Acids & Bases: Aspirin pKa, pH in Stomach & Intestine

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Discussion Overview

The discussion revolves around the pKa of acetylsalicylic acid (aspirin) and its ionization state in different pH environments, specifically in the stomach and small intestine. Participants explore the relationship between pH and pKa, the applicability of the Henderson-Hasselbalch equation, and the implications of varying pKa values.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant states that aspirin will be ionized in the small intestine and almost unionized in the stomach based on the pKa and pH values.
  • Another participant seeks clarification on the quantitative implications of pKa in relation to the pH of the stomach and small intestine.
  • Some participants discuss the Henderson-Hasselbalch equation, noting its relevance to buffer systems and questioning its applicability to the context of aspirin in gastric acid.
  • There is a suggestion that if the pKa of aspirin were lower (e.g., 1.5), it might change the ionization state in the stomach, prompting further inquiry into the relationship between pH and pKa.
  • One participant asserts that pH is directly proportional to pKa, which is challenged by another participant who argues this is a misrepresentation of the Henderson-Hasselbalch equation.
  • Participants discuss the ratio of ionized to unionized forms of an acid as expressed in the Henderson-Hasselbalch equation, emphasizing the importance of this ratio in determining ionization states.
  • There is a debate about whether the Henderson-Hasselbalch equation is appropriate for weak acids in strong acid environments, with differing opinions on its validity in this context.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the Henderson-Hasselbalch equation to the situation involving aspirin and gastric acid. There is no consensus on whether pH is directly proportional to pKa, and the discussion remains unresolved regarding the implications of varying pKa values on ionization states.

Contextual Notes

Some participants highlight limitations in applying the Henderson-Hasselbalch equation to strong acids and weak acids, suggesting that the context may affect the validity of the equation. There are also unresolved questions regarding the quantitative relationship between pH and pKa.

erisedk
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Homework Statement


The pKa of acetylsalicylic acid (aspirin) is 3.5. The pH of gastric juice in the stomach is about 2-3 and the pH in the small intestine is about 8. Aspirin will be
Answer is ionised in the small intestine and almost unionised in the stomach.

Homework Equations

The Attempt at a Solution


I understand from the pH values given that the stomach is acidic and small intestine basic. So, I could already qualitatively figure out what the answer would be. However, is it quantitatively correct to say that aspirin is unionised in the stomach because the pH of stomach is less than the pKa of aspirin? What if the pKa of aspirin was 1.5? Would it then be ionised in the stomach?
Basically, I just want someone to clarify what pKa tells us in this context.
 
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I was just looking for a relationship between pH and pKa. Found it!
 
erisedk said:
I was just looking for a relationship between pH and pKa. Found it!

Wouldn't hurt if you will show what you have found. This is by no means a trivial relationship (actually there is NO relationship between just pKa and pH).
 
The Henderson Hasselbalch equation. I know it's for buffers, but pH is directly proportional to pKa and I was looking for some quantitative relationship. If you could expand more though, I'd more than appreciate it.
 
erisedk said:
What if the pKa of aspirin was 1.5? Would it then be ionised in the stomach?
Also, maybe answer this question? I think it wouldn't based off pH directly proportional to pKa, but it's not a linear proportion so I'm not very sure.
 
erisedk said:
pH is directly proportional to pKa

That's a serious misrepresentation of the HH equation.

However, HH equation is a good start here. It contains \log \frac {[A^-]}{[HA]} expression which is what is important. Rearrange the equation in such a way this ratio is expressed as a function of both pH and pKa. When this ratio is small, acid is mostly unionized. When this ratio is high, acid is mostly ionized. This is quantitative.

Actually you were quite close to the correct answer in your first post:

erisedk said:
is it quantitatively correct to say that aspirin is unionised in the stomach because the pH of stomach is less than the pKa of aspirin? What if the pKa of aspirin was 1.5? Would it then be ionised in the stomach?

When pH = pKa exactly half of the acid is ionized (and the other half unionized). When pH < pKa, there is more unionized form.
 
Ohhhh, that's really cool!
[A-]/[HA] = 10pH-pKa
So it's the difference that counts.
Thanks :)
 
On second thought, shouldn't the HH equation be viable only for a buffer? Gastric acid in the stomach is HCl which is a strong acid and aspirin (a weak acid) couldn't possibly form a buffer with HCl (or for that matter, the weak base of the small intestine). So, isn't using HH incorrect for this situation?
 
HH equation is derived just by rearranging dissociation constant definition (reaction quotient), so it holds for any acid. It just happens to be a form that makes buffer calculations easy.
 
  • #10
Ok!
 

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