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Ionisation of a drug

Author: RxPG, Posted on Thursday, August 05 @ 14:52:03 IST by RxPG  

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Pharmacology

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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