Blood Acid/pKa question

[rskhan29]

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Last question, I hope!

Will an acid with pKa=4 be mostly deprotonated (with 50%+ percent dissociation) at pH=7.4? I would think not, since if you work it out with Ka you will see that very few of them do dissociate. But how come most fatty acids (pKa~4) in the blood (pH~7.4) have deprotoanated at their acid group? Does it have something do with bases in the blood acting to deprotonate it?

 

[TieuBachHo]

How do you come up with this question? 😀 ... Actually, the biological system work differently as compared to the an isolated system. The body homeostatis has the ability to maintain its internal environment. If the acidic level in the body becomes too high, it would cause a lots of problems. So whichever the mechanisms involved, it will act to counter that.

 

[BerkReviewTeach]

Last question, I hope!

Will an acid with pKa=4 be mostly deprotonated (with 50%+ percent dissociation) at pH=7.4? I would think not, since if you work it out with Ka you will see that very few of them do dissociate. But how come most fatty acids (pKa~4) in the blood (pH~7.4) have deprotoanated at their acid group? Does it have something do with bases in the blood acting to deprotonate it?

When the pH > pKa, then the solution is basic enough to remove the proton. It will be roughly 99.96% dissociated. You can solve this exactly using the Henderson-Hasselbalch equation:

pH = pKa + log [A-]/[HA]

7.4 = 4.0 + log [A-]/[HA]

3.4 = log [A-]/[HA]

10exp3.4 = [A-]/[HA]

so [A-] = 2500 [HA] (given that 10exp3.4 = 2500)

There are 2500 deprotonated species for every 1 protonated species. This is why in biological systems at all locations except the stomach and lysosomes have carboxylic acids in their deprotonated form. It also explains why amino acids exist predominantly as zwitterions at pH = 7.4.