Conductometric Titrations in 1,1,3,3=Tetramethylguanidine Study of Weak Acids, Indicators, and Nitroaromatic Compounds Melvin L. Anderson1 and Robert N. Hammer Chemistry Department, Michigan State University, East Lansing, Mich. 48823 The feasibility of conductometric titrations of some representative acids in 1,1,3,3-tetramethylguanidine (TMG) has been demonstrated using tetra-n-butylammonium hydroxide as standard base. Colors of 21 indicators in acidic, neutral, and basic TMG solutions were determined and eight of these are recommended for acid-base titrations. Conductance titration curves are presented and discussed. Single end points were obtained for p-toluenesulfonic and benzoic acid, for phenol, and for ammonium and tetramethylguanidinium bromide. Salicylic acid appears to be a dibasic acid in TMG; maleic acid is dibasic and citric acid tribasic in TMG. The multiple end points found for 0-nitrophenol, 1,3-dinitrobenzene, and 1,3,5-trinitrobenzene are consistent with addition of hydroxyl ions to the aromatic ring.
ALTHOUGH 1,1,3,3-TETRAMETHYLGUANIDINE (TMG) Was first prepared many years ago ( I ) , its properties are not well known. Recently, however, some work has been reported using TMG as a ligand in coordination complexes (2) and as a solvent for the titration of weak acids (3-5). The latter studies have demonstrated the feasibility of potentiometric and indicator titrations in TMG as a nonaqueous solvent while the work reported here covers conductometric titrations of some selected compounds. In this laboratory a general study of 1,1,3,3tetramethylguanidine as a nonaqueous solvent ( 6 ) as well as an investigation of TMG as a dissociating solvent for some 5-substituted tetrazoles (7) have been done. EXPERIMENTAL Apparatus. Conductometric titrations in TMG were done with a Serfass Model RC M15 bridge. Solutions were contained in a 48- x 87-mm beaker closed with a rubber stopper through which passed the tip of a 10-ml buret, a nitrogen inlet, and a nitrogen outlet. Sample and titrant solutions were protected from the atmosphere with drying tubes containing Ascarite and Drierite. During titration, solutions were mixed with a small magnetic stirrer. No temperature control was used because it was found that the small temperature increases involved in the titrations had negligible effect on the measured conductances. The two conductance cells used interchangeably in these studies have been described (6). Chemicals. Stock TMG, purified as described previously (6), was used as the titration solvent. Methanol solutions 1 Present address, Rocky Flats Division, The Dow Chemical Co., Golden, Colo. 80401.
(1) A. Berg, Compr. Rend., 116, 887 (1893). (2) R. Longhi and R. S . Drago, Inorg. Chem., 4, 1 1 (1965). (3) T. R. Williams and J. Custer, Talunta, 9, 175 (1962). (4) T. R. Williams and M. Lautenschleger, ibid., 10, 804 (1963). (5) J. A. Caruso, G. G. Jones, and A. I. Popov, Anal. Chim. Acta, 40,49 (1968). (6) . , M. L. Anderson and R. N. Hammer, J . Chem. Eng. Data, 12, 442 (1967). (7) J. A. Caruso, P. G. Sears, and A. I. Popov, J . Phys. Chem., 71, 1756 (1967).
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ANALYTICAL CHEMISTRY
I
I
I
I
MOLES OH-/ MOLE AClO
Figure 1. Conductometric titrations of ptoluenesulfonic acid A . 0.0200M CHICBHSO~H E . 0.0104M CH~C~H~SOIH
Indicator color change a. Curcumin b. Phenolphthalein c. 2-Nitrodiphenylamine
of the base, tetra-n-butylammonium hydroxide (Eastman), were used in 0.2 to 0.8M concentrations standardized with aqueous benzoic acid or potassium acid phthalate. Only about 3 ml of base were required for 50 or 60 ml of acid solution in each titration, but corrections were made for dilution nevertheless. The conductances of given amounts of methanol in TMG were determined and a standard curve drawn ; the appropriate (and relatively small) methanol conductance values were then subtracted from measured conductances in all titrations. When indicators were used in titrations, they were added as oven-dried (110' C) research-grade materials. Initial indicator concentrations were approximately 0.0001M except in a few cases where a somewhat higher concentration was necessary to develop sufficient color intensity. The acids used in the conductometric titrations were stock materials purified, if necessary, by sublimation or distillation until their melting or boiling points agreed closely with literature values. p-Toluenesulfonic acid was used as the monohydrate as it was difficult to dehydrate completely (8). Experiments showed that water of hydration from the monohydrate had no effect on the results of the acid-base titrations. The method of preparation of tetramethylguanidinium bromide has been described (6). Procedure. Acid solutions in TMG were prepared under a dry nitrogen atmosphere. On removal of the solution from the drybox, 50 or 60 ml were pipetted into theconductance cell, the cell was stoppered, and dry nitrogen bubbled through the stirred solution for 30 minutes. The titration itself was then carried out with continued stirring and nitrogen purging. Solution temperatures ranged from 24.5 O to 26.0' C . (8) A. I. Popov and J. C . Marshall, J . Inorg. Nucl. Chem., 19, 340 (1961).
Table I.
Indicator Crystal violet *Tropeoline 00 p - Aminoazobenzene Bromphenol blue Alizarin red S Bromthymol blue Neutral red Metacresol purple *Phenolphthalein *Thymolphthalein *Curcumin Alkali blue Methyl blue * Clayton yellow Basic fuchsin 2,4Dinitrodiphenylamine * 2,4-Dinitroaniline 2,CDinitrophenylhydrazine
* 2-Nitroacetanilide * 2-Nitrodiphenylamine
CNitroaniline * Exhibit significant color change.
Colors of Indicators in 1,1,3,3-Tetramethylgnidine Neutral (TMG) Acidic (TMGH+) Yellow Yellow Yellow Yellow Yellow Yellow Blue Blue Blue-violet Blue-violet Blue Blue Yellow Yellow Blue-black ( ?) Blue-black ( ?) Colorless Colorless Pale geen Pale green Blue-violet Red-violet Orangebrown Red-brown Colorless Colorless Orange-red Orange-red Orange-yellow Orange-yellow Red-orange Red-orange Pink Orange-pink Green-black (?) Green-black ( ?) Pale yellow Pale yellow Yellow Yellow Green-yellow Green-yellow
Basic (OH-) Orange-yellow Violet Yellow Blue Blue Blue Yellow Blue-violet ( ?) Pink-violet Blue Yellow Green-black Yellow-black Red-violet Orange-yellow Orange-brown Red-violet Violet-brown (?) Orange Violet Yellow
RESULTS AND DISCUSSION
Table 11. Absorption Maxima in Spectra of Indicators Behavior of Indicators. Twenty-one indicators were tested, each in pure (neutral) TMG as well as in acidic and basic solutions. The acidic solutions were either 0.001 or 0.01M in TMGH+Br-. The basic solutions were prepared by addition of sufficient 25 tetrabutylammonium hydroxide in methanol to give 0.01M base in TMG. The presence of the small amount of resultant methanol had no effect on the color of the indicators. The indicators and their colors are listed in Table I. Eight of them, marked with an asterisk, exhibited significant color changes on going from a neutral to a basic solution. These eight indicators should therefore be usable in acid-base titrations. The appreciable color change shown by phenolphthalein appears to be in some disagreement with reported results of Williams and Lautenschleger ( 4 ) on potentiometric titrations in TMG. The latter workers did find in addition that alizarin yellow and azoviolet gave good visual end points in titrations of benzoic acid. Agreement between the color changes of some of the indicators listed in Table I and the end points in conductometric titrations will be discussed below. Metacresol purple and 2,4-dinitrophenylhydrazineare listed in the table as having questionable colors because of changes in color with time and lack of reproducibility of the colors. Perhaps these two indicators react with the solvent. The indicators from crystal violet to basic fuchsin in Table I cover the aqueous pH transformation range from about pH 1 to 13. The remaining compounds, all nitroaromatic dyes, have pK, values from about 14 to 18.5 as determined in mixed aqueous-nonaqueous systems (9). Thus it is seen that except for tropeoline 00, the useful indicators all have high pK, values (or possess isoelectronic points above pH 7 in aqueous solution). This is probably due to the strongly basic nature (IO,11)of the solvent.
(9) R. Stewart and J. P. O'Donnell, J . Am. Chem. SOC.,84, 493 (1962). (IO) S. J. Angyal and W. K. Warburton, J. Chem. SOC.,1951,2492. (11) T. E. Mead, J. Phys. Chem., 66, 2149 (1962).
Indicator Curcumin
Clayton yellow
Wavelength maxima, mp Acidic Neutral Basic 563 565 592 461 467 479 ... 417 409 362 368 369 529 533 542 423 42 1 388
Two indicators that gave an appreciable visual color change even at 0.001M OH- were curcumin and clayton yellow. Visible and near ultraviolet absorption spectra of acidic, neutral, and basic solutions of these indicators were recorded and the wavelength maxima are given in Table 11. As is evident from the table, acid-base behavior in TMG could be studied spectrophotometrically by proper selection of indicators. Caruso et al. (5) have found that curcumin can function as an end point indicator in TMG with an accuracy comparable to that of potentiometric titrations using a hydrogen electrode. Titration Curves. The results of the titrations are given in Figures 1 to 9, each of which will be discussed below. The curves are plotted in units of specific conductance us. moles of base per mole of acid. Data points are as found and in most cases straight lines are drawn to intersect at the theoretical base-acid integral ratios. Titrations of Some Organic Acids. Organic acids which can be titrated in water should be even more highly dissociated in a basic solvent like TMG. This indeed appears to be true with such representative acids as p-toluenesulfonic, benzoic, and salicylic acids, Tetramethylguanidine solutions of the latter two acids have been titrated potentiometrically (4). Conductance titration curves are shown in Figure 1 for ptoluenesulfonic acid at two different initial acid concentrations. The end points are exceedingly sharp and within experimental error of 1 :1 stoichiometry. Figure 1 also gives the color transformation ranges for the three indicators, curcumin, phenolphthalein, and 24trodiphenylamine, which were used in separate titrations ofp-toluenesulfonic acid. VOL 40,
NO. 6, MAY
1968
941
W v)
0
I 0.5
I
I
1.0
1.5
J
2.0 MOLES OH-/MOLE ACID
MOLES OH*/MOLE A C I D
Figure 2. Conductometric titration of benzoic acid (0.01030M) Indicator color change a. Clayton yellow b. Thymolphthalein The first change in indicator color corresponds closely with the conductometric end point. Apparently the base reacts first with the stronger acid, p-toluenesulfonic acid, and then with the weaker indicator acid. Of course, in a solvent with a dielectric constant of 11.00 (7), p-toluenesulfonic acid will likely be a weak electrolyte due to ion pairing. Caruso et ai. (7) have found ion-pair formation even for picric acid in TMG. The leveling effect (7) of the basic solvent should make p-toluenesulfonic acid comparable in dissociation to that of picric acid. No quantitative determinations of acid ionization or ion-pair dissociation were made from these conductance titrations. The conductometric titration curve for benzoic acid is given in Figure 2. Also shown is the behavior of the two indicators, thymolphthalein and clayton yellow, in separate titrations of benzoic acid. Apparently, clayton yellow is a stronger acid than benzoic acid in that its color change occurred immediately upon addition of base. Thymolphthalein, on the other hand, could be used as a visual indicator for benzoic acid, although the initial color change occurred slightly before the measured conductometric end point. A separate experiment extending the addition of base beyond a 2 :1 base :acid ratio showed no additional break in the conductance curve (important below in discussion of nitroaromatic compounds). Salicylic acid was of interest because of the possibility of titrating both the first [KI (aq) = 10-31 and second [Kz (as) 10-131 (12) protons in a leveling basic solvent such as TMG. The curve depicted in Figure 3 shows that this was indeed possible. However, in addition to the leveling effect of the solvent, the ability to titrate the second proton of salicylic acid may be because TMG is a weaker acid than water, so that higher basicities can be attained by addition of titrant. Although the data subsequent to the first end point appear to describe a curve rather than two straight lines, this curvature was completely reproducible for different samples of the acid. However, considering that Williams and Lautenschleger (4) observed only one inflection in their potentiometric titration of salicylic acid, it is difficult to state with certainty that the hydroxyl proton of the acid is sufficiently ionized for titration in TMG. Titrations of Nitroaromatic Compounds. Because a nitro group on an aromatic ring is electron-withdrawing, protons on the benzene ring itself or on the substituents tend to be (12) “Handbook of Chemistry and Physics,” 45th ed., p D-77, R. C. Weast, Ed., Chemical Rubber Co., Cleveland, Ohio,
1964.
942
ANALYTICAL CHEMISTRY
Figure 3.
Conductometric titration
of salicylic acid
(0.006468M)
acidic. This acidity should be even more pronounced in basic solvents, and Williams and Custer (3)have already found 2,4-dinitrophenol as well as m-and p-nitrophenol to behave as acids in TMG. In the present work o-nitrophenol was titrated conductometrically (Figure 4, curve A ) . The expected end point occurred at a 1:1 mole ratio, but there is also a second sharp break in the curve at a 1 :2 acid-base stoichiometry. That the second end point is real was shown by reproducible results using the two titration cells with different constants (6) and by variation of the concentration of basic titrant. Possibly, after titration of the hydroxyl hydrogen, one of the ring protons is sufficiently acidic to be titrated. A better explanation may be that a hydroxyl ion adds to the ring. For instance, Fritz et al. (13) found that some nitroaromatic amines gave two end points in titrations in pyridine with the base triethylbutylammonium hydroxide. With 1,3,S-trinitrobenzene, for example, the second end point was attributed to OH- addition to the benzene ring. Referring again to Figure 4, it is seen that the color change for thymolphthalein corresponds to the first end point of onitrophenol. With HIn denoting the acid form of thymolphthalein, the consecutive reaction occurring in the titration can probably be written as follows (disregarding ion pairing) : 0-
OH
HIn 0-
+ OH-
-+
In-
4- HOH
(2)
0-
Further insight into reactions such as Equation 3 may be possible through ultraviolet or visible absorption studies or by proton magnetic resonance spectra. The fact that water is a product in titrations of acids with (CaH8)4NOHapparently is not a problem because of the slowness of hydrolysis of TMG (6). Thus, any effect on the titration curves caused by deliberate addition of small amounts of water would be expected to be only that of dilution. (13) J. S. Fritz, A. J. Moye, and M. J. Richard, ANAL.CHEM., 29, 1685 (1957).
0
0.5
1.0
1.s
2.0
2.5
3.0
MOLES O H 7 M O L E ACID MOLES OH-/MOLE
Figure 4. Conductometric titrations A . 0.01001M o-nitrophenol B. 0.01023M phenol
Figure 5. Conductometric titration of 1,Sdinitrobenzene (0.01013M) I
Further validation of the second break in the conductance titration of o-nitrophenol was obtained by titration of phenol where only a single end point at 1 :1 stoichiometry was found. For direct comparison with o-nitrophenol, the titration curve for phenol is shown in Figure 4, curve B. Within experimental error, the conductance is linear from the 1:1 end point to as high as a 1:3 acid:base ratio. Thus, ring addition is more facile in the case of nitro-substituted aromatics. It is suggested here that the potentiometric titrations of the nitrophenols by Williams and Custer (3) may also have given second end points had base addition been continued. An attempt to compare the titration of o-nitrophenol with that of nitrobenzene met with little success. Apparently nitrobenzene is too weak an acid in TMG to give a satisfactory conductometric titration. Two polynitrobenzenes, 1,3-dinitro- and 1,3,5-trinitrobenzene, were titrated. A typical conductometric titration of 1,3dinitrobenzene is shown in Figure 5 . The marked slope changes at 1 :1 and 1 :2 acid :base ratios are evidence for the reaction of each mole of the dinitrobenzene with two moles of hydroxyl ion. Because there likely are no acidic protons in 1,3-dinitrobenzene, the hydroxyl ion may again be adding to the ring or perhaps there is reaction of hydroxyl ion with each of the nitro groups. That the latter may be true is shown by the titration of 1,3,5trinitrobenzene in Figure 6. Although the conductance values immediately following the first end point appear somewhat anomalous, it was possible to plot the data showing distinct breaks at the 1 :1, 1 :2, and 1 :3 acid:base ratios. Thus, the number of nitro groups substituted on a benzene ring, in the present examples at least, corresponds to the number of end points which were obtained. The anomalous data between the first and second end point found for 1,3,5trinitrobenzene could be caused by impurities or some unknown slow reactions. Similar multiple end points have been reported in the titration of polynitrobenzenes using ethylenediamine as a solvent. Favini and Bellobono in both potentiometric (14) and conductometric (15) titrations with sodium aminoethoxide in ethylenediamine found 1,3-dinitrobenzene to have two end points and 1,3,5trinitrobenzene to exhibit three. They also studied other polynitrobenzenes and toluenes and obtained generally similar results. It is also of interest that they found only single end points for phenol and for benzoic acid, as in the (14) 0. Favini and I. R. Bellobono, Ann. Chim. (Rome), 50, 825 (1960). (15) G. Favini and I. R. Bellobono, ibid., 51, 841 (1961).
ACID
I
I
I
I
1
MOLES O H 7 M O L E A C I D
Figure 6. Conductometric titration of 1,3,54rinitrobenzene (0.005246M)
present work. By combination of titration results with spectrophotometric studies on some polynitrobenzene systems, Favini and Bellobono (14) interpret the titration behavior as consistent with ring addition. This addition, in turn, gives rise to polarization of the individual nitro groups as ,0(-) =N(+) . Therefore it is believed that analogous \@-)
mechanisms can also be postulated for the titrations of polynitrobenzenes in TMG. On the other hand, reaction products have not been isolated by us or others (13-15) in order to prove conclusively the ring addition mechanism. Titrations of Inorganic Acids. Other results (6) indicate that ammonium salts function as acids in TMG. Solutions of ammonium salts therefore should be titrated readily. That this is so is shown by Figure 7, curve A , a typical conductance titration for ammonium bromide. A single sharp break is obtained at a 1 :1 mole ratio. Tetramethylguanidinium bromide, as one would expect, also titrates as an acid as indicated by Figure 7, curve B. After the 1 :1 end point, the gradual curvature described by the data was definitely reproducible although the low solubility of TMGHBr prevented a thorough study. From the data it is conceivable that the titration curve could be drawn with one or even two additional end points at the 1 :2 and 1 :3 acid :base mole ratios, respectively, However, this does not appear to be justified because structures such as TMGHz2+Brz2-or TMGHBr .HBr can be excluded based on bromine analysis (6).
Titrations of Polycarboxylic Acids. It was desired to titrate some representative polyprotic acids in TMG to determine if multiple end points could be obtained. For this purpose, citric acid and maleic acid were selected. The titraVOL 40, NO. 6, MAY 1960
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MOLES O H 4 MOLE A C I D
0
I.q
1.0 I
2.0 I
I .O[
3.0 I
4.0 I
I
I
I
I
1
I
2.0 MOLES O H Y M O L E A C I D
Figure 8. Conductometric titrations of citric acid
A . 0.001019M ammonium bromide B. 0.000640M tetramethylguanidinium bromide
A . 0.00106M CBHsO, B. 0.000590M CtjHsO;
tion of citric acid, a triprotic acid, may give some indication of the possibility of differential titration of mixed acid solutions. This should be true because the consecutive dissociation constants of citric acid in aqueous solution differ only by about single orders of magnitude from each other. The pK, (aq) values for citric acid are 3.08, 4.74, and 5.40 (12) at 18" C. On dissolution of citric acid monohydrate in TMG it was not possible to obtain complete solution of a given sample, despite the dilution. Perhaps the solvent forms a film of liquid around the solute to prevent solubility equilibrium. Nevertheless, two titrations of citric acid were made at two different acid concentrations. These conductance curves are given in Figure 8, where it is easily seen that three end points were obtained for each concentration of acid. That there are only three end points is shown by the completely linear conductance behavior well beyond a theoretical 1 :4 acid :base stoichiometry. It is interesting that each of the three acidic protons in citric acid can be titrated with base in TMG. Similar behavior would probably be obtained in other basic solvents. In fact, quite analogous conductometric titrations of many carboxylic and phenolic acids have been reported by van Meurs and Dahmen (16, 17). Their titrations were done in pyridine and N,N-dimethylformamide. Maleic acid is a diprotic acid whose aqueous dissociation constants are separated by several orders of magnitude. The pK, (as) values are 1.83 and 6.07 (12). As expected, two easily distinguishable end points were found in a conductance titration in TMG as shown in Figure 9. Neutral or Basic Compounds. A number of compounds, generally weak acids or weak bases in aqueous solution, could not be titrated satisfactorily as acids in TMG. These compounds were urea, acetamide, water, benzene, nitrobenzene, and 2-nitroacetanilide. Their titration curves were essentially indistinguishable from blank titrations. Quantitative Reliability of Titrations. Most of the conductometric titrations discussed above lend themselves to determination of the percentage recovery of the acid. These percentages are listed in Table 111, in terms of millimoles of acid taken and found. As can be seen, the recoveries are reasonably good in most cases with a maximum error of approximately i2 %. (16) N. van Meurs and E. A. M. F. Dahmen, Anal. Chim. Acta, 19, 64 (1958). (17) N. van Mews and E. A. M. F. Dahmen, ibid., 21, 443 (1959). 944
MOLES O H Y M O L E A C I D
Figure 7. Conductometric titrations
ANALYTICAL CHEMISTRY
f
o8 .0u:
0.5
I
.o
1.5
3.0
2.5
2.0
MOLES O H 7 N O L E
3.5
ACID
Figure 9. Conductometric titration of maleic acid (0.00590M)
Table 111. Percentage Recovery of Acids
Millimoles Taken Found
Acid
p-Toluenesulfonic acid Benzoic acid Salicylic acid o-Nitrophenol Phenol 1,3-Dinitrobenzene
1.21
0.624 0.617 0.289 0,6006
0.6006 0.512
1.22 0.619
0.618 0.294 0.6009 0.5969 0.518
Recovery, 100.8 99.2 100.2 101.8 100.05 99.38 101.2
0.608
0.614
101.0
1.01
0.989
97.9
Utility of 1,1,3,3-Tetramethylguanidine. The quantitative results of Table I11 coupled with the qualitative results of other titrations and those using indicators appear to make TMG a practical solvent for conductometric titrations of a wide class of acidic compounds. In addition, TMG should be practical for studies of the acidities and various reactions of nitroaromatic compounds. ACKNOWLEDGMENT
The authors thank, the American Cyanamid Co., Wayne, N.J., for their gift of 1,1,3,3-tetramethylguanidine. Many helpful discussions were held with A. I. Popov. RECEIVED for review August 28, 1967. Accepted February 26, 1968. Presented in part to the Division of Inorganic Chemistry, 145th Meeting, ACS, New York, N. Y., September 1963. Taken from the Ph.D. dissertation of M. L. Anderson, Michigan State University, East Lansing, Mich., 1965.
