From Protonation to Partitioning: Equilibrium-Driven Phase Behavior Extraction of Amphetamine from Urine

Marina Sadek

Co-Presenters: Individual Presentation

College: Hennings College of Science Mathematics and Technology

Major: BA.CHEM/PREPROF

Faculty Research Mentor: Mongelli, Matthew  Stokes-Huby, Heather

Abstract:

AbstractAcid–base controlled liquid–liquid extraction is used to selectively isolate amphetamine from aqueous urine matrices by making use of protonation equilibria of the primary amine functional group. Amphetamine with a pKₐ ≈ 9.9 exists mainly in its protonated ammonium form BH⁺ under physiological and acidic conditions, which results in high aqueous solubility and minimal partitioning into nonpolar organic solvents. To observe these interactions, urine samples are acidified to suppress the extraction of amphetamine while also removing neutral lipophilic interferers. As a result, basification shifted the equilibrium toward the neutral free base (B), which decreased hydration and increased affinity for the organic phase, promoting efficient transfer into an immiscible organic solvent. This pH-dependent speciation directly looks over the distribution coefficient (D) and extraction recovery, which allows selective control of amphetamine partitioning through equilibrium displacement instead of through chemical derivatization. This system gives a mechanistically simple model where extraction efficiency is quantitatively linked to acid–base equilibria, solvation energetics, and phase transfer thermodynamics, and these results show how controlled proton activity can be used to optimize analytical recovery and matrix cleanup for basic drugs in biological fluids. This relationship, taken together, shows how selective separation and chemistry in complex aqueous matrices can be rationalized through classical equilibrium and solvation thermodynamics, thereby reinforcing the enduring relevance of fundamental acid-base chemistry in modern analytical applications.¹–³Key words: Amphetamine, Acid–base extraction, Protonation equilibria, Distribution coefficient (D), pH-dependent solubility, Phase partitioning, Organic solvent extraction, Urine matrix, Solvation energetics, Phase transfer thermodynamics, and Equilibrium displacement