3. Physiology
        3.1. Acid and base
            3.1.1. Basics of acid-base
3.1.1.3. Henderson-Hasselbalch equation
[WG21:p736]

pH and pK

For any acid/base,

HA <----> H+ + A-

K = [H+][A-]/[HA]

Thus,

pK = pH - log{[A-]/[HA]}

--> pH = pK + log{[A-]/[HA]}
* i.e. Henderson-Hasselbalch equation

NB:

 

Henderson-Hasselbalch equation

--> pH = pK + log{[A-]/[HA]}

(See above)

Carbon dioxide and pH

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

Henderson-Hasselbalch equation for H2CO3

pH = pK + log {[HCO3-]/[H2CO3]}

When [H2CO3] is changed to [CO2]
--> pK becomes pK' to denote two related but different values

Also,

pKa varies with temperature
--> pKa = 6.1 at 37C

Thus,

pH = 6.1 + log {[HCO3-]/[CO2]}

Also,

CO2 solubility = 0.0301 mmol/L/mmHg of pCO2

Thus,

pH = 6.1 + log {[HCO3-]/(0.0301 x pCO2)}

NB:

 



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