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

Hint:

Put pH = pKa into Henderson-Hasselbalch; log of 1 is zero.

Answer 1

The Correct Option is A

Step 1: Recall the definition. pKa is the negative logarithm of the acid dissociation constant of a weak electrolyte and represents the pH at which the drug is half ionised.
Step 2: Apply the Henderson-Hasselbalch relationship. For a weak acid, \[ pH = pKa + \log\frac{[\text{ionised}]}{[\text{unionised}]} \]
Step 3: Set pH equal to pKa. Then \(\log\frac{[\text{ionised}]}{[\text{unionised}]} = 0\), so the ratio of ionised to unionised drug equals 1.
Step 4: An equal ratio means the drug is exactly 50% ionised and 50% non-ionised, which is option a. The other options give incorrect non-equal splits.