Aspirin (acetylsalicylic acid) undergoes hydrolysis in the presence of moisture or heat to yield salicylic acid and acetic acid. The reaction is accelerated at higher temperatures and follows first-order kinetics, making it suitable for accelerated stability testing.

The concentration of salicylic acid formed can be determined spectrophotometrically using ferric nitrate, which reacts with the phenolic group of salicylic acid to produce a purple complex measurable at 540–547 nm.

The first-order degradation rate constant (k) is calculated using the equation:
log(% Drug Undecomposed) = -kt / 2.303 + log C₀

The Arrhenius equation is used to calculate activation energy (Ea):
log k = log A - Ea / (2.303RT)

By plotting log k vs. 1/T, the slope of the straight line gives -Ea / 2.303R, from which the activation energy can be determined. This data allows estimation of the shelf life (t₉₀) of aspirin at room temperature.