justadream

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I get that

H2O + CO2 <=> H2CO3 <=> HCO3- and H+

So if you add more CO2, the entire equilibrium shifts to the right (producing more H+).

But doesn't this also produce an equal amount of HCO3- (a base)?

So why is there a net acidic effect of adding CO2?
 

Cawolf

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The buffer system is dependent on the ratio of H2CO3 to HCO3-.

So adding CO2 or H+ that drives the reaction to the middle (carbonic acid) will cause the ratio of acid to base to increase - decreasing the pH.

pH = pka (H2CO3) + log ([HCO3-]/[H2CO3])
 
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justadream

justadream

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@Cawolf

But how does that equation "account for" the fact that whenever you generate HCO3-, you make a H+ as well?

In other words, is the [H+] irrelevant?
 

Cawolf

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Sort of.

Free H+ would rapidly combine with present bicarbonate to form H2CO3.

It is in equilibrium based on the amount of bicarbonate present in the blood.

So if the pCO2 or H+ concentration is too high for the bicarbonate load to handle, then the pH will decrease.

It isn't irrelevant, but it is best to look at the buffer system as a whole.

That is why if I have a patient who is highly acidotic, I give IV NaHCO3 to support the buffer system.
 
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kraskadva

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LeChatlier's principle.
If you've got more of the reactant on one side, it will drive the reaction to the other side.
The soluble anion does not cancel out the H+, but driving the reaction back towards the middle by increasing the amount of the anion (by giving NaHCO3) will remove the H+.