Amantadine (weak base, pKa = 10.8) is a sparingly water-soluble drug used in treating some viral diseases. At which ONE of the following pH values is amantadine most soluble in water?

Question

BlackTom AI · Accepted Answer

A weak base is most water soluble in its protonated (ionized) form; for a base the Henderson–Hasselbalch equation relates pH, pKa and the ratio of unprotonated (B) to protonated (BH+): pH = pKa + log([B]/[BH+]).

Step-by-step reasoning:
1) Relevant concepts:
- Amantadine is a weak base (pKa = 10.8). The protonated form BH+ is ionic and much more water-soluble than the neutral base B.
- To maximize solubility, maximize the fraction of the drug present as BH+ (i.e., keep pH well below pKa).

2) Use the Henderson–Hasselbalch form to find fraction protonated:
[B]/[BH+] = 10^(pH − pKa).
Fraction protonated = [BH+]/([B]+[BH+]) = 1 / (1 + 10^(pH − pKa)).

3) Calculate intermediate values for each choice (pKa = 10.8):
- pH = 6.5: exponent = 6.5 − 10.8 = −4.3 → 10^(−4.3) ≈ 5.01×10^−5.
  Fraction BH+ ≈ 1 / (1 + 5.01×10^−5) ≈ 0.99995 (≈ 99.995% protonated).
- pH = 7.5: exponent = −3.3 → 10^(−3.3) ≈ 5.01×10^−4.
  Fraction BH+ ≈ 0.9995 (≈ 99.95%).
- pH = 8.5: exponent = −2.3 → 10^(−2.3) ≈ 5.01×10^−3.
  Fraction BH+ ≈ 0.995 (≈ 99.5%).
- pH = 9.5: exponent = −1.3 → 10^(−1.3) ≈ 0.0501.
  Fraction BH+ ≈ 0.952 (≈ 95.2%).

4) Why the correct answer is correct:
At pH 6.5 the drug is essentially completely protonated, so the ionic, highly water-soluble form predominates. That guarantees the greatest aqueous solubility compared with the higher pH choices.

5) Why other options are incorrect:
Although pH 7.5 and 8.5 still give high protonation, they produce slightly lower fractions of BH+ than pH 6.5, so solubility is slightly less. pH 9.5 gives the lowest protonation of the choices and therefore the lowest solubility.

Conclusion: The most soluble pH among the options is a. pH = 6.5, which matches the provided answer.

Amantadine (weak base, pKa = 10.8) is a sparingly water-soluble drug used in treating some viral diseases. At which ONE of the following pH values is amantadine most soluble in water?Single choice

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