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Lactic acid vs lactate in sweat

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Lactic acid vs lactate in sweat

Postby juanfhj » Mon Jan 24, 2011 6:49 am

I thought about telling if one could detect lactic acid in sweat as indicative of aerobic threshold in athletes, by electrochemical means, maybe just qualitatively.

References are careful to tell that sweat contains lactate, not lactic acid (lactate is to lactic acid as nitrate is to nitric acid.). However, according to this article http://www.bentham.org/open/tocorrj/articles/V003/38TOCORRJ.pdf, artificial sweat is synthesized with lactic acid "as per standard ISO 3160-2, (20g/l NaCl, 17.5 g/l NH 4 Cl, 5g/l acetic acid and 15 g/l d,l lactic acid with the pH adjusted to 4.7 by NaOH)".

Does this mean that the lactic acid is just meant to produce lactate, and the acidity is due to the acetic acid? Doesn't the NaOH adjust for acetic acid too?

The article notes that sweat exhibits corrosion potential and corrosion current, and various electrochemical curves. Are those properties characteristic of an acid, or NaCl and lactate as salts? In such case, why can't I detect lactic acid like one observes an electric potential in citric acid with copper and zinc electrodes ("lemon battery")? Would the presence of Na with acetic acid be in equilibrium with sodium acetate?

I'm aware that if lactic acid were detected so easily, it would be trivially known long ago, so I'm likely speaking out of daring ignorance, but it's rather an idea I toyed with. Where does this "reasoning" fail?
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Re: Lactic acid vs lactate in sweat

Postby JackBean » Mon Jan 24, 2011 11:51 am

juanfhj wrote:(lactate is to lactic acid as nitrate is to nitric acid.)

simply, it's salt of lactic acid ;)

the lactic acid has pK_a approx. 3.86, so, if the pH is 4.7, then there will be mostly lactate rather than lactic acid.

I don't know, what did you mean by
juanfhj wrote: Doesn't the NaOH adjust for acetic acid too?

but the pH is given by acetic acid, which is partially titrated by NaOH forming sodium acetate and thus it's buffering solution ;)

You should be able to detect the potential, but the addition by lactate will be probably pretty small in comparison to the acetic acid.
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Postby juanfhj » Mon Jan 24, 2011 3:57 pm

Thanks for your answer. Does the acetic acid contribute more to the pH than lactic acid, given that there's 5g/l acetic and 15g/l lactic?
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