Question

Sarah Mathews arrived at the Emergency Room with her daughter Molly, where the little girl immediately underwent a physical exam and lab samples were obtained for analysis. By this time, she was almost unarousable, and was breathing rapidly and deeply. The physician on duty, Dr. Pedro Martinez, intubated Molly’s trachea for airway protection and carried out hyperventilation, which he said was “to avoid hypoventilation and a worsening of her metabolic acidosis.”

“What does that have to do with aspirin?” asked Sarah, anxiously.

Dr. Martinez replied, “Well, aspirin was originally a trademark for acetosalicylic acid, which can inhibit a pathway, leading to inflammation, but it is also a weak organic acid. That means at high levels it can lower the pH of your blood from its normal value of about 7.4—and any level below about 7 begins to be dangerous. As you can see, the nurse is also starting to administer activated charcoal through a nasogastric tube to absorb any residual aspirin in Molly’s stomach and prevent its entry into the bloodstream.”

“Oh,” said Sarah, “Our son Paul was trying to explain pH to my husband and me last night—but what do you mean by dangerous, and what can you do to get it back up again?”

They were interrupted by another nurse who came in with lab results. Dr. Martinez frowned as he looked over the results. They revealed a pH of 6.8 and a plasma salicylate level of 100 mg/dL, together with a number of other electrolyte abnormalities. He hadn’t seen a pH that low for some time. It certainly explained Molly’s rapid and deep respiration.

Question:

Write a reaction equilibrium for the dissociation of acetic acid (using HA to represent acetosalicylic acid). Use the reaction to explain why the pH of Molly’s blood changed when she ingested so much aspirin.

What is the window of pH within which humans can survive? Use any reliable resources you have available to answer and cite your source (the format of your citation does not matter, you can simply provide a website URL if you’d like).

By what factor does the [H3O+] of Molly’s blood (pH 6.8) differ from normal (pH 7.4)? Hint: refer to your answers to the questions in Part I.

Why is Molly breathing so rapidly and deeply when she arrives at the Emergency Room, despite being nearly comatose? Why isn’t it helping make her feel better?

What are some possible ways the doctor could reverse the change in Molly’s blood pH? Hint: think about Le Chatlier’s principle. Which way do we want the reaction to shift to save Molly?

Answer 1

The dissociation of acetic acid can be represented as:

As can be seen from the dissociation equilibrium, adding any more of aspirin{HA} would alter the equilibrium and would cause the reaction to proceed forward that would reult in production of more H⁺ ions . This is in accodance with the Le Chatelier's princilpe where addition of reactants favours forward reaction ,while decrease in reactants or increase in products woud push the reaction backward.This is the cause of the soaring pH.

Our body can normally function at narrow pH range of 7.3 to 7.5

Since pH is a logarithmic function, each unit change in pH is indicative of ten times the change in concentration of H⁺ ions .

The difference in Molly's pH = 7.4 - 6.8 = 0.6 units

This value indicates that the H⁺ ions concentration has gone up by a factor of 6 or has increased by six times.

To reverse the change incurred , the following can be done-

1. Additionally, acetosalicylate which is a salt of acetosalicylic acid can be administered.This will increase the product concentration in the reaction thus shifting the equilibrium to the reverse direction where H⁺ ions would be consumed.
2. Some complexing agents can be given that would scavenge the aspirin molecules.
3. To nuetralise the acidity and the excess H⁺ ions , mild bases can be taken like dilute baking soda solution[sodium bicarbonate] .

We require the reaction to be shifted in the reverse direction where the H⁺ ions would be consumed back in the reaction.