Question.
The extent to which ionisation 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.
1.pKa of a drug is the pH at which the drug is 50% ionized.
2.Small changes of pH near the pKa of a weak acidic drug will not affect its
degree of ionisation.
3.Knowledge of pKa of a drug is useful in predicting its behaviour in various
body fluids.
4.Phenobarbitone with a pKa of 7.2 is largely ionized at acid pH and will be
about 40% non-ionised in plasma.
Answer
4.Phenobarbitonewith a pKa of 7.2 is largely ionized at acid pH and will be
about 40% non-ionised in plasma.
Reference
KDT 5th Edition Page 12 Figure 2.3 & 4th Edition Chapter 2 Pages 11,12 Fig 2.3
Harper 25th Edition Page 23 Figures 3.6, 3.7
Quality
Thinker, needs basic concepts
Status
New
QTDF
KDT, Harper
Discussion
pKa of a substance is the pH at which it is 50 % ionized. And small changes of
pH near its pKa will not affect the ionization of Acidic as well as Basic Drugs.
And of course the knowledge of pKa is needed in predicting the behaviour of
various drugs in body fluids. Just because we know the pKa of Aspirin we are
able to understand why it is selectively concentrated in the gastric mucosa at
concentration much higher than that of the gastric lumen. Because we know the
pKa of Phenobarbitone, we are able to Use Forced Alkaline diuresis for its
excretion
Explanation
1.pKa of a drug is the pH at which the drug is 50% ionized.
2.Small changes of pH near the pKa of a weak acidic drug will not affect its
degree of ionisation.
3.Knowledge of pKa of a drug is useful in predicting its behaviour in various
body fluids.
4.Phenobarbitone with a pKa of 7.2 is largely unionized at acid pH and is more
ionised in Alkaline pH. Remember Forced Alkaline Diuresis for Phenobarbituric
ACID !!!!)
Comments
This Question can be solved easily if one knows the basic concepts of Acid, Base
and pH
Tips
• As already told, if one has strong correct concepts in Acid Base and
Electrolyte Balance, about 5% of Questions can be attended with minimum fuss
• This Question carries a chance of being wrongly interpreted and considering
Answer 4 as correct. Be careful !!! Phenobarbitone is Phenobarbituic ACID and is
not a Basic Drug !!!
Source: RxPG AIPG 2003 Book by TargetPG, Author : Dr.J.Mariano Anto Bruno
Mascarenhas
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