Interactive Henderson Equation

Many students of chemistry have wondered if putting the mass action expression in logarithmic format should have warranted immortalization of the names Henderson and Hasselbalch.

Po, Henry N. and Senozan, N.M., 2001

Modified Henderson Equation

Henderson’s Year: The year 1908 was a good year for acid-base balance – thanks to Henderson. In that year he appreciated the buffering power of CO2 and went on to apply the law of mass action to produce his simple formula which, rearranged, looks like this:

[H+] [HCO3] = K [CO2] [H2O]

This can be simplified: [H2O] remains constant and physicians are much more familiar with PCO2. This gives us the version used in the Interactive Equation:

[H+] [HCO3] = k x PCO2

Easy to understand: most school children (even medical students !) could  grasp its meaning . The simplicity, however, also guaranteed that it was of little “interest”. Without any logarithms it performs the same function as the subsequent derivation – the Henderson-Hasselbalch Equation – see below

No Hasselbalch! Click on the Blue PCO2 buttons (SBE is constant); click on the Blue pH Buttons (PCO2 is constant).

Acid-Base Balance Tutorial - Henderson Equation
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× K
= [H+]
× HCO3-

With Hassel(balch)

Next year, 1909, was the start of a downhill slide: Sorensen introduced the negative logarithm (pH). This paved the way for Hasselbalch who in 1916 combined Henderson’s excellent equation with Sorensen’s pH to produce the dreaded Henderson-Hasselbalch equation.