Materials Required:
● Salicylic acid
● 0.1 N HCl
● Ferric nitrate solution
● Distilled water
● Spectrophotometer
● Cuvettes, test tubes, beakers, conical flasks
● Pipettes and measuring cylinders
● Digital weighing balance
● Water baths maintained at 40°C, 60°C, and 70°C

Procedure:

1. Prepare a standard solution by dissolving 100 mg of salicylic acid in 100 mL distilled water to obtain a 1 mg/mL standard solution.
   
2. Prepare an acid solution by adding 8.5 mL concentrated HCl to distilled water and make up to 100 mL (handle with care).
   
3. For the calibration curve, pipette known concentrations of salicylic acid into test tubes, add ferric nitrate reagent, mix well, and measure absorbance at 540–547 nm.
   
4. Plot absorbance versus concentration to obtain the calibration curve.
   
5. Prepare an aspirin solution of known concentration and incubate in water baths at 40°C, 60°C, and 70°C for 10, 20, and 30 minutes.
   
6. Withdraw aliquots at each time interval, cool them, add ferric nitrate reagent, and measure absorbance.
   
7. Calculate the percentage of undecomposed drug and plot log(% drug undecomposed) vs. time to confirm first-order kinetics.
   
8. From the slope, determine rate constant (k) at each temperature.
   
9. Plot log k vs. 1/T (Arrhenius plot) to determine activation energy (Ea).
   
10. Estimate shelf life (t₉₀) using the first-order equation.