Knowledge of possible variability in the dissociation constant (pKa) among preparations of phenol red-polyacrylamide was needed during development of a fiber optic probe to monitor fetal blood pH. Determination of the pKa through absorbance-wavelength spectra on opaque 50% glycerol suspensions of phenol red-resin required second derivative spectrophotometry. The apparent pKa was 8.0 for both the dye and several batches of its resin derivative. Spectrophotometry of comparable aqueous solutions of phenol red yielded a pKa of 7.78 ± 0.02, at μ=0.25M.