How is the 0.03 Factor in the Henderson-Hasselbalch Equation Determined?

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Discussion Overview

The discussion revolves around the Henderson-Hasselbalch equation, specifically focusing on the factor of 0.03 used to calculate the concentration of CO2 from the partial pressure of CO2 (PCO2). Participants explore the implications of this factor, its derivation, and its relevance in calculating bicarbonate concentration ([HCO3]) in a physiological context.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the origin of the 0.03 factor in the equation [CO2] = 0.03 * PCO2, seeking clarification on its significance.
  • Another participant suggests that pO2 is irrelevant for the calculation and emphasizes the need to find the concentration of H2CO3, which may be approximated as the concentration of dissolved CO2.
  • A participant mentions using the HH equation to find [HCO3] and expresses uncertainty about how to determine the concentration of CO2 or H2CO3 without given values.
  • It is noted that the concentration of CO2 can be calculated using Henry's law, which relates the amount of CO2 to its partial pressure and solubility coefficient.
  • One participant states that the 0.037 factor represents the solubility coefficient of CO2 in plasma under physiological conditions, with units of mmol/mmHg/cc.
  • There is a discussion regarding the value of pK, with one participant suggesting it is 6.1 while another provides a different value of 6.4, indicating potential corrections for activity coefficients.

Areas of Agreement / Disagreement

Participants express differing views on the value of pK and the relevance of pO2 in the context of the HH equation. The discussion remains unresolved regarding the exact derivation and implications of the 0.03 factor and the appropriate pK value.

Contextual Notes

There are limitations regarding the assumptions made about the concentration of H2CO3 and the applicability of Henry's law. The discussion also highlights the dependence on specific physiological conditions and the potential for variation in pK values based on different sources.

Wek
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I'm given the following values: pH = 7.48, PO2 = 120mmHg, and PCO2 = 18mmHg.
I have to find the [HCO3] using these values. There's one thing I don't understand about the Henderson-Hasselbalch equation and that is how to calculate the [CO2]. I found out that [CO2] = 0.03*PCO2. My question is, what is the 0.03 that is multiplied to the PCO2 to get the [CO2] value?

One last thing, the pKa in the HH equation, is it of the HCO2?

Thanks
 
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As far as I can tell pO2 is irrelevant. You need to find concentration of H2CO3 (could be you are to assume it is identical with concentration of dissolved CO2, that's a common approximation). Then you should use HH equation, pKa1 describes equilibrium of the reaction

H2CO3 <-> H+ + HCO3-

and you have it either in your textbook or in any tables with Ka values.
 
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Yes, I used the HH equation to find the [HCO3] and it came out to be 13. I' not given any concentration for CO2 or H2CO3. In this case, how would you be able to find the concentration of CO2 or even H2CO3, if its possible at all? The value I used for [CO2] was something I found on Cornell's website but it was not explain why they used that, which is what I'm asking.
 
Concentration of CO2 is something that can be calculated - for known partial pressure - using Henry's law.
 
Last edited by a moderator:
Wek said:
Yes, I used the HH equation to find the [HCO3] and it came out to be 13. I' not given any concentration for CO2 or H2CO3. In this case, how would you be able to find the concentration of CO2 or even H2CO3, if its possible at all? The value I used for [CO2] was something I found on Cornell's website but it was not explain why they used that, which is what I'm asking.

0.037 represents the solubility coefficient of C02 in plasma under physiological conditions. Units are mmol/mmHg/cc

Amount of C02 is a linear function of the product P(C02) and aforementioned solubility coefficient.
I believe pK is 6.1 in this case.
 
DanP said:
Amount of C02 is a linear function of the product P(C02) and aforementioned solubility coefficient.

That's Henry's law.

I believe pK is 6.1 in this case.

My tables give 6.4. It doesn't mean 6.1 is incorrect, could be that's value already corrected for the activity coefficients of all ions assuming ionic strength of plasma.
 
Last edited by a moderator:

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