The extent to which ionization of a drug takes place is dependent upon pKa of the drug and the pH of the solution in which the drug is dissolved. Which of the following statements is not correct:

• A. Small changes of pH near the pKa of a weak acidic drug will not affect its degree of ionization.
• B. pKa of a drug is the pH at which the drug is 50% ionized
• C. Phenobarbitone with a pKa of 7.2 is largely ionized at acid pH and will be about 40% non-ionized in plasma.
• D. Knowledge of pKa of a drug is useful in predicting its behaviour in various body fluids.