Aspirin Metabolism in the Body

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The aspirin hydrolysis reaction is aspirin + water ⇌ salicylic acid + acetic acid. This Demonstration investigates the effects of the aspirin dosage on both chemical equilibrium and rate of aspirin hydrolysis. We also take into account the effects of varying blood plasma concentrations of acetic acid and salicylic acid on the position of chemical equilibrium. Notably, the extreme ends of the (number of moles of acetic acid) slider correlated to the average levels of plasma acetic acid in healthy patients and in diabetic patients, allowing us to elucidate the effects of diabetes on chemical equilibrium of this reaction in vivo. The effects of body temperatures (ranging from hypothermia to hyperthermia) on the rate of reaction are shown to illustrate how varying body temperatures can affect the absorption of this drug. It is also known that as the concentration of salicylic acid in a solution containing ions is increased, salicylic acid forms blue-violet complexes with these ions; thus, we altered the color of the salicylic acid bar in the chemical equilibrium bar chart, making the blue darker as the concentration of salicylic acid (and thus, the absorption of salicylic acid according to the Beer–Lambert law) increased.

Contributed by: Matthew Yacoub, Jihyun Park and Kevin Hsieh (May 8)
Open content licensed under CC BY-NC-SA


This Demonstration considers both chemical equilibrium and reaction rate. The chemical equilibrium is represented by the temperature-dependent equilibrium constant equation, a function of product and reactant concentrations. Le Châtelier’s principle determines the effect on equilibrium of changing concentrations. We consider the effects of salicylic acid concentration and absorbance, in accordance with the Beer–Lambert law. This determines the color of a solution containing ions.

The Arrhenius equation is used to determine the value of the rate constant at any given temperature, with the value of , the exponential pre-factor, derived from experimental values of the rate constant and activation energy of aspirin hydrolysis. The Arrhenius equation is temperature-dependent, allowing us to study the dependence of rate on body temperature. We show three plots representing the rate of the reaction. The reaction is a pseudo first order, which enables us to make use of the first-order reaction integrated rate law (which depends on the initial concentration of aspirin). The first plot uses the integrated rate law to determine the concentration of aspirin over time. The second plot multiplies the value of the first plot by the Arrhenius-derived rate constant to show the rate of hydrolysis over time. The third plot shows the natural log of the value of the first plot for a linear graph of aspirin concentration vs. time.


[1] A. R. Fersht and A. J. Kirby, "Hydrolysis of Aspirin. Intramolecular General Base Catalysis of Ester Hydrolysis," Journal of the American Chemical Society, 89(19), 1967 pp. 4857–4863. doi:10.1021/ja00995a007.

[2] "Aspirin." National Institute of Standards and Technology, (Apr 4, 2023)

[3] Libretexts. "The Equilibrium Constant." LibreTexts Chemistry. (Apr 4, 2023) _Chemistry _Textbook _Maps/Supplemental_Modules_ (Physical_and _Theoretical _Chemistry)/Equilibria/Chemical_Equilibria/The_Equilibrium_Constant.

[4] Libretexts. "Reaction Rates." LibreTexts Chemistry. (Apr 4, 2023) A_Chemistry _-_The _Central _Science _ (Brown_et _al.)/14%3 A_Chemical _Kinetics/14.02%3 A_Reaction _Rates.

[5] R. F. Smith, S. Humphreys and T. D. Hockaday, "The Measurement of Plasma Acetate by a Manual or Automated Technique in Diabetic and Non-diabetic Subjects," Annals of Clinical Biochemistry, 23(3), 1986 pp. 285–291. doi:10.1177/000456328602300307.

[6] M. Vrtačnik, V. Mesec and N. Gros, "2. Comparison of Aspirin Hydrolysis at Ambient and Elevated Temperature." Chemical Kinetics. (Apr 4, 2023)


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