1.
Part A.Acetaminophen is a weak organic acid with a pKa of 9.5. What percentage of a given dose will be in the lipid soluble form at an intestinal pH of 7.5%?
Part B.Psueduoephedrine is a weak organic base with a pKa of 9.8. What will be the ratio of non-lipid soluble drug to lipid soluble drug at a stomach pH of 2.8? As the dose of psudoephedrine which you have just take moves from your stomach with a pH of 2.8 to your jejunum with a pH of 7.8, will the absorption of the drug into your blood stream increase or decrease? Do the math to prove it.
1.
Part A
In the case of weak acids and weak bases the ability to move from aqueous to lipid or vice versa varies with the pH of the medium.
• Henderson-Hasselbalch equation (can be use to predict the effect of pH change on ABSORPTION):
weak Acid: pH = pKa + log [A-]/[HA]
Non ionized form [HA] of a drug is lipid soluble .
Acetaminophen is a weak organic acid with a pKa of 9.5. What percentage of a given dose will be in the lipid soluble form at an intestinal pH of 7.5 :
we have,
pH = pKa + log [A-]/[HA]
7.5 = 9.5 + log [A-]/[HA] ===> log [A-]/[HA] = -2
[A-]/[HA] = 0.01
[A-] = [HA] * 0.01
thus, lipid soluble form : 1/1.01 *100 = 99 %
Part B.
Psueduoephedrine is a weak organic base with a pKa of 9.8. What will be the ratio of non-lipid soluble drug to lipid soluble drug at a stomach pH of 2.8?
Non ionized form [A] of a drug is lipid soluble, and ionized form protonated form will be non-lipid soluble[HA] .
we have,
pH = pKa + log [A-]/[HA]
2.8 = 9.8 + log [A ]/[HA] ===> log [A]/[HA] = -7
[A]/[HA] = 10-7
thus , ratio of non-lipid soluble drug to lipid soluble drug at a stomach pH of 2.8 ; [HA]/[A] = 107
As the dose of psudoephedrine which you have just take moves from your stomach with a pH of 2.8 to your jejunum with a pH of 7.8 .
here,
pH = pKa + log [A-]/[HA]
7.8 = 9.8 + log [A ]/[HA] ===> log [A]/[HA] = -2
[A]/[HA] =0.01
[A] = 1%
So, it can be stated that absorption of the drug into your blood stream increase
1. Part A.Acetaminophen is a weak organic acid with a pKa of 9.5. What percentage of...
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