Lactic acid vs lactate in sweat

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Discussion Overview

The discussion revolves around the measurement of pH in sweat to estimate the anaerobic threshold, specifically focusing on the presence of lactic acid versus lactate. Participants explore the implications of using artificial sweat and the chemical principles involved in measuring pH and concentrations of lactic acid and lactate.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested, Homework-related

Main Points Raised

  • One participant notes that many references indicate lactate is found in sweat, while questioning the use of lactic acid in artificial sweat as per ISO standards.
  • Another participant inquires about the pKa of lactic acid and how to calculate the concentrations of lactate and lactic acid using the Henderson-Hasselbalch equation, suggesting that pH measurement alone may not suffice for this purpose.
  • A different participant asks about the information that could be derived from using sweat in a half-cell setup and expresses uncertainty regarding the Nernst equation and its application to a lemon battery.
  • One participant critiques another for lacking understanding of general chemistry concepts related to their inquiry, suggesting that foundational knowledge is necessary for addressing their questions.

Areas of Agreement / Disagreement

Participants express differing levels of understanding regarding the chemistry involved in measuring pH and the implications for lactic acid and lactate in sweat. There is no consensus on the best approach to measuring these concentrations or the application of the Nernst equation.

Contextual Notes

Participants highlight limitations in understanding chemical principles, such as the relationship between pH and half-cell potentials, and the specific calculations needed to relate pH to concentrations of lactic acid and lactate.

Who May Find This Useful

This discussion may be useful for individuals interested in biochemistry, sports science, or analytical chemistry, particularly those exploring the biochemical properties of sweat and methods of measurement.

juanfhj
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I want to measure the pH of sweat in order to estimate the anaerobic threshold by detection of lactic acid.

Many references point out that it's lactate, not lactic acid, that is found in sweat.

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).

Also, I want to measure the pH by the potential sensed by copper-zinc electrodes in series etched on a PCB. Would that work?

Thanks.
 
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juanfhj said:
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).

What is pKa of lactic acid? Do you know how to calculate concentration of lactate and lactic acid knowing pH and pka?

Hint: Henderson-Hasselbalch equation.

You will be not able to find lactate/lactic acid concentrations measuring pH.
 
I get it. Another question: what information could a half-cell using sweat as the acid (Nernst equation) give about the sweat properties? I'm not really familiar with reduction potentials, chemical activities of reductants and oxidants, and their relation to pH. Also, I don't understand if the Nernst equation applies to the lemon battery, in which I don't see an apparent salt bridge.
 
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Seems to me like you are struggling with the general chemistry, not with the biological aspects of your problem.

Nernst equation describes any half cell. Half cell potential can be a function of pH, can be not. Sorry, but this is not a place for a full lecture on the Nernst equation, I suggest you learn about things you have listed (and looks like they are crucial for what you are trying to do) on your own, they are explained in many general and analytical chemistry books. Come back with specific questions.