See the DrugPatentWatch profile for aspirin
What is aspirin’s pKa?
Aspirin (acetylsalicylic acid) is a weak acid. Its commonly cited pKa for the carboxylic acid group is about 3.5 (often reported in the ~3.3–3.5 range depending on conditions like solvent and temperature).
Which part of aspirin is responsible for that pKa?
The pKa value refers to the acidic proton on aspirin’s carboxylic (–COOH) group. After deprotonation (–COO− form), aspirin becomes more soluble in water, which is why the pKa is relevant to things like dissolution and absorption.
Why does pKa sometimes vary by source?
Different references may report slightly different pKa values because measurements can vary with:
- solvent (water vs. buffered solutions),
- ionic strength,
- temperature,
- and the experimental method used to fit the acid–base equilibrium.
How pKa affects aspirin in the body
Because aspirin’s pKa is near the lower end of physiological pH (blood is around pH 7.4), a large fraction of aspirin exists in the deprotonated (ionized, –COO−) form at typical body pH. This influences absorption and distribution.
If you need a specific pKa for a calculation
Tell me the context you’re using (e.g., Henderson–Hasselbalch calculation, formulation/dissolution, or a specific reference like “in water at 25°C”), and I can help match the right pKa value to that condition.