A reinvestigation of a colorimetric method aimed at quantifying minute amounts of carboxylic acids present in polystyrene and poly(n‐alkyl methacrylate)s

Abstract

A reinvestigation of a method (Christoff, M.; Yamaki, S. B.; de Oliveira, M. G.; Atvars, T. D. Z. J. Appl. Polym. Sci. 2004, 92, 830-838) aimed at quantitatively determining trace amounts of residual carboxylic acid functional groups in polymethacrylates and polystyrene has been carried out. The original technique was based on the monitoring of an absorption band in the UV-visible region (610 nm) that appears after known amounts of polymer are added to a benzene solution containing the basic form of the Nile Blue A dye. It is shown here that the assumption upon which the methodology is based, that is, the occurrence of a 1:1 stoichiometric and complete proton transfer of the carboxylic acid to the dye, is invalid and yields greatly underestimated values for the COOH contents, possibly by 2-3 orders of magnitude. In addition, temperature strongly influences the equilibrium constant for the acidbase reaction. Lastly, preparation and storage of the analytical solution was found to be problematic, supporting earlier claims that hydrolysis of Nile Blue A base does occur when preparing the benzene solution and/or over storage, yielding Nile Red dye whose absorption in the visible part of the spectrum significantly overlaps. Based on the above results, further use of this technique, at least in its quantitative version, should be discouraged.