Ultraviolet Absorption of Local Anesthetics with an Aromatic Amino Group as a Function of pH

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Abstract
The ultraviolet absorption of benoxinate, butethamine, chloro-procaine, metabutethamine, metabutoxycaine, procaine, proparacaine, propoxycaine, and tetracaine hydrochlorides in solutions with a pH of 0.1 to 10 was determined. From this data, the apparent dissociations of the aromatic amino group and the isosbestic points of these compounds were determined. The isobestic point is defined as the wavelength where the absorbancy indexes are the same for two materials that are interconvertible, as in the hydrogen ion activity, redox potential or tautomeric equilibria. The effect of chemical structure and pH of the solution on the ultraviolet absorption is discussed as well as the conditions most suitable for analysis of these compounds.