Potentiometric Titration of Weak Acids in Anhydrous Ethylenediamine M. L. MOSS, JOHN H. ELLIOTT, AND ROBERT T. HALL Hercules Experiment Station, Hercules Powder Company, Wilmington 99, Del. A new approach to the problem of titrating phenols and other acids too weak to be titrated in aqueous medium is based on use of an anhydrous, strongly basic solvent such as ethylenediamine and an even more basic titrant. The behavior of phenols or carboxylic acids when titrated in this system is analogous to the behavior of carboxylic acids or mineral acids when titrated in water. With mixtures of phenols and carboxylic acids, two separate end
T
ITRATION of the acidic components of certain resins of commercial importance is complicated by several factors. The dark color of some of these resins makes it impossible to determine the end point visually. This difficulty due to color alone may be overcome by electrometric titration, or the indicator end point may be detected by using a photoelectric device (12). However, even the electrometric end point may not be sufficiently sharp to be detected with the desired precision. Titration to an arbitrarily established reading on a glass electrode pH meter is not capable of high precision or accuracy if the sample does not give a sharp inflection in the titration curve. Work in this and other laboratories (9) has shown that the poor end point with dark-colored resins is due to the presence in the resin of very weakly acidic components, phenolic in nature, which are too weak in neutral solvents to be susceptible to titration themselves but which obscure the end point given by the stronger carboxylic acids. Hence, neither the total acids nor the stronger acid fraction can be satisfactorily determined, The work reported here is concerned with a practical solution of this problem of titrating such resinous materials containing weakly acidic components. Although all the possibilities of the proposed method have by no means been explored, it is believed that the results demonstrate the soundness of the principles involved. The problem of titrating dark-colored resins can be considered to consist of two phases: (1) determination of acids that are too weak to give satisfactory inflections and, therefore, cannot be titrated in the conventional media; and (2) titration of mixtures of acids whose strengths are not greatly different. According to the Brghsted theory (S),the dissociation of an uncharged acid in a solvent is a reaction of the type
d+S==SSH++B11 here A is the acid, S the solvent, S H the solvated proton, and B - the conjugate base of the acid. If this equilibrium is to be shifted appreciably to the right, the solvent must be more basic than the conjugate base of the acid. Thus, in a basic solvent, weak acids would show a much greater acid strength than in water or alcohols. It has been reported, for example, that in liquid ammonia carboxylic acids behave like strong acids (6). Use of a basic solvent, therefore, seemed a logical approach to the titration of very weak acids-e.g., phenol, which cannot be titrated satisfactorily in either aqueous or alcoholic solutions. This principle was applied some years ago by Hall and Conant ( 5 ) and Hall (4)to the analogous case of the titration of very weak bases, using glacial acetic acid as the solvent, but does not appear to have been applied before to the titration of weak acids. In addition to being strongly basic, the solvent should have a small autopro-
points generally occur, corresponding to the two materials of different acid strength. Dark-colored resins such as Vinsol, which contains both carboxylic and phenolic constituents, give a curve having two points of inflection when titrated in this new system. Resorcinol and salicylic acid give t w o end points correspondingto each of their two acidic hydrogens. In ethylenediamine, amino acids behave as typical unsubstituted carboxylic acids.
tolysis constant to minimize solvolysis effects. It is evident that the titrant used must be even more basic than the solvent. The problem of determining each of two acids whose strengths are not greatly different was discussed theoretically by Auerbach and Smolczyk (1). who showed that in the titration of a dibasic acid two points of inflection will not appear unless the ratio of the dissociation constants is greater than 16. MacInnes (10)points out, however, that if the two inflection points are to be determined experimentally with any accuracy, this ratio must be considerably greater. In this investigation, it was decided to carry 0ut:electrometric titrations in a nonaqueous, basic solvent. It was hoped that a solvent could be chosen of such basicity that carboxylic acids would behave as strong acids while the phenols, having lower
A. REFERENCE ELECTRODE 8. INDICATOR ELECTRODE C. MAGNETIC STIRRER D. 50 ML. BURET E. T I T R A N T F. ASCARITE + DEHYDRITE C. 12/5 BALL JOINTS H. 10/30 f JOINTS
+
Figure 1. Buret and Electrode Assembly
V O L U M E 20, NO, 9, S E P T E M B E R 1 9 4 8
H
A.
CHERRY AMPLIFIER 8. POTENTIOMETER C. STORAGE BATTERY G. GALVANOMETER
785 by FTillard and Boldyreff (14). Thus, irnniersed in the titrant, the reference electrode is connected electrically with the solution being titrated through the buret tip. This affords the advantage, not existing in other reference elrctrodes tried, of a continual flushing of the reference electrode t o prevent diffusion. Renewal of the liquid junction occurs a t the end of the buret tip, serving as a salt bridge, n-ith addition of each new increment of titrant. Figure 2 shows the arrangement of electrical apparatus used.
llthough the resistance of the system is low, an amplifier is used to increase sensitivity of the measuring system. Bmplifiration is especially desirable when hydrogen and calomel electrodes are used. A Leeds & Sorthrup Cherry amplifier (Catalog Xo. 7673), portable potentiometer (Catalog No. 7655), and galvanometer (Catalog No. 2 4 2 0 4 ) were used. The hydrogen-calomel electrode combiI o o o o ~ nation mas employed for much of the work, although it is less convenient than the antimony electrodes described. Several types of Figure 2. Arrangement of Electrical Apparatus hydrogen electrodes were used, including thr type described by Sewbery (111,coated with Du Pont liquid bright gold KO.4618. After being fired, the gold electrodes were olatinized in the usual xav. Hvacid strengths, would behave as weak acids and, hence, two indrogen IT& introduced throuih a sintered-glass bubbliig tube flection points would be obtained. Ethylenediamine was chosen placed under the electrode. as the solvent because it may be obtained commercially and is a Thc most practical and serviceable hydrogen electrodes congood solvent for resins. The relatively high dielectric constant, sisted of stainless steel tubes, 10 mm. in diameter and 100 mm. in length, with sintered metal disks a t one end. The hydrogen nas about 16 (Y), is desirable because ion-pair formation will take bubbled through, making contact with the solution a t the surface place to a lesser extent than in media of loTTer dielectric constant. of the disk. These one-piece electrodes !yere mounted with Furthermore, it has been frequently observed that electrometrir sealing wax and paraffin in ground-glass joints to fit the titration measurements in low dielectric-constant media are difficult to flask. They were satisfactory as received with a bright surface, no platinizing being necessary. make because of electrode instability and slow attainment of With the hydrogen-calomel electrode system in ethylenediaelectrode equilibria. mine, potential differences as high as 1.6 volts required use of an As solvents, other amines including diamylamine, pyridine, extra working battery to oppose the output of the amplifier in and ethanolamine were tried. None of these, however, gave order to bring the readings below 1.11volts and nithin the range of the potentiometer. end points as distinct as those obtained in ethylenediamine. h pH nieter equipped with a hvdrogen or antimony electrode K i t h ethylenediamine as the solvent, the obvious titrant to use may be used for the e.m.f. measurements, although a potentiomwould be an ethylenediamine solution of an alkali metal derivaeter graduated in millivolts is preferred. The glass electrode itself tive of the ethylenediamine. As preliminary attempts to prepare is unstable in this system, whether filled with the usual aqueous solutions or with a buffer in ethylenediamine. I t does register a mch a solution were not promising, an ethylenediamine solution sharp change in potential a t the end point but drifts back immrof sodium aminoethoxide was chosen. diatelp in the direction of its original potential. Several methods of detecting the end point in this system !yere studied. Various indicators, including phenolphthalein and thySOLUTIONS AND REAGENTS mol blue, change color when titrated although the color changes Ethanolamine (Eastman Kodak practical) was distilled three are not considered sufficiently sharp for visual TTork particularly times, as recommended by Kohlraysch and Ypsilanti (8),through with dark-colored samples. Phenol gives a veiy sharp en'd point a 2-foot Vigreux column. Dry ethylenediamine was prepared if titrated using trinitrobenzene a' the indicator and B photometer from the commercial 70% aqueous grade (Carbide and Carbon) by the method of Putnam and Kobe ( I S ) using sodium hydroxide is used for measuring the color change, preferably at about 450 and, finally, distilling over sodium. A4more recent method of nip. The behavior of the indicator is not the same as in water, inBromley and Luder ( 2 ) employs activated alumina and mav asmuch as in ethylenediamine it changes from orange on the acid offer some advantages over the earlier method. side to colorless on the alkaline side. For the weak acid mixtures studied, the potentiometric The effect of moisture is important and best results are obmethod of titrating gave the most satisfactory end points of the tained with anhydrous ethylenediamine prepared by distilling various methods considered. -4lthough the conductometric over sodium. As the mater concentration increases, sharpness of method would have been the most convenient electrometric the end points deteriorates progressively and the system assumes method to use in ethylenediamine, it did not give a satisfactory essentially aqueous characteristics. For example, a t 20% moisend point with phenol in thir: medium. -4potentiometric method ture, phenol gave an end-point break of only about 20 mv., was, therefore, adopted. measured with the hydrogen electrode, in comparison with 180 to 200 mv. in 99 to 100% ethylenediamine. Removal of even the APPARATUS AND ELECTRICAL SYSTEM last 1 % of moisture is advantageous. The titration flask, buret, and electrode assembly as shown in The titrant was prepared by dissolving approximately 2.5 Figure 1 were designed to esclude atmospheric moisture and cargrams of sodium, which had been TTashed successively in ethanol bon dioxide. By using a magnetic stirrer, an opening in the flask and ethanolamine, in 100 ml. of ethanolamine with cooling, and f o r a stirrer shaft is eliminated. Both hydrogen and antimony indiluting to 500 ml. with ethylenediamine. Potassium and lithdicator electrodes were used; the latter was somewhat more conium were also tried but did not offer any apparent advantages. venient. Some excess ethanolamine, 10 to 20% of the total volume, was The antimony electrode consists of a rod prepared by sucking required in order to obtain a solution that would remain clear on thermometer joint and the molten metal into a male 10/30 standing. This solution was standardized against benzoic acid chipping off the glass below the joint after cooling. The reference by the following procedure, which was also used for titrating the electrode is a similarly prepared antimony rod mounted in the samples. buret below the stopcock, a variation of the electrode suggested I datively
786
ANALYTICAL CHEMISTRY
Titration Procedure. Weigh a sample of 0.1 to 1gram, transfer to the titration flask, and dissolve in 75 ml. of ethylenediamine, using the magnetic stirrer. Flush out the buret tip and the reference electrode and connect the flask to the buret. Carry out the potentiometric titration in the usual way adding increments of 0.1 to 0.3 mi. of titrant as the end point is approached. Plot the titration curve taking the inflection as the equivalence point.
Table I.
Titration of Mixtures of Benzoic Acid and Phenol in Ethylenediamine
sample Benzoic Acid, Gram No. Present Found Difference
1
0.2500
2
0.5000
DISCUSSION
Because carboxylic acids are stronger acids in liquid ammonia than in water, it was expected that an amine solvent should enhance the acidic properties of phenols to such an extent that they might then be readily titrated, perhaps even in the presence of carboxylic acids. This would be analogous to the titration of mixtures of mineral and carboxylic acids in water. Even though ethylenediamine itself is basic, the titrant used is more strongly basic and acts, therefore, as a base in the presence of the diamine.
I
0.1000
0.247 0.247 0.501 0.502
-0,003 0.003 +0.001 0.002
0.101 0.108 0.108
+0.001 0.008
0.008
Present 0.2500
0.1000
Phenol, Gram Found Difference
0.254 0.254
+0.004 0.004
0.102
+0.002
0.500 0.503
0,000 +0.003
0.492
-0.008
0.104
0,5000
0.004
obtained in aqueous medium, the striking effect of the basic solvent and the stronger titrant is readily apparent. In this system, phenol behaves like a carboxylic acid in water and the carboxylic acid behaves like a strong mineral acid in water. The possibility of determining phenols and carboxylic acids simultaneously is, therefore, promising. The curve for the phenol-benzoic acid mixture (Figure 6) has two distinct breaks and very nearly the same form &s the curve for the mixture of hydrochloric and acetic acids in water shown in
Figure 4. Results obtained on mixtures of benzoic acid and phenol are listed in Table I. In such mixtures, the changes in e.m.f. for the carboxyl end points amount to about 20 to 100 mv. compared to 100 to 150 mv. for the phenol end points. The + BENZOIC ACID magnitude of the breaks appears to be influenced by such factors as composition of the sample, the electrodes used, and ‘95% Ethanol dryness of the solvent. From these and similar data, it appears that best results are 0 5 10 I5 obtained when the ratio of carboxyl acidity to phenol acidity ML NaOH. 0 5 1 is comparatively high, although it is possible to measure Figure 3. Titration of Weak Acids in Aqueous Medium with total acidity irrespective of their relative proportions~ Glass-Calomel Electrodes In ethylenediamine, the difficulty of analyzing samples high in phenolic constituents occurs in the carboxyl end point, which is somewhat obscure in such mixtures. The uncertainty of The over-all reaction involved in the titration of a carboxylic acid the first end point then enters into both the phenol and carboxyl in this solvent with sodium aminoethoxide may be represented by results. I t does not, however, affect the calculation of total acidthe following equations: ity represented by the second end point, which is considerably RCOzH H Z N - C ~ I ~ - N H ~ ~ H Z N - C Z H ~ - S H ~RCOa+ more distinct than the first. Calculation of either carboxyl or HzN- CzH4ONa +N a + HZX-CZHIOphenol alone involves taking the difference between the two end points, and thereby requires an accurate determination of each. HzN-M4-NHa+ HzN-CzHrO- +H&-CzH4--OH (acid) (base) The same situation is encountered in titrating mixtures of hydroHzN-CzHd-SHz chloric and acetic acids in water. The results reported in Table I are typical of what may be obThe neutralizatipn reaction then merely involves transfer of tained without unusual precautions and do not necessarily reprethe proton from the cation t o the anion with regeneration of the two free bases. For purposes of comparison, several titration curves for neutralization reactions in aqueous medium are shown in Figure 3. Although 95% ethanol was used as solvent for the phenol-benzoic acid mixture, this system is essentially aqueous in its behavior. As indicated, it is virtually impossible to titrate unsubstituted phenol in water solution, as there is no appreciable inflection point. A similar curve is obtained with amino acids such as glycine. Generally speaking, in a mixture containing two sufficiently acidic components of relatively different acid strengths, it is possible to determine the two acids simultaneously by obtaining two separate end points. This situation is illustrated by the titration curves shown in Figure 4 for the mixtures of acetic and hydrochloric acids in aqueous medium with which separate end points are obtained for each of the acids present. When titrated in the presence of each other, the weak acid gives the sharper end point although the reverse 20 30 10 -0 is true when the acids are titrated separately. ML. N a O H , 0.2 ?? Figure 5 shows titration curves obtained in the ethyleneFigure 4. Titration of Hydrochloric and Acetic Acid Mixtures in Water with Sodium Hydroxide diamine system. Comparing these with the previous curves I
P”
+
+
+
+
+
787
V O L U M E 20, NO. 9, S E P T E M B E R 1 9 4 8
Table 11. Acid Numbers of Various htaterials Determined by Titration in Ethylenediamine Acid No., Mg. KOH/Gram __ BENZOIC &CIDJ
I
10
I
20
)
Compound Resorcinol Salicylic acid Methyl p-hydroxybenzoate E t h y l p-hydroxybenzoate Vinsol resin Belro resin Glycine a-Aminocamoic acid Anthranilic acid p-Aminobenzoic acid Boric acid
Found Second end point 1000 415 810 374 .. 353 125 2i7 176 .. 77 .. 419 .. 412 .. 414 873 1829 2788 (third)
First end point 528
Theoretical First Second end end point point 510 1019 406 812 369 .. 348 ..
..
7i.l 427 409 409 906
..
.. .. .. I
.
18i2 2718 (third)
,
30
ML. T I T R A N T , 0 . 2 N
Figure 5. Titration of Phenol and of Benzoic Acid in Ethylenediamine with Hydrogen-Calomel Electrodes
with benzoic acid and rosin. The descending nature of the curves in Figures 7,8, and 9 is characteristic of the electrode system used. Several substituted phenols containing more than one acidic hydrogen were also titrated in ethylenediamine (see Figure 8). Resorcinol gives an interesting curve with two breaks, the first much more pronounced than the second, indicating the second
sent the limit of accuracy obtainable. As the primary interest to date has been in basic principles, no effort has been made to exhaust the Dossibilities of mecision and accuracy. Although the development work has bee; concerned 0 700mainly with the titration of phenol and carboxyl acidity in the presence of each other, the principal objective was -GLYCINE to devise means for titrating acids too weak to be titrated by existing methods. Weak acids of this kind occur in certain commercial resinous materials such as Vinsol E-AMINOCAPROIC ACID resin, a paraffin hydrocarbon-insoluble fraction of pine i wood resin, whose titration curve in ethylenediamine is 9 0600given in Figure 6. The general shape of the Vinsol curve LL 9 is similar to that obtained with the mixture of phenol and W benzoic acid. This confirms the results of the other work performed in this laboratory, which indicate the presence of both phenolic and carboxylic acid groups. From the data, it appears that acids of two different strengths are present in approximately equivalent amounts. In alcoholic medium the titration curve for Vinsol shows no 0500 sharp inflection. L 1 5 10 15 Another potential application of the method being M L TITRANT, O Z C . ' studied is the titration of amino acids. In aqueous soluFigure 7, Titration of Amino Acids in Ethylenediamine with tion these compounds are difficult to titrate because of Antimony Electrodes their amphoteric character. In ethylenediamine, however, the buffering effect of the amino group is completely hydrogen to be extremely weakly acidic. X o evidence of either eliminated and the amino acid can he titrated as readily as any acidic hydrogen is found on titrating in aqueous medium. With other carboxylic acid. Curves for glycine and e-aminocaproic salicylic acid, the situation is similar to that found with a mixacid, shown in Figure 7, are of the same general form obtained ture of phenol and benzoic acid; the e.m.f. change is greater a t the second end point and somewhat more abrupt than at the first. Because of the general interest often expressed among analysts in the possibility of titrating the third hydrogen of phosphoric and boric acids, several titrations were undertaken with these compounds. Phosphoric acid precipitated in ethylenediamine and in ethanolamine and, therefore, could not be titrated. In the case of boric acid, which is much weaker than phosphoric acid, there were three detectable inflections, as shown in Figure 9, close to the calculated values. Although the end points are not sufficiently distinct for accurate results, the curve shows the possible advantages of titrating weak acids in a basic solvent. Acid numbers obtained in ethylenediamine for various acidic materials are shown in Table 11. Most of the samples listed cannot be titrated satisfactorily by ordinary methods. Results are rounded off to the nearest whole 10 20 30 number, as the purity of most of these materials was not deML T I T R A N T , 0 2 termined. In cases where more than one end point was observed, Figure 6. Titration of Vinsol Resin and of Phenol-Benthe second (and third) acid numbers indicate total acidity tizoic Acid Mixture in Ethylenediamine with Hgdrogentrated up to those end points. Calomel Electrodes v)
t
ANALYTICAL CHEMISTRY
788 Although ethylenediamine is somewhat objectionable as a solvent because of its caustic properties and tendency to fume in air, it can be handled conveniently in the apparatus described. It offersobvious advantages over liquid ammonia, which was f i s t considered. Existing methods employing various neutral nonaqueous solvents for the titration of dark-colored resins leave much to be desired, even with the best electrical equipment. In the opinion of the authors, any outstanding progress in the field will probably depend first on a drastic departure from conventional solvent-titrant combinations.
0.3C
In
OgOOn 3
2
020
5 W
0 IC
5
ML. T I T R A N T , 0 2 5
IO
Figure 9. Titration of Boric Acid in Ethylenediamine with Antimony Electrodes
I
10 M L . TITRANT, 0.2fi
1
20
Figure 8. Titrations in Ethylenediamine with Hydrogen-Antimony Electrodes
The most promising approach to the problem would seem to be to work in a basic solvent so &s to take advantage of the ‘hewer” concepts of acids and bases. ACKNOWLEDGMENT
The authors wish to acknowledge the many helpful suggestion made by Martin Kilpatrick, Hercules Powder Company consultant, during the course of this work. LITERATURE CITED
(1) Auerbach, Fr., and Smolczyk, E., 2 . physik. Chem., AIIO, 65
(1924).
(2) Bromley, IT. H., Jr., and Luder, W. F., J . Am. Chem. ~ S O C .66, . 107 (1944). (3) BrZnsted, J. N., 2. physik. Chem., A169,52(1934). (4) Hall, N. F., J. A m . Chem. Soc., 52, 5115 (1930). (5) Hall, X. F., and Conant, J. B., Ibid., 49, 3047 (1927). (6) Hammett, L. P., “Physical Organic Chemistry,” p. 261, New York, McGraw-Hill Book Co., 1940. (7) International Critical Tables, 1st ed., Vol. 6, p. 85, Ken- P o r k , McGraw-Hill Book Co., 1929. (8) Kohlrausch, K. W, F., and Ypsilanti, Gr. Pr., Z. p h y s i k . (’hem. B29, 279 (1935). (9) Lykken, L., Porter, P., Ruliffaon, H. D., and Tuemmler, 1:. D., IND. ENG.CHEM.,ANAL.ED.,16,219(1944). (IO) MacInnes, D. A , “Principles of Electrochemistry,” p. 305. NenYork, Reinhold Publishing Corp., 1939. (11) Newbery, E., Trans. Elect~ochem.Soc., 64, 209 (1933). (12) Osborn, R. H., Elliott, J. H., and Martin, A. F., IXD.ENG. CHEM., AKAL.ED.,15, 642 (1943). (13) Putnam, G. L., and Kobe, K. .1.,Trans. Electrochem. Soc., 74, 609 (1938). (14) Willard and Boldyreff, J . A m . Chem. Soc., 51,471 (1929). RECEIVED September 23, 1947. Presented before the Division of Analytioal and Micro Chemistry a t the 112th 3Ieetinp of the . i ! d E R I c a r CHEMICAL SOCIETY, Ye17 York, N . Y .
