Question

You are going to carry out a gastric lavage (stomach pump) in order to try and get rid of the aspirin that has not yet been absorbed. The procedure involves washing the organ with a solution of a certain pH containing activated charcoal to bind the aspirin. The pH of the stomach is typically around 2.0. Which of the following do you think would be the most effective lavage? Hint: think about the pH at which aspirin would cross the plasma membrane of the stomach cells in order to be absorbed.

A) Lavage of charcoal in a pH 8 solution
B) Lavage of charcoal in a pH 1 solution
C) Lavage of charcoal in a pH 3 solution

Answer 1

Here the answer would be Lavage is performing at pH 8 because of the following reasons.

This is the structure of aspirin. When aspirin is swallowed inside the stomach it is acted upon by stomach Acids, HCl, Esterase (enzymes that catalyses hydrolysis of ester).

Chemical equation of acid hydrolysis of aspirin is as follows

acetyl salicylate + H₂O --> salicylic acid + acetic acid

Since the pKa value of salicylic acid is 2.97 ,at pH 2 which is stomach pH 90% of acid remains protonated while 10% of salicylic acid remains in deprotonated form.

During the aspirin overdose, emergency treatment that is involves a “gastric lavage” (stomach pumping). This procedure changes the pH of the stomach to about 8 %.

salicylic acid that is deprotonated as well as the % of salicylic acid that is protonated at pH = 8 , salicylate: ~100% and salicylic acid: only trace amounts.

We know that uncharged molecules are more readily absorbed through cell membranes of the stomach than charged molecules and this explain how the gastric lavage helps to prevents aspirin absorption by the stomach. At pH 8, effectively all of the salicylate generated from aspirin is charged and therefore unable to be cross the stomach cell membranes.