Question

At pKa=pH-

• A. Conc. of drug is 50% ionic and 50 % non-ionic
• B. Absorption of drug is 50% ionic and 50% ionic
• C. Conc of drug is 75% ionic and 25 % non-ionic
• D. Conc of drug is 25% ionic and 75 % non-ionic

Answer 1

The Correct Option is A

In biochemistry, the term pKa refers to the acid dissociation constant, which is a quantitative measure of the strength of an acid in solution. The pKa value is the pH at which half of the species in solution is deprotonated, meaning that the concentrations of the protonated (non-ionic) and deprotonated (ionic) forms of the drug are equal.

When pKa equals pH, it implies that the drug exists in a state where 50% is in the ionic form and 50% in the non-ionic form:

• pH = pKa: At this point, the concentration of the drug in its ionic form is equal to the concentration of the drug in its non-ionic form.
• This equilibrium is mathematically represented by the Henderson-Hasselbalch equation:
  pH = pKa + log₁₀([A⁻]/[HA])
  where:
  • [A⁻] represents the concentration of the ionic form of the drug.
  • [HA] represents the concentration of the non-ionic form of the drug.

Therefore, when pKa equals pH, the ratio [A⁻]/[HA] becomes 1, meaning:

• The concentration of drug is 50% ionic as [A⁻] = [HA].
• The concentration of drug is 50% non-ionic.

Thus, the correct answer is: Conc. of drug is 50% ionic and 50% non-ionic.