Indicator Titrations in Sulfolane Arden P. Zipp Chemistry Department, State University College, Cortland, N . Y.13045 THEREHAS BEEN a great deal of recent interest in the solvent properties of sulfolane (tetramethylene sulfone). A potentiometric study ( I ) has shown that sulfolane and its derivatives, 3-methylsulfolane (3-MS) and 2,4-dimethylsulfolane (2,4-DMS), perform very well as titration media for the differentiation of acids and bases of widely varying strengths. The occurrence of steep titration curves, as was found for most titrations in that study, has been shown (2) to be characteristic of conditions where good indicator end points may be expected. For this reason, the behavior of several indicators in sulfolane has been investigated and the results of this study are reported here.
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EXPERIMENTAL Chemicals. Sulfolane and 3-MS (Phillips Petroleum) were each doubly-distilled from sodium hydroxide in vacuo (3). The middle one-half from each distillation was used (sulfolane bp 110-111 “C 1 torr, 3-MS bp 101-102 “C 1 torr), the specific conductivity of the final sulfolane fraction being less than 4 X lo-* mhos. All indicators were used as 0.2% solutions in 3-MS. The tetramethylammonium hydroxide (0.2N in methanol) which was used for the titration of acids was prepared by diluting a 25% stock solution obtained from Eastman Kodak and was standardized against benzoic acid. Perchloric acid was diluted to approximately 0.2N with dioxane and standardized with 1,3-diphenylguanidine. Apparatus and Procedure. Titrations were performed manually with a Beckman Zeromatic pH meter using a standard calomel and glass electrode pair. A salt bridge of tetramethylammonium perchlorate in 3-MS was used to achieve electrical contact between the titration vessel and the calomel electrode, which was maintained in 1M aqueous tetramethylammonium chloride. Titrants were delivered from a 10-ml microburet graduated in 0.05 ml with all titrations being performed under nitrogen. RESULTS AND DISCUSSION Seven common acid-base indicators, methyl orange, thymol blue (thymosulfophthalein), bromcresol purple, phenolphthalein, azo violet [4-(p-nitrophenylazo)-resorcinol),alizarin yellow, and curcumin, were investigated in the titration of both acids and bases in sulfolane. Methyl orange, bromcresol purple, and thymol blue proved to be effective for the titration of weak bases [1,3-diphenylguanidine pK, (H20)= 10.01, while for very weak bases [p-toluidine pK, (H20) = 5.081, only methyl orange showed a detectable end point and that was deemed inadequate for analytical purposes because of the gradual nature of the transition from yellow to red. Thymol blue showed its customary yellow to violet color change and methyl orange its usual yellow to red transition, while in the region of the equivalence point bromcresol purple displayed a series of colors as shown in Figure 1. (1) D. H. Morman and G . A. Harlow, ANAL.CHEM., 39, 1869
(1967). (2) I. Gyenes, “Titrations in Non-Aqueous Media,” D. Van Nostrand Co., Princeton, N. J., 1967. (3) M. Della Monica, U. Lamanna, and L. Senatore,J . Phys. Chem., 72, 2124 (1968).
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Figure 1. Titration of 0.2N diphenylguanidine in the presence of bromcresol purple indicator In the titrations of weak acids (benzoic acid pK, = 4.2) phenolphthalein, curcumin, bromcresol purple, azo violet, and thymol blue performed satisfactorily but did not give detectable color changes for very weak acids (phenol pK, = 9.89). The yellow to blue color change of thymol blue was the sharpest of those examined while the usual colorless to pink transition for phenolphthalein was very faint and difficult to recognize visually. Azo violet and bromcresol purple exhibited the series of color changes shown in Figures 2 and 3 while curcumin was notable in changing from yellow through orange to purple (which coincided with the potentiometric end point in this instance) then back to orange. The diversity of color changes which these indicators exhibit would require that the color change corresponding to the end point be determined for a particular titration before the indicator could be used routinely. This same color variation could be useful, however, in the titration of a mixture of acids or bases, once the specific color changes had been assigned for each end point. The reproducibility of end-point detection and the necessity of applying blank corrections were determined for each indicator by comparing end points for several indicator titrations with those found potentiometrically in the absence of indicator. All the indicators so examined were found to be free from appreciable blank corrections and, once the desired color change had been selected for a particular indicator, the reproducibility from titration to titration was very good (+0.3z) In.this regard, thymol blue and methyl orange ANALYTICAL CHEMISTRY, VOL. 42, NO. 8 , JULY 1970
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Figure 2. Titration of 0.2N benzoic acid in the presence of azo violet indicator
Figure 3. Titration of 0.2N benzoic acid in the presence of bromcresol purple indicator
performed best in the titration of bases because of the distinct character of their single color changes while thymol blue was preferable in the titration of acids for the same reason. This preference for thymol blue has also been reported for both acid and base titrations in DMF ( 4 ) and for titrations of acids in tetramethylurea (5). The effect of water on the detection of end points was investigated for both indicators and the potentiometric method, because of their sensitivity to the presence of water in some solvents (2). The presence of 2 0 z water by volume caused a decrease in the potential span from 610 rnV to 360 mV in the titration of benzoic acid with TMAH, although the number of milliliters of titrant required for neutralization was unchanged. Under the same conditions, the indicators methyl orange and thymol blue gave distinct end points with their customary color changes. In conclusion, the behavior of the indicators studied here suggests that potentiometric methods of end-point detection may still be preferable for those applications which depend on the special characteristics of sulfolane. For example, the
very weak acidity and basicity of sulfolane make it possible to titrate extremely weak acids such as phenol (pK, = 9.89) and bases such as p-chloroaniline [pK, (H20) = 2.61 but none of the indicators examined gave satisfactory end points for either phenol or p-toluidine [pK, (HzO) = 5-11. However, where a large number of titrations of sufficiently strong acids or bases are to be performed in this solvent, indicators may be employed profitably because of the greater convenience associated with their use.
(4) J. S. Fritz, ANAL.CHEM., 24, 306 (1952). (5) S. L. Culp and J. A. Caruso, ibid.,41,1876 (1969).
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RECEIVED for review March 2, 1970. Accepted May 4,1970.
Correction Acid-Base Microtitrations Based on Serial Dilution In this article by J. R. Robinson et al. [ANAL.CHEM., 42, 495 (1970)l there is an error on page 496, line 4 of the Experimental Section. Roger Gilmont Instruments of Great Neck, N. Y., was incorrectly identified as Roger Gilman Instruments.
