Epinephrine ionization

Because most drugs exist in a reduced form, there may be an increased instability when the solution is consistently introduced into an atmosphere of 20% oxygen. The pH of the solution may effect the oxidation of phenolic and sulfhydryl group containing drugs because it is principally the ionized form of these drugs that participate in the oxidation (11). For example, epinephrine is only slowly oxidized at pH < 4 but rapidly degrades under alkaline pH conditions. 

Chemically, NE is classified as a catecholamine. A catechol is a 1,2-dihydroxybenzene, and NE is a p-hydroxyethylaminodihydroxybenzene. Epinephrine is the N-methyl derivative of NE, and they both have acidic and basic functional groups. Physiologically, however, they behave as a base, being more than 90% protonated at pH 7.4 (pKa = 9.6) and functioning as an ionized acid. 

---