. Thiophmol, which is niorc acidic in pyridine than phrnol, and which has both ortho positions open, exhibits a conventional conductance curve similar to those s1ion.n in Figure 3. This may bc explained on the basis that sulfur atoms form much weaker hydrogrn bonds than do oxygen atoms and the acid-anion coniplrx is not formed. It is possible to obtain two distinct inflections in thp conductometric titration curvc’ of a iiiisture of phenol nith a hindered phenol such as the 2,6-di-fertbutyl-4-methylphenol due to the diffcrent species formrd in solution. Conductance Curves in Solvents of Low Dielectric Constant. Conductance curvc”~for :i yarir3ty of phenols in t,oluene a r r shown in Figure 5 . These titrations ivcre p r r f o r n i d using the vibi,ating wcd clectronic~trr doscribed. The potassium anrl sodinni titrants, which were used in the solvents of iiitcrnitdiate dirlrctric constant, precipitated in the solvents of low dielectric constant. Tetrabutylammonium hydroxide [vas found to be a satisfactory titrant for phenols in these solvcnts. Attempts to use other quaternary animonium titrants such as the tetramethyl-, trtraethyl-, tetrapropyl-, aiid cetyltrimcthylaninionium hydroxides were not as successful, because of t,heprecipitation which occurred in some cases. The titrant was prepared a t 1.O.V strength, in order to add as small an amount of isopropyl alcohol as possible to the solvrnt during the course of the titration. The amounts of phenols titrated \Yere such that the amount of
isopropyl alcohol present in the solution a t the end of the titration was about 0.1% by volume. Figure 5 shows that the titration curves obtained in toluene, using the direct current measurement, are similar to those obtained in the solvents of intermediate dielectric constant with the alternating current bridge. Again the maxima are present a t the niid-point of the titration curve. Hindered phenols such as 2,4,6-trimethylphenol do not cxhibit maxima (Figure 6). Hindered phenols (both ortho positions occupied) also do not exhibit a n end point in inert solvents with this titrant, whereas an end point was obtained in solvents of higher dielectric constant using the potassium and sodium titrants (Figure 3). Another striking diff‘ereace between the inert solvents and the solvents of intermediate dielectric constant is that the height? of the conductance midpoint maxima increase in the inert solw n t s as the strength of the acid derreases. Thus the alkyl-substituted phenols show more pronounced niauima than does phenol, or phenols v ith negative groups attached to the ring. This is just the reverse of the behavior in pyridine (Figure 2). The cffcct of small amounts of polar w l w n t s such as water and isopropyl alcohol on the shape of the titration curve is shown in Figure 7 . A very pronounced maximum occurs in dried toluene (0.002% by weight of n ater) which is much less evident in the undricd toluene (0.0275 by w i g h t of nater). As increasing amounts of isopropyl alcohol are added to the solution, the conductance maximum dccreases until. a t 17, isopropyl alcohol, the inaxiniuni has disappeared, and the end point of the titration cannot be detected. Katc>r and isopropyl alcohol are strong hydrogm-bonding solvents and conipetc I\ ith the phenolate ions, thus effectively destroying the acid-anion cmnplex which is responsible for the cwnductancc maximum. Titrations in other inwt solvents such as xylentx, carbon tetrachloride, and Iienzenc. exhibit results similar to those obtained in toluene. Precipitation \vas more evident in xylene and carbon tctracnliloride than in toluene and benzene. 3laxima appear to be more pronounced in xj-lenc than in toluene, but preripitation frequently occurred in xylene after the mid-point had been reached. The titrations in benzene rr ere very similar to those in toluene. Carbon tetrachloride gave the poorest results of all of the incrt solvents. The conductance midpoint maxima ~ e r much e less distinct.
-45 ion pair formation beconws iniportant, large rc4ative differences exist in the dissociation constants of the ion pair betn een the tetrabutylammoniuni ion and the phenolate ion and the acidanion complex. It may he concluded that the sanie sort of ion pair betn een a n acid-anion complex and cation i b rwponsible for the conductance mnuimum in thesc inert solvents as n as prescnt in thc solvclnts of internicdiatc dielcctric constant. LITERATURE CITED
(1) Alni. Soc. Testing llateriale, Philadelphia, Pa., “=\STr\l Standards 011 0.04 0.08 0 . 10 0 . 12 VOLUME O F TITRANT, ml.
Figure 6. Steric effect on conductance curves of phenols in toluene
Petroleum Products and Lubricants,” D 663-55. I). 281. 1954. ( 2 ) Bryant: b. J. R., ir-ardrop, .1.I T . H., J . Chevi. SOC. 895 (1957). ( 3 ) Grunrvald, E., ;\s.IL. CHFXI. 26, 1606 (1934). (4IHal1, Y , F., Spengernan. if-. F., Trans. TtTisconsiri ;Irad. Sci. 30, 51 (1957). ( 5 ) Harlon-, G. .I.,Bruas, D. R., . \ s . ~ L . CHEJI. 30, 183:1 (1958).
( 6 ) Harlow, G . A , , Soble, C. AI., \Tyld, G . E. A , , Ibid., 28, i 8 7 (1956). ( 7 ) Hiuuchi. T.. Rehni. C. 11.. I h i d . . 27. 408 11955). (8) Ishidate, 31.. Masui, 11 , J . Phariii. Soc. Japan 73, 487 (1953). (I)) Karrman, K. J., Johansson, G., Jlikrochznj. d r f n 1573 (1956). (10) La Mer, Y. Ii., Domes, H . C , J . L4m. C h r m . Soc 53. 888 11931). (11) Lane, E. S., d n a l y k 80,’675 (1955). (12) Lippniaa, E. T., J . . A ~ a l . rhem. (C.7.S.S.R.)10, 169 (1955j. (13) Alarvott, A. J . Research -Vat/. Bur. Standards 38,’k27 (194i). 11.1) Masui. 11..J . Pharnj. Soc. J a m r r 75, 1519 (1955). ~
0 . 08 0. 10 0. 1 2 VOLUME O F TITRANT, ml
0.04
Figure 7. Effect of polar solvents on titration of phenol A. 6. C. D. E. F.
0.002% water
0.0270 water
0.01 % isopropyl alcohol 0.02% isopropyl alcohol 0.1 70isopropyl alcohol 1 .O% isopropyl alcohol
I n comparing the results obtained in the two solvent classes, it must be remembered that different techniques mere used in each. S o t only do the solvents differ widely in character, but also the method of measurement and the titrants used ncre different. The success of the quaternary ammonium titrant in the inert solvcmts might be attributed to the fact that the low dielectric constants of these solvents caused greatcr ion pair formation to occur than ivould be possible in the solvents of higher dirlectric constant.
R E C E I ~ Efor D review February 25, 3958. -1ccepted June 23, 1958. Division of Analytical Chemistrv, 133rd Meeting, ACS, San Franrisro, calif., -4pril 1958.
Inexpensive Automatic Recording Thermobalance-Correction I n the articlc on an “Inespensive Automatic Recording Thermobalance” [ASAL.CHEM.30, 56 (195S)l. referelwe to a n article describing a similar instrument n-as inadvertently omitted. The article was by P. L. Waters. Coal Rescarrh Section, Coninionnealth S G n tific and Industrial Organisation, Australia, and appeared in .\’atitre (/,ondon) 178, 324 (1956). and Jownal of Scientific Instruments, 35, 41 (1958).
\VI.:~LEI.\V. KESDLASDT Department of Chemistry and Chemical Engineering Texas Tec~hnologicalCollege Luhhocak, Tes.
VOL. 3c), NO. 1 1 , NOVEMBER 1958
1839
