[Aymeric]

Hello,

From my old biochemistry lessons I’ve been under the impression that lactic acid qualifies as a weak acid. Could anyone confirm this for me?
If this is indeed correct, my second question would be: how dissociated does it get in physiological human pH, i.e. is it possible to know the proportion of lactic acid vs. lactate at 37 C° and pH = 7.4?

I’ve looked this up online and only came across calculation methods involving Ka and Kb which are way above the concepts I was taught back in school.

Thanks for your help.

[owk9688]

Lactic acid is a weak acid, yes. The pKa of lactic acid is 3.86 (the lower the pKa, the stronger the acid. In general, strong acids are the acids having pKa values less than or equal to 0

For the second part of your question, you can use the Henderson Hasselbach equation to get this, but an easier way is just to estimate it based on the pKa. For every 1 unit of difference between pKa and pH, the ratio of protonated vs deprotonated acid will change by a factor of 10. Therefore, at pH = 7.86, you would have 10,000 times as much lactate (deprotonated) as lactic acid (protonated). So, at pH= 7.4, this would be slightly smaller (7.4 - 3.86 = 3.54  10^3.54 = 3467 )

So, at physiological pH, you have essentially 3500x as much deprotonated lactic acid as protonated. It is, however, still considered weak.

[Borek]

is it possible to know the proportion of lactic acid vs. lactate at 37 C° and pH = 7.4?

I’ve looked this up online and only came across calculation methods involving Ka and Kb which are way above the concepts I was taught back in school.

I am afraid if you are interested in quantitative results you need to learn the quantitative approach - and good understanding of Ka, Kb, being equilibrium constants of the acid/base dissociation constants, is a must.

So the answer to your question is: yes, calculating thing you need is rather simple, once you understand concepts behind. If you will need them more often, I suggest reading any reasonably good GenChem textbook, equilibrium section. Acid/base/pH are part of it.

Good thing is, when the pH is given finding ratio of protonated to not protonated acid is one of the simplest tasks to tackle.

[Aymeric]

Thanks everyone!
Fortunately owk9688’s answer is more than enough. I was wondering about this because of the claims that lactic acid produces acidity in muscle cells at the end of glycolysis. So I wanted to know how much lactic acid actually becomes lactate and releases protons into the cytosol, only to find out just a few minutes ago that what comes out of pyruvate reduction by NADH is not lactic acid but lactate, which thus has no extra proton to release in the first place, so it cannot be held responsible for lowering cytosolic pH.

Thanks again!