Stoichiometric concentration and chemical potential.
It has been recommended to use SI units in clinical chemistry. A consequence of this is that pH is reported as the excess chemical potential (or standard chemical potential) of hydrogen ions with the unit kJ/mol. On the basis of the excess chemical potential of H+ it is possible to calculate the hydrogen ion concentration in the system = the equilibrium concentration of H+ = the concentration of free H+. This quantity must be clearly distinguished from the stoichiometric concentration of H+ = the excess concentration of total H+, which indicates the amount of added or removed H+. The latter quantity with opposite sign has been called the excess concentration of base, but the designation "stoichiometric concentration of H+" seems to be more logical. The general principles for description of a component in a chemical system are based on (1) an extensive quantity (the stoichiometric amount of substance), and (2) an intensive quantity (the excess chemical potential); the product of these has the dimension of energy.
D000136:Acid-Base Equilibrium; D001769:Blood; D002624:Chemistry, Clinical; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009626:Terminology as Topic