Description: Aspirin has been around for over 100 years. Like many NSAIDs it con
ID: 532511 • Letter: D
Question
Description: Aspirin has been around for over 100 years. Like many NSAIDs it contains a carboxylic acid. This is interesting to us because that means it can exist in two different forms, as a carboxylic acid at low pH and as a carboxylate at high pH.
Instructions: Your job for this case study is to investigate the pKa of aspirin’s carboxylic acid. Once you have that number in hand compare it to the pH of both the stomach (use the middle of the stomach pH range) and the small intestine (again assume the middle of the range). With all these numbers in hand, estimate the ratio of carboxylic acid to carboxylate for aspirin in both the stomach and the small intestine. Please show your work.
Any help?
Explanation / Answer
Aspirin (acetyl salicylic acid) with chemical formula C9H8O4 has a carboxylate with a PKa of 3.5. Aspirin is weak acid and represented by [HA] . It partially ionizes in water:
[HA] + H2O ====> [A-] + H3O+
The pH of the stomach is 1.6,
PKa of aspirin is 3.5
Since we are given both the pH of the stomach and the PKa of aspirin, we can use the Henderson-Hasselbalch equation to solve for the ratio of [HA] to [A-].
pH = pKa - log([HA] / [A-])
log [HA] / [A-] = pKa – pH
log[HA] / [A-] = 3.5 – 1.6
log([HA] / [A-]) = 1.9
([HA] / [A-]) = 79.4
The ratio of protonated aspirin to its conjugate base is 79 to 1.
So one-eightieth (1/80) of the total aspirin taken will be in the conjugate base form and available for immediate absorption in the stomach.
The ratio of carboxylic acid to carboxylate for aspirin in the stomach is 79 to 1.
The pH of the intestine is 6.4,
PKa of aspirin is 3.5
Since we are given both the pH of the intestine and the PKa of aspirin, we can use the Henderson-Hasselbalch equation to solve for the ratio of [HA] to [A-].
pH = pKa - log([HA] / [A-])
log [HA] / [A-] = pKa – pH
log[HA] / [A-] = 3.5 – 6.4
log([HA] / [A-]) = - 2.9
([HA] / [A-]) = 0.00125
The ratio of carboxylic acid to carboxylate for aspirin in intestine is 125 to 10000.