Effect of acidity and alkalinity on the distillation of phenol: interferences of aromatic amines and formaldehyde with the 4-aminoantipyrine spectro-photometric method for phenol

As determined by the 4-aminoantipyrine (4-AAP) spectrophotometric method, the distillation of phenol is quantitative over the range from about pH 6 to very strongly acidic solutions. Recovery from alkaline solutions decreases with increasing alkalinity. Aromatic amines can interfere with the 4-AAP method by producing colors. The extent of the interference varies markedly with different aromatic amines and is much greater for the extraction method than the direct method. The interference can be considerably reduced by distillation from a strongly acidic solution (10 ml of concentrated sulfuric acid per 500 ml); for large amounts of aromatic amines, double distillation may be necessary. Formaldehyde can interfere by reacting with phenol and repressing the color development. This interference can be eliminated by treatment with ammonium sulfate and sodium hydroxide to form hexamethylenetetramine, followed by acidification and distillation.