Po, Henry N. and Senozan, N.M., 2001
Modified Henderson Equation
Henderson’s Year: The year 1908 was a good year for acidbase balance – thanks to Henderson. In that year he appreciated the buffering power of CO_{2} and went on to apply the law of mass action to produce his simple formula which, rearranged, looks like this:
[H^{+}] [HCO_{3}^{–}] = K [CO_{2}] [H_{2}O]
This can be simplified: [H_{2}O] remains constant and physicians are much more familiar with PCO_{2}. This gives us the version used in the Interactive Equation:
[H^{+}] [HCO_{3}^{–}] = k x PCO_{2}
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 HendersonHasselbalch Equation – see below
No Hasselbalch! Click on the Blue PCO_{2} buttons (SBE is constant); click on the Blue pH Buttons (PCO_{2} is constant).

Add Hasselbalch
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 HendersonHasselbalch equation.