Proton balance and electrical neutrality

5 Proton balance and electrical neutrality. For bulk solutions in their natural condition the overall charge of all the soluble chemical species is zero, therefore, this constraint can be imposed if it is not possible to use an MBE. The example in the section on carbonate equilibria (Section 5.2.6.4) provides an example of the use of an electrical neutrality equation (ENE) to calculate pEL 

Another constraint equation often used in equilibrium problems is the proton balance equation (PBE) (Pankow, 1991). It provides a means of keeping account of protons in the system. A PBE can be formulated by writing an MBE in which the concentration of each species in the EPM table is multiplied by the stoichiometric coefficient of H in the EPM table. For example, the PBE of a diprotic acid H2L where the components that define the species are H2L and H+ would be 

At first glance this may appear to be the ENE however, if the components used in the EPM were L2 - and H+ the PBE would be 

The PBE can be thought of as a proton reference level relative to the aqueous solution components chosen to define the equilibrium problem. A more detailed discussion of the PBE is given by Morel (1983) and Pankow (1991). 

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