The absorbance spectrum of the immobilized Congo Red
shows a red shift in comparison to its free form. The absorbance
maxima at two extreme pH values are at 504 and 600 nm for
the former and 493 and 567 nm for the later. The same shift is
observed for the isobestic points. Similar observations have been
reported by others [10]. The ratio of the peak-height absorption
maxima of immobilized Congo Red is 1.20 and that of the free
dye is 1.76.