Differential Congo red staining: the effects of pH, non-aqueous solvents and the substrate

Congo red is an acid-base indicator dye. In free solution the colour and absorption characteristics of Congo red depend not only on the pH but are also governed by the nature of the solvent environment. In tissue sections stained by Congo red, alteration of the pH and the use of non-aqueous solvents can effect differential colouring of the tissue components. Stained sections of unmodified and chemically substituted celluloses show that differential red or blue coloration reflects the acidic or basic character of the substrate. In stained tissue sections, secondary protein structure and porosity of the substrate may also influence their colour. The effect of non-aqueous solvents is probably to modify the ionization state of the dye-substrate complex, thus altering the colour of the Congo red. Such solvents may also change the aggregation or solvation states of the dye, with consequent modification in the colour of tissue components.