You are here
Home > Medical Calculator > Henderson Hasselbalch equation

Henderson Hasselbalch equation


HCO3: meq/L
PCO2: mmHg


PCO2 is equivalent to PaCO2

Normal values:
HCO3: 22-26 meq/L
PCO2: 35 - 45 mmHg
pH: 7.35 - 7.45

The Henderson-Hasselbalch equation describes the relationship of pH as a measure of acidity with the acid dissociation constant (pKa), in biological and chemical systems. The equation is especially useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions.

Quick derivation:
Carbonic acid (H2CO3) equilibrium:
[H+] x [HCO3-] <---> [H2CO3] <---> [CO2] x [H2O]

[H+] x [HCO3-] = K x [CO2] x [H2O]
K = equilibrium constant

pH = pK + log ( [HCO3-] / [CO2] )

pK = -log K
-log H+ = -log pK -log (0.03 x PCO2)/ HCO3-
pH = pK -log (0.03 x PCO2)/ HCO3-
pH = pK +log HCO3-/ (0.03 x PCO2)
pK = 6.1 (bicarbonate buffer system)

pH = 6.1 +log (HCO3-/ (0.03 x PCO2))


The authors make no claims of the accuracy of the information contained herein; and these suggested doses are not a substitute for clinical judgment. Neither GlobalRPh Inc. nor any other party involved in the preparation of this program shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user’s use of or reliance upon this material.PLEASE READ THE DISCLAIMER CAREFULLY BEFORE ACCESSING OR USING THIS SITE. BY ACCESSING OR USING THIS SITE, YOU AGREE TO BE BOUND BY THE TERMS AND CONDITIONS SET FORTH IN THE DISCLAIMER. Read the disclaimer