Spectrophotometric determination of catechol, epinephrine, dopa, dopamine and other aromatic vic-diols

Abstract

Aromatic vic-diols such as catechols, epinephrine, norepinephrine, dopa, and dopamine can be determined spectrophotometrically by nitration with sodium nitrite in slightly acidic solution. The method is selective for aromatic vic-diols in which either the 3- or 4-position is unsubstituted and these positions are not sterically blocked. Tungstate or molybdata ions increase the color intensity, probably by stabilizing the nitrated phenol by complex formation. Molybdate is more effective than tungstate. For most diphenols the optimum pH is around 5.0–5.5 (acetate buffer) but this depends on the diphenol as well as on the particular buffer system. The optimum time for nitration (2–30 min) also depends on the diphenol. After nitration for the optimum duration, the solution is made basic; an absorption band appears at about 500 nm with a molar absorptivity of ca. 10 4 1 mol -1 cm -1. The system follows Beer's law provided that molybdate ion is present and both the pH and reaction time are optimized. The minimum detectable amount of diphenol is about 5 nmol, for an absorbance of 0.01 in a 1.00-cm cell. Phenol, hydroquinone, resorcinol, pyrogallol, phloroglucinol, guaiacol, and veratrole, do not interfere.