Using activities to correct the Henderson-Hasselbalch equation

The Henderson-Hasselbalch equation is central in chemistry teaching and has many practical applications. The equation, however, has many inherent approximations which limit its application. Here, we focus on one particular approximation, the use of concentrations instead of activities. We show that this can be easily corrected for in a modification which extends the useful range of the equation to moderately strong electrolytes (I < 0.5). Without this correction, the calculated pH of a typical phosphate buffer is too high by up to 0.4. The correction can be easily automated in a spreadsheet and is straightforward to implement into the chemistry and biochemistry teaching and laboratory curriculum. It introduces students to the concept of activities in thermodynamic equilibrium, and to the Debye-Hückel equation. It further emphasizes the importance of using activities instead of concentrations when the ionic strength exceeds 0.005 M, in contrast to the approach found in textbooks where activities are introduced, but then ignored 'for the sake of simplicity', even in example calculations where the use of concentrations is clearly not appropriate. In this contribution we intend also to stimulate discussions about how to teach chemical equilibria, Brønsted-Lowry acid-base reactions and titrations, buffer solutions, the concept of activity and the concept and definition of pH.