weak acid
Summary
Acid that only partially dissociates in solution (pKa typically 3-7). Pharmacologic relevance: weak acids are uncharged (lipid-soluble, absorbed) at low pH and ionized (trapped) at high pH; alkalinizing urine traps them and accelerates renal excretion in overdose.
Detail
By Henderson-Hasselbalch, when pH < pKa a weak acid is mostly protonated/uncharged (HA) and crosses lipid membranes, while at pH > pKa it dissociates into A- (charged, trapped). This explains why weak acids (aspirin/salicylates, phenobarbital, methotrexate) are well absorbed in the stomach but reabsorbed less in alkaline urine. In overdose, sodium bicarbonate is given to alkalinize urine and 'ion-trap' the acid in the tubular lumen, increasing excretion. Conversely, weak bases (amphetamines, TCAs, PCP) are trapped in acidic urine, although urinary acidification is rarely used clinically due to acidosis risk. Key high-yield: 'AB salt' mnemonic — Alkalinize for Acids, acidify for Bases; pKa determines the pH at which 50% of drug is ionized.
Sources
- First Aid for USMLE Step 1 2024
- Katzung Basic & Clinical Pharmacology
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