pKa is the pH at which:

Ans. B. 50 percent of the drug is ionized50 percent of the drug is ionizeda. Drugs are weak acids or basesb. Their ionization depends on pH of solution and pKa of drug (pH at which a drug is 50% ionized)c. Acidic drugs ionize at basic pH while basic drugs ionize more at acidic pHd. One-unit change in pH from pKa causes ten times change in ionization