Degradation kinetics of neostigmine in solution

The degradation kinetics of neostigmine were studied in aqueous solutions w ith varied pH from 1.5 to 9.9 under accelerated storage conditions. The sta bility of neostigmine in solutions containing propylene glycol or polyethyl ene glycol 400 was also investigated. The reaction order of neostigmine in these aqueous and solvent systems followed pseudo-first-order degradation k inetics. The degradation rates of neostigmine under various buffer concentr ations within the investigated pH range were obtained. They indicated that the degradation was independent of the species of buffering agent. Maximum stability of neostigmine was determined at pH 5.0 buffer species conditions . The activation energy could be estimated from the Arrhenius plot as 15.72 kcal/mole. The half-life of 883.7 days was estimated at room temperature i n 0.1 M, pH 4.9 acetate buffer solution (mu = 0.5). Ultraviolet (UV) irradi ation at 254 nm of the neostigmine solutions in pH 4.9 acetate buffer showe d an accelerated degradation in comparison with light-protected samples. In corporation of propylene glycol into the neostigmine solution at pH 4.9 enh anced the stability; however, an adverse effect on the stability of neostig mine was noted when a polyethylene glycol 400 solvent system was used.