Some drugs are formulated as their salt form. The salt form is used because:

Question

BlackTom AI · Accepted Answer

Salt formation increases the fraction of drug present as an ion, which typically raises aqueous solubility and therefore the dissolution rate.

1) Relevant knowledge points and concepts
- Weak acids/bases ionize depending on pH (Henderson–Hasselbalch relationship).
- Salts of drugs are pre-ionized ionic species that are often more water‑soluble than the free base/acid form.
- Dissolution rate (Noyes–Whitney) is proportional to the solubility at the particle surface (Cs): dM/dt = (D·A/h)·(Cs − C), so increasing Cs increases dissolution rate.

2) Step‑by‑step reasoning
- For a weak base B with pKa = 8, fraction ionized at a given pH = 1/(1 + 10^(pH−pKa)). 
- At pH 7: fraction ionized = 1/(1 + 10^(7−8)) = 1/(1 + 0.1) = 0.909 (≈91% ionized → high solubility).
- At pH 9: fraction ionized = 1/(1 + 10^(9−8)) = 1/(1 + 10) = 0.091 (≈9% ionized → low solubility).
- Converting the drug into its salt (e.g., BH+ Cl−) effectively supplies the ionized form over a wider pH range, so measurable solubility is higher across pH values where the free base would be largely unionized.

3) Calculations shown
- Example above shows how ionization fraction changes with pH and pKa; higher ionized fraction correlates with higher aqueous solubility.
- Noyes–Whitney: increasing Cs (solubility at the surface) increases dM/dt (dissolution rate), assuming diffusion coefficient D, surface area A and boundary layer thickness h are constant.

4) Why the correct answer is correct
- Salt formation improves solubility (makes more drug dissolve) and, because dissolution rate depends on solubility, it also improves dissolution over a range of pH values where the free form would be less ionized.

5) Why other options are incorrect
- Option a (only improved solubility) is incomplete because improved solubility leads to improved dissolution as well.
- Option b (only improved dissolution) is incomplete because the primary effect is on solubility; dissolution improvement follows from that.

6) Conclusion
- Both statements are true; therefore the correct answer is c.

Some drugs are formulated as their salt form. The salt form is used because:单项选择题

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