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).
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.