Measurement of pH near dissolving enteric coatings

Previous authors have predicted the pH at the surface of enteric coatings using mathematical models. The purpose of our work is to experimentally determine the pH. A modified laser spectrometer was constructed to excite fluorescein in the medium above a dissolving enteric coating--hydroxypropylmethylcellulose phthalate (HP) in buffer (pH 6.5 and 7.0) in the absence and presence of forced convection. As predicted by mathematical models, the pH near dissolving enteric coatings is reduced relative to bulk pH. The reduction in pH is 0.2 to 0.4 depending upon the pH of the buffer relative to the pK(a) of the polymer. The pH near (i.e. about 250 microm from the surface) HP-50 is lower than that near HP-55 under the same conditions. The experimental data agree to a first approximation with data from mathematical models.