Is Acetylsalicylic acid stable? An undergraduate laboratory experiment

CECILIA M. PERALTA; VIRGINIA DAVÍN; CRISTINA ALMANDOZ

Congreso; III Reunión Internacional de Ciencias Farmacéuticas; 2014

Aspirin (ASA) is the main salicylate belonging to the AINEs family drugs. It has been widely used due to its analgesic, anti-inflammatory and antipyretic properties. The decomposition reaction of ASA generates salicylic (AS) and acetic acids, which takes place in alkaline medium. Among others, surfactants have the capacity to modify chemical reaction rates and equilibrium, and play an important role in pharmaceutical formulations. The purpose of this work was to investigate, in three days, the ASA hydrolysis in presence and absence of surfactant?s micelles. A kinetic study of the alkaline hydrolysis of ASA at 37°C in presences of cationic (hexadecyltrimethylammonium bromide, CTAB) and anionic (sodium dodecyl-sulfate, SDS) surfactants (0-0.02 M) were evaluated. Hanna E25 conductivity-meter and Cary 50-UV-Visible spectrophotometer, were used. Day 1: Critical micelle concentration (cmc) determination. The obtained cmc values were 7.82×10-3M (SDS) and 1.07×10-3M (CTAB). Day 2: UV-spectrophotometric study of ASA hydrolysis (without surfactants). The reaction rates were studied at 296 nm (λmax AS). The kinetic reactions were carried out at four pHs and analyzed using the expressions for pseudo first-order kinetics, ln⁡(A_(∞ )- A_t )=- k_obs t+ln⁡〖A_∞ 〗 k_obs=k [OH] At and A∞: absorbance at different times and t=∞, respectively. Rate constants, k_obs, increase linearly with an increase in [NaOH]. Day 3: Micellar media effect. Higher and lower concentrations of surfactant than cmc, were used. Solutions with low [CTAB] were shown a catalytic effect. However, this reaction was inhibited at higher surfactant concentrations. In the case of anionic micelle, a very low catalytic effect occurs. Both surfactant behaviors were more evident at pH 12.5. The ASA hydrolysis was determined (k = 9.52×10-2 M-1min-1). The presence of both surfactant showed a catalytic effect on ASA decomposition reaction. The proposed technique is useful in chemical kinetics teaching. These experiences allow pharmacy students a comprehensive study of the drugs stability.