With a drug of pKa 3, plasma pH 7.4 and stomach pH 1.2, where would you expect higher concentrations?

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Study for the Pharmaceutics Xenobiotics Across Bio Membrane Test. Prepare with flashcards and multiple-choice questions, each providing hints and explanations. Get ready for your pharmacy exam!

Multiple Choice

With a drug of pKa 3, plasma pH 7.4 and stomach pH 1.2, where would you expect higher concentrations?

Explanation:
Ionization depends on pH relative to pKa. For a weak acid (pKa = 3), the non-ionized form dominates when pH < pKa, and the ionized form dominates when pH > pKa. In the stomach, pH is 1.2, well below 3, so the drug is mostly unionized. The unionized form is more lipophilic and crosses membranes easily, concentrating there. In plasma, pH is 7.4, well above 3, so most of the drug is ionized and less able to diffuse into tissues. Thus the higher concentration of the unionized form would be in the stomach.

Ionization depends on pH relative to pKa. For a weak acid (pKa = 3), the non-ionized form dominates when pH < pKa, and the ionized form dominates when pH > pKa. In the stomach, pH is 1.2, well below 3, so the drug is mostly unionized. The unionized form is more lipophilic and crosses membranes easily, concentrating there. In plasma, pH is 7.4, well above 3, so most of the drug is ionized and less able to diffuse into tissues. Thus the higher concentration of the unionized form would be in the stomach.

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