March 28, 2024, 06:14:23 AM
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Topic: Calculating the pH/ Bicarbonate change when modelling CO2 addition to blood  (Read 2821 times)

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Offline gilpster

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Hi everyone,

I'm a doctor/programmer not a chemist and i'm stuck with a chemistry problem while trying to calculate the addition of CO2, H+, or HCO3- to a simplified model of human blood (we can assume the blood is water)

I know that CO2 dissociates to carbonic acid then bicarbonate and hydrogen ions:

CO2 + H2::equil:: H2CO3  ::equil:: H+ + HCO3-

which I can simplify:

CO2 + H20 ::equil:: H+ + HCO3-
(I'm guessing we know the pKa for this)

I know that we can use a modified version of the henderson-hasselbach equation to then calculate [H+] (and then pH of course)

[H+] = k x PCO2 / [HCO3-]

If I ignore the effects of CO2 binding to haemoglobin and amino groups, then adding CO2 (or H+ / HCO3- etc.) to the blood should then allow me to calculate the change in pH according to the above equation.

The problem is that some of the CO2 will dissociate to H+ and HCO3- according to the first equation, so before I calculate the pH I need to solve the math problem: Given that I know the starting concentrations of H+, CO2, and HCO3-, can I calculate the new concentrations after addition of a know quantity of one of these chemicals?

This matters greatly when giving Na+·HCO3- to a patient.  It produces large amounts of CO2 which the patient then needs to exhale and can cause respiratory failure (so I need to be able to model this)

Please forgive me if I have made some elemental errors in chemistry understanding - it really isn't my knowledge base and I don't have access to a biochemist.

Kenneth

Offline Borek

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Please start here:

http://www.chembuddy.com/?left=pH-calculation&right=toc

While it doesn't address your problem directly, it is a discussion of the general method of dealing with acid/base equilibria, and it is exactly the problem you are facing now. The only thing left out is the dependence between concentration of the dissolved CO2 and the partial pressure of the CO2 - but this is given by the Henry's law.
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