True about pKa is:

Ans. a. pH at which ionized fraction of drug equals to unionized fractionRef: Goodman Gilman 12th E/P 18pKa is the pH at which as drug is 50% ionized and 50% unionized, i.e. the fraction of ionized drug equals to the fraction of unionized drug.The effect of pH on drug ionization and unionization can be calculated for acidic and basic drug by Hasselbalch equations given below:Basic drug: Log10 (Unionizeddrug)(Ionizeddrug) =pK4 -PHAcidic drug: Log10 (Ionizeddrug)(Ionizeddrug) =pKa -PHWhat is pKa ?If the ionized drug=unionized drug, i.e. 50% each, then the equation becomes,Log10 1=PKa -PHOr,0=pKa -pHOr. pKa =pHHencepKa is the pH at which the drug is 50% ionized and 50% unionized.