Potentiometric Titration of Very Weak Acids Influence of Titrant Solvent GERALD A. HARLOW and GARRARD E. A. WYLD Shell Development Co., Emeryville, Calif. ,The inflections obtained in the potentiometric titration of very weak acids in acetone and pyridine are greatly affected by the acidity of the titrant solvent. Tetra-n-butyl ammonium titrants prepared in isopropyl alcohol are superior to titrants made up in ethyl alcohol, methanol, and water. Methanol is as harmful as water in this respect.
T
importance of preparing strongbase titrants in solvents of low acidity is not generally recognized. Many investigators who have studied the titration of very weak acids in nonaqueous solvents realize the importance of keeping water out of the solvent, but few seem to realize the harmful effect oi other solvents such as methanol. For this reason a few comparative titration curves, obtained with titrants made up in various media, are presented here. Titration procedures and preparation of titrants have been described (1-3). A glass-calomel electrode system was used throughout. The titrant concentration was approximately 0.1N except where indicated otherwise. The curves in Figure 1 represent titration of phenol in acetone with tetra-n-butyl ammonium titrants made up in water, methanol, ethyl alcohol, and isopropyl alcohol. The titrations are approximately equal and were designed to introduce about 10% of titrant into the acetone a t the end point. The curves in Figure 2 represent titration of phenol in pyridine with the four titrants. The solvent a t the end point contains about 2.5y0titrant. Titration curves for another very weak acid, 2,6di-tert-butyl-4-methylpheno1, are shown in Figure 3. The solvent is again pyridine and a t the end point about 2.5y0of the titrant has been introduced. The large differences in the size of the inflections obtained in the titration curves can be explained on the basis of differences in the acidity of water and the various alcohols (4). As these solvents are all acidic in character, they limit the basicity which can be established in their presence. Under the titration conditions methanol and water are stronger acids than ethyl alcohol, which in turn is a stronger acid than isopropyl alcohol. HE
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Y o l u r e a i T i t r a i t . ml.
Figure 3. Titration of 2,6-di-terfbutyl-4-methylphenol in pyridine with tetra-n-butyl ammonium hydroxide and alcoholates
Figure 1. Titration of 0.2 meq. of phenol in acetone with tetra-n-butyl ammonium hydroxide and alcoholates
600
L o . 02 ml.
curve A
500
Volume of T i t r a n t
Volume of T l l i n n l
Figure 2. Titration of 0.04 meq. of phenol in pyridine with tetra-n-butyl ammonium hydroxide and alcoholates
It is apparent that a titrant prepared in isopropyl alcohol is better for very weak acids than one prepared in water, methanol, or ethyl alcohol, and that a less acidic solvent than isopropyl alcohol would be even more desirable. Unfortunately, in the case of quaternary ammonium titrants such as tetra-nbutyl ammonium compounds, titrants prepared in less acidic solvents appear to be less stable. A compromise must thus be sought between acidity and stability of the titrant. The detrimental effect of the titrant solvent on the size of the inflections obtained is a function of amount and acidity of titrant. The volume of titrant introduced during the titration can be kept small by using a small sample and relatively concentrated titrants. The influence of titrant concentration is shown in Figure 4 for the
Figure 4. Titration of 0.1 meq. of phenol in pyridine with 0.081 and 1 .ON tetra-n-butyl ammonium isopropylate in isopropyl alcohol
titration of 0.1 meq. of phenol in pyridine with 0.08 and 1 . O N tetra-n-butyl ammonium titrants. The small inflection a t the mid-point of curve A is probably due to the acid-anion association complex of the type reported by Van der Heidje (6). Such association is not apparent when larger amounts of isopropyl alcohol are present (curve B ) , LITERATURE CITED
Cundiff, R. H., Markunas, P. C., ANAL. CHEM.
28, 792 (1956).
Deal, V. Z., ffyld, G. E. A . , Ibid., 27, 47 (1955).
Harlow, G. A,, h'oble, C. M., Wyld, G. E. A., Ibid., 28, 787 (1956). Hine, J., Hine, M., J.Am. Chem. Soc. 74,5266 (1952).
Van der Heijde, H. B., Anal. Chim. Acta 16, 392 (1957).
RECEIVEDfor review May 20, 1957. Accepted August 30, 1957. VOL. 30, NO. 1, JANUARY 1958
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