A weak base with a pKa around 7 is in the stomach (pH 1.2). What is its ionization state in the stomach?

A weak base with a pKa around 7 is in the stomach (pH 1.2). What is its ionization state in the stomach?

• A. Ionized
• B. Unionized
• C. Neutral
• D. Both

Answer: (A)

When pH is far below the pKa, a weak base is mostly in its protonated, charged form. For a base, the relationship is pH = pKa + log([base]/[conjugate acid BH+]). Here pH is 1.2 and pKa is about 7, so pH − pKa = −5.8. This means [base]/[BH+] ≈ 10^(−5.8) ≈ 1.6 × 10^−6, so the conjugate acid BH+ vastly dominates. Nearly all of the molecule is protonated and ionized (BH+), not unionized. In the stomach, this ionized form will have very limited membrane permeability, which is why ionization state matters for absorption.

When pH is far below the pKa, a weak base is mostly in its protonated, charged form. For a base, the relationship is pH = pKa + log([base]/[conjugate acid BH+]). Here pH is 1.2 and pKa is about 7, so pH − pKa = −5.8. This means [base]/[BH+] ≈ 10^(−5.8) ≈ 1.6 × 10^−6, so the conjugate acid BH+ vastly dominates. Nearly all of the molecule is protonated and ionized (BH+), not unionized. In the stomach, this ionized form will have very limited membrane permeability, which is why ionization state matters for absorption.