Potentiometric Titration of Very Weak Acids Tetrabutylammonium Hydroxide as Titrant in Nonaqueous Media G. A. HARLOW, C. M. NOBLE', and GARRARD E. A. WYLD Shell Development Co., Emeryville, Calif. aqueous solvent and the solution was shaken with finely POWdered silver oxide. Although this method has been previously used with success to prepare these hydroxides in aqueous solution (i'), it was completely unsuccessful in the present case. Attempts to concentrate the commercially available 1N solution of tetrabutylammonium hydroxide by evaporation and crystallization were also unsuccessful. An ion exchange process utilizing an exchange resin and an isopropyl alcohol solution of tetrabutylammonium iodide was found to be very satisfactory. The saturated alcoholic solution was slowly passed through a column containing Amberlite IRA400 resin which had previously been converted to the hydroxide form by washing with aqueous potassium hydroxide solution, then rinsed with isopropyl alcohol to remove the water. This process resulted in the conversion of the quaternary ammonium iodide to quaternary ammonium hydroxide.
Tetrabutylammonium hydroxide in nearly anhydrous isopropyl alcohol solution has a number of advantages over more conventional titrants for very weak acids in nonaqueous solvents. The tetrabutylammonium salts of most weak acids are more soluble in organic solvents than are the corresponding sodium or potassium salts; thus, difficulties due to precipitation are minimized. The titrant can be used with the glass electrode without the pronounced loss in sensitivity in the highly alkaline region which is encountered with titrants containing sodium and potassium. The titrant can be prepared by passing a solution of tetrabutylammonium iodide in isopropyl alcohol through an anion exchange column which has been converted to the hydroxide form with potassium hydroxide.
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An exchange column 4.5 em. in diameter and 52 em. long was found to be sufficiently large for the preparation of 1400 ml. of the hydroxide, Potassium hydroxide rather than sodium hydroxide was chosen as the strong alkali to convert the exchange resin to the basic form because the presence of small amounts of potassium ion in the titrant would be less objectionable than the same amount of sodium ion. No tests were conducted to determine the minimum amount of potassium hydroxide required, but 10 liters of 1,V aqueous solution appeared to give satisfactory conversion. Six liters of distilled water was used to rinse the column free of potassium hydroxide and 5 liters of anhydrous isopropyl alcohol was used to remove the excess water. A saturated solution of tetrabutylammoniurn iodide in isopropyl alcohol was prepared and passed through the column very slowly (not more than 5 ml. per minute). hlore rapid throughput resulted in incomplete conversion. The effluent was collected in a receiver which was protected from atmospheric carbon dioxide.
HE first successful potentiometric titration of very weak acids (those comparable in strength to phenol) was reported by Moss, Elliot, and Hall (6) in 1948. These workers used ethylenediamine as the solvent and a solution of sodium aminoethovide in ethylenediamine as the titrant. In later publications other compounds have been suggested for use as titrants. Sodium methoxide (3)and potassium methoxide in methanolbenzene ( 2 )were employed by Fritz and others. Deal and Wyld ( 1 ) reported that potassium hydrovide in isopropyl alcohol solution could be used for the titration of very weak acids in ethylenediamine and dimethyl formamide. It was also shown that tetrabutylammonium hydroxide in isopropyl alcohol containing 10% water gave inflections for the titration of very weak arids in the above solvents which were similar in appearance to those obtained with potassium hydroxide in anhydrous isopropyl alcohol. These results were obtained with a glass indicating electrode which loses sensitivity in the presence of potassium ions. Because quaternary ammonium ions do not reduce the sensitivity of the glass electrode, tetrabutylammonium hydroxide should have given considerably larger inflections than potassium hydroxide. There was reason to believe that the relatively high water content of the quaternary ammonium hydroxide titrant was counteracting the advantage of greater electrode sensitivitv.
In order to determine the efficiency of the conversion of the iodide to the hydroxide, 400 ml. of the saturated iodide solution
TETRABUTY LABIAIOVIUM HYDROXIDE TITRANT
Tetrabutylammonium hydroxide is available commercially in l d l aqueous soluticn. Unfortunately, water has a detrimental effect on the titration of very weak acids and the presence of only a few per cent in the solvent is sufficient to reduce markedly the siz? and sharpness of the inflections obtained. If the solution is diluted with a less acidic solvent such as isopropyl alcohol to give a 0.2N solution for titration purposes, the water content is still 20%. It was felt, therefore, that a satisfactory evaluation of tetrabutylammonium hydroxide as a titrant could be made only after a method had been developed for preparing it in a nearly anhydrous solvent which is less acidic than mater. Preparation. The first attempts to prepare nonaqueous quaternary ammonium hydroxide titrants were along conventional lines A quaternary ammonium halide was dissolved in a non1
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Figure 1. Ion exchange conversion of tetrabutylammonium iodide to hydroxide in isopropyl alcohol
Present address. Shell Chemical Corp , Houston. Tex.
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ANALYTICAL CHEMISTRY
788 was passed through the column and the effluent collected in 100ml. fractions. The column was then rinsed Il-ith isopropyl alcohol and further fractions were collected. Each fraction was analyzed to determine the alkalinity and iodide content. Results of the determination of hydroxide content are s h o m in Figure 1, The iodide content, which amounted t o no more than a trace in the fraction, indicated that the conversion was nearly complete. The first 1000 ml. of solution to come through the column had an average free hydroxide content of 0.303.V and represented over 90% of the theoretical yield. S o t all of the commercially available tetrabutylammonium iodide was satisfactory as the starting material. However, that obtained from Matheson, Coleman and Bell, Inc., and from Southwestern Analytical Chemicals, Austin, Tex., was satisfactory. Attempts were also made to prepare tetrabutylammonium hydroxide in pyridine, but were not successful. The pyridine emuent from the ion exchange column appeared to be very unstable, rapidly growing dark in color and losing its strong alkali content. Characteristics. The water content of the 0 2N tetrabutylammonium hydroxide was determined by means of the Karl Fischer reagent and found to be about 0.5%. I t is likely that the water content could be reduced by more thorough dehydration
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of the ion exchange column during its preparation if this should prove desirable. The stability of the titrant compares favorably m-ith that of solutions of similar strength of potassium hydroxide in isopropyl alcohol. Over a period of 6 weeks the normality of the tetrabutylammonium hydroxide dropped from 0.2349 to 0.2331 : during this same period a solution of potassium hydroxide changed from 0.1893 to 0.1821. As in the case of the potassium hydroxide titrant, care must be taken during storage to prevent the absorption of carbon diovide from the air. Some hatches of the titrant appeared to decompose sloivly upon standing. The titration curves obtained with these solutions show the presence of a base weaker than tetrabutylammonium hydroxide, probably the tertiary amine. Although this component does not interfere with the titration of very Jveak acids when the titrant is standardized against benzoic acid, it complicates the determination of mixtures of strong and veak acids. APPARATUS AIVD PROCEDURE
Most of the titrations were performed automatically with the aid of the modified Precision-Dow Recordomatic Titrator. The titration assembly employed for these titrations was similar to the one previously described (4)in which the cell, the solvent reservoir, the electrodes, and the stirrer are all housed in a fume hood while the titrator and the titrant reservoir are outside of the hood area. In cases where the titration was performed manually, a Precision-Shell Dual AC Titrometer was used in conjunction with a titration assembly which has been described elsewhere (1 ), The platinum-oxygen indicating electrode was prepared by the polarization procedure ( 4 ) . The procedure used for the titrations consisted of dissolving the sample in 20 ml. of the solvent and titrating with approximately 0.2N titrant (4). The solvents used were of the best grade available commercially and were used directly in cases
Phenol
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Titration of phenol-acetic acid-hydrochloric acid mixture in ethylenediamine Glass-platinum electrodes
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Figure 3. Titration of phenol in ethylenediamine
Figure 4. Titration of p,p’-dihydroxydiphenylmethane in ethylenediamine
Platinum electrodes
Platinum electrodes: curves shifted vertically for clarity
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V O L U M E 28, NO. 5, M A Y 1 9 5 6 where blank titrations indicated the absence of acidic impurities. This was the case with the alcohols, acetone, and pyridine. Piperidine and ethylenediamine were found t o contain acidic impurities and these solvents were purified by passing them through a column of activated alumina (8lcoa F-20). TITRATION OF VERY WEAK ACIDS
In Ethylenediamine. Tetrabutylammonium hydroxide in isopropyl alcohol was compared with potassium hydroxide in isopropyl alcohol as a titrant for the determination of very weak acids in ethylenediamine. The titration curves obtained from a mixture of phenol, acetic acid, and hydrochloric acid titrated with the aid of a glass indicating electrode are shown in Figure 2. The differencein the size of the inflections for phenol in the two curves is due largely to the effect of the potassium ion on the response of the glass electrode in strongly basic media. When an indicating electrode which is not sensitive to the potassium ion is used for the titration of phenol the two titrants yield curves which are similar in appearance. This can be seen from Figure 3, ahich shows curves obtained with a platinum-oxygen electrode. The titration curves in Figure 2 shorn another characteristic difference in the two titrants. The curve obtained with tetrabutylsmmonium hydroxide has an incipient inflection for hydrochloric acid, whereas no sign of such an inflection appears in the curve obtained qqth potassium hydroxide. This difference in the degree of resolution which can be achieved with the two titrants is even more noticeable in some other solvents. The titration of p,p’-dihydroxydiphenylmethane,a very weak dibasic acid, illustrates another influence of the titrant cation on the appearance of the titration curve. The differentiation of the two replaceable hydrogens of p,p’-dihydroxydiphenylmethane when this compound is titrated in ethylenediamine with tetrabutylammonium hydroxide is shoan in Figure 4. A similar titration with potassium hydroxide (also shown) yields rz curve with a single inflection. The resolution obtained with the tetrabutylammonium hydroxide is due to the fact that the salt formed when the first hydrogen of the p,p’-dihydroxydiphenylinethane is titrated io insoluble in the ethylenediamine,
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Titration of phenol-acetic acid mixture in pyridine Glass-calomel electrodes
while the completely neutralized compound is soluble. This titration was one of the rare cases in which a precipitate formed during titration with quaternary ammonium hydroxide titrant. Even in this case, however, the final product of the titration r a e soluble. In Pyridine. A comparison of potassium hydroxide and tetrabutylammonium hydroxide in pyridine solvent was made by titrating very weak acids with the aid of both glass and platinum indicating electrodes (Figures 5 and 6). As can be seen from Figure 5 , obtained with a glass electrode, good resolution was achieved with both titrants, but a much larger inflectioii nas obtained with the tetrabutylammonium hydroxide. The difference in the size of the phenol inflection is too great to be explained simply on the basis of the potassium ion effect on the response of the glass electrode. The fact that some other fartor i s also
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Figure 6.
Titration of phenol in pyridine Platinum electrodes
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ANALYTICAL CHEMISTRY
involved ie apparent from the titration curves for phenol (Figure 6), which were obtained with the aid of a platinum-oxygen indicating electrode. Some solubility difficulties are encountered when potassium hydroxide is used as a titrant in pyridine. There is a tendency for a precipitate to form when the first few drops of titrant are added to the pure solvent, as in the case of a blank titration. Apparently a certain amount of isopropyl alcohol is necessary to keep the potassium hydroxide in solution, because the precipitate redissolves upon the addition of more titrant. N o difficulties of this type are encountered when tetrabutylammonium hydroxide is used as the titrant. The titration of p,p‘-dihydroxydiphenylmethanein pyridine with the two titrants results in curves which are very different in appearance. Figure 7 shows that two well-defined inflections are obtained when tetrabutylammonium hydroxide is used, but only a single inflection when potassium hydroxide is used. These
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Figure 10. Titration of phenol-acetic acid-hydrochloric acid mixture in isopropyl alcohol GlassAplatinurn electrodes; curves shifted for clarity
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0.2N tetrabutylarnrnonium hydroxide titrant in isopropyl alcohol, glasscalomel electrodes 1. Isopropyl alcohol 2. E t h y l alcohol 3. Methanol
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Figure 9.
Titration of phenol in isopropyl alcohol Platinum electrodes
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titration curves cannot be explained on the basis of solubility as were the curves which were obtained in ethylenediamine (Figure 4),because precipitation does not occur in pyridine. In Alcohols. Alcohols are not generally considered as suitable solvents for the titration of very weak acids, although they have found considerable application for the titration of stronger acids. Very little information can be found in the literature which would permit the titration characteristics of different alcohols to be compared. An attempt was made to titrate phenol in methanol and ethyl and isopropyl alcohols with tetrabutylammonium hydroxide. The resulting curves, shown in Figure 8, indicate that only the isopropyl alcohol gives a detectable inflection. Apparently both methanol and ethyl alcohol are more acid than isopropyl alcohol and thus less suited for the titration of very weak acids. This view is supported by the results of Hine and Hine (j),who found the order of increasing acidity to be isopropyl alcohol, ethyl alcohol, and methanol, as measured by a colorimetric procedure. A comparison of the titration curves obtained when phenol is titrated in isopropyl alcohol with potassium hydroxide and tetrabutylammonium hydroxide with the aid of a platinum indicating electrode is shown in Figure 9. Because the platinum electrode is not subject to alkali ion error, the differences between the two curves must be due to some other effect. The low dielectric constant of isopropyl alcohol (D = 18.3) suggests the possibility of ion pair formation. The curves for the titration of mixtures of a strong, weak, and very weak acid with tetrabutylammonium hydroxide and potassium hydroxide are shown in Figure 10. Separate inflections are obtained for hydrochloric acid, acetic acid, and phenol,
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V O L U M E 28, NO. 5, M A Y 1 9 5 6 although the inflection for phenol is not sharp. These curves illustrate the value of a solvent such as isopropyl alcohol which is not strongly basic for the resolution of acid mixtures. In Other Solvents. The curves obtained when phenol is titrated with tetrabutylammonium hydroxide in chloroform, ethyl ether, acetone, methyl ethyl ketone, and piperidine are shown in Figure 11. The most promising of these solvents is methyl ethyl ketone, because it not only gives good inflections for phenol but permits excellent resolution of acid mixtures. -4lthough potassium hydroxide can be used as a titrant in this solvent, tetrabutylammonium hydroxide is preferable because it gives better inflections when the glass electrode is used and because of the greater solubility of its salts. Titration curves for a mixture of hydrochloric acid, acetic acid, and phenol in methyl ethyl ketone are shown in Figure 12. Curve 1, in which tetrabutylammonium hydroxide was used as titrant, and curve 2, in which potassium hydroxide was used, were both obtained xith the platinum-oxygen indicating electrode. These curves are similar to one another in shape and in voltage span and quite different from curve 3, which was obtained with the potassium hydroxide titrant and a glass indicating electrode. Apparently the reduced span and small phenol inflection of curve 3 are due to the effect of potassium ion on the glass indicating electrode.
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Figure 12. Titration of phenol-acetic acid-hydrochloric acid mixture in methyl ethyl ketone
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Figure 11. Titration of phenol in various solvents with tetrabutylammonium hydroxide in isopropyl alcohol Glass-calomel electrodes; curves shifted for clarity
DISCUSSION
The use of tetrabutylammonium hydroxide in the present investigation in no way implies that it is the best quaternary ammonium hydroxide for nonaqueous titrations. It was chosen as a convenient starting point for the exploratory study of these compounds. There is also reason to believe that more convenient and less expensive methods of preparing such titrants can be developed. Because of the wide variety of quaternary ammonium hydroxides, specific effects of structure may be pronounced in Some cases. This should be especially true in titrations conducted in solvents of very low dielectric constant where ion associa-
tion is important. The possibility of applying such specific effects to differentiate acids should not be overlooked. The curves obtained when phenol is titrated in nonaqueous solvents with tetrabutylammoniurn hydroxide support the contention that the hydroxide ion is a sufficiently strong titrant for very weak acids. It has also been shown that very weak acids can be successfully titrated with tetrabutylammonium hydroxide in solvents such as pyridine, isopropyl alcohol, ethyl ether, acetone, and methyl ethyl ketone, which are not generally considered as very basic solvents. The main purpose of this investigation was t o obtain information on the general characteristics of tetrabutylammonium hydroxide; therefore, no single titration solvent was studied in detail. It is apparent, however, that some of the solvents tested possess such interesting and useful properties that further study should prove profitable. This is especially true of pyridine and methyl ethyl ketone which combine three very important qualities of a good titration medium: (1) They are good solvents for a wide variety of materials; (2) they are sufficiently weakly acidic to permit the titration of very weak acids; and (3) they are sufficiently weakly basic to be good differentiating solvents. LITERATURE CITED
(1) Deal, V. Z., R7yld, G. E. A., ANAL.CHEM.27, 47 (1955). (2) Fritz, J. S.,Keen, R. T., Ibid., 25, 179 (1953) (3) Fritz. J. S.,Lisicki, N. M., Ibid., 23, 589 (1961). (4) Harlow, G. A., Koble, C. M., Wyld, G. E. A., Ibid,, 28, 784 (1956). (5) Hine, J., Hine, A I . , J. Am. Chem. SOC.74, 5266 (1962). (6) Moss, I f . , Elliot, J., Hall, R., ANAL.CHEM.20,784 (1948). (7) Weaver, J. R., Lykken, L., I b i d , 19, 372 (1947). RECEIVEDfor review October 3, 1955. Accepted February 6, 1955.
