Question

A patient appears in the ER. He is hyperventilating and his blood pH is 7.6. It turns out he has taken an accidental overdose of aspirin.

Aspirin is Acetylsalicylic acid.   In the stomach it is hydrolyzed to acetic acid and salicylic acid. The salicylic acid acts on the central nervous system to increase respiration rates.

a)What does hyperventilation do to the pH of the blood stream?

b)Why?

c)Patients suffering from an aspirin overdose are treated with activated charcoal to absorb the aspirin. Sodium bicarbonate is also given when the stomach is pumped to increase the solubility of the salicylic acid in the stomach. Why does this help? Hint: the pKa of salicylic acid is 3.0 and the pH of stomach is about 2. What is the fraction of salicylic acid that is charged at pH = 2 and pH=7?

Thanks in advance!

Answer 1

a) Hyperventilation causes the blood to become more alkaline, i.e. have higher pH than normal.

b) pH is increased in blood as during hyperventilation, the body expels carbon dioxide more rapidly, which dissolves in the bloodstream to give carbonic acid. Without the species responsible for acidity, the pH of blood increases. This alkalinity also results in increased binding affinity of hemoglobin to oxygen.

c) ASA is a weak acid and sodium bicarbonate is a weak base. When the base is pumped into the stomach, it reacts with the excess ASA present, neutralizing it and thereby forming sodium salicylate salt with the salicylic acid. All salts are highly polar due to the permanent charge-separation in them, resulting in enhanced solubility of salts in water. Though ASA is an acid, due to the extensively non-polar moeity attached to the polar carboxylic acid group, aspirin as such does not have great solubility in water.

pKa = -logKa, which gives Ka of salicylic acid as 10^-pKa which gives 0.001. Since salicylic acid is a weak acid, it exists in an equilibrium with its undissociated species after dissociation, which is represented by

HA <-----> H⁺ + A^- and its equilibrium constant is given by Ka = ([H⁺][A^-])/[HA]

At a pH of 2, the concentration of protons in the medium is given by 10^-pH = 0.01 and at a pH of 7. it will be 10^-7. Since salicylic is a monoprotic acid, there will be a 1:1 molar ratio of protons and conjugate base. Therefore, at a pH in the stomach, almost no salicylic acid will be dissociated and hence all of the excess ASA will remain insoluble in water, making it impossible to be expelled out. Thus, addition of sodium bicarbonate induces the acid-base reaction apart from salt formation and so helps in removing ASA from the body. LeChatlier's principle dictates that presence of a one species in a medium retards the formation of more of the same by the dissociation of association of some other molecules. Thus, salicylic acid's percentage dissociation to give protons in a medium already having 0.01M protons will give concentration of the undissociated species as 0.001 = (0.01)x(0.01)/[HA] = 0.1M. Now, the percentage dissociation is given by concentration of dissociated w.r.t associated resulting in a total 0.0001% dissociation at pH = 2. Similarly, at pH = 7, there will be 10^-7M of protons and the precentage dissociation will be 99.09%.