The result is that an acid dye can approach an acid material closely enough that hydrogen bonding or van der Waal's forces can bind them. The scenario above is not, in fact, hypothetical. It is the explanation for the inclusion of sodium chloride into ethanol solutions of congo red for the demonstration of amyloid. Its inclusion enables a higher degree of hydrogen bonding than would otherwise be the case. The salt is therefore an accentuator in the sense of increased staining.
Mucopolysaccharides are ionic complexes and react with alcian blue ionically.
The addition of magnesium as an ionic salt preferentially inhibits staining with the dye by competing with it for the negative charge on the mucosubstance. Since different acid mucopolysaccharides require different concentrations of magnesium ions to overcome the attraction between alcian blue and the mucosubstance, it can be used to differentiate between them. How phenol accentuates staining is still not completely clear. The explanations tend to be speculative, and are not particularly convincing. It has been suggested that the mild acidity produced by solutions of phenol is the cause of intensified staining, and adding phenol is fundamentally pH control.