ABSTRACT
This chapter uses conventional procedures to develop and compare equations for the hydrogen ion concentration in solutions containing concentrations equal to or larger than zero of a monoprotic weak acid and/or its conjugate base. The discussion begins with an overview of reactions to be discussed, including the deprotonation of a monoprotic weak acid and the acid-base behavior of water. These reactions and associated constants are used to develop an exact equation for the hydrogen ion concentration in terms of the deprotonation constant for the acid, the autoprotolysis constant for water, and the diluted concentrations of the acid-base pair. The discussion continues with a description of an iterative procedure to solve the exact equation for the hydrogen ion concentration. Hydrogen ion concentrations calculated in this way are then used to calculate fractions and concentrations of forms of weak acids in fully protonated and fully deprotonated forms. The discussion continues with descriptions of conventional procedures for approximate calculations as well as a more recently developed approximation procedure called unified approximation. Finally, numerical and graphical results are used to show that whereas conventional approximation procedures give unacceptably large errors for both very small and large deprotonation constants, unified approximation procedures give errors less than about 5% for a much wider range of deprotonation constants and concentrations of the acid-base pair.
