Revelations from charge-balance modeling.

In biological fluids the charge is dominated by strong ions with pH-independent complete dissociation. The requirement of electroneutrality in combination with the principle of mass conservation and rules of dissociation as understood from physical chemistry including auto-dissociation of water allows definite specification of charge in any water-based fluid with known composition. Herein, two important publications are revisited using charge-balance modeling. In the first case it is shown that knowing a fixed pH difference between two types of fibroblasts and knowing also the pH-dependent buffer-capacities in the two cells allows quantitative assessment of buffer concentrations and strong ion differences. In the second example it is shown that the difficult problem of finding by calculation or titration the titratable acidity in urine modeled as a phosphate solution with high ionic strength can be solved with just measuring urine SID and total phosphate concentration without knowing either the correct dissociation constant or pH in urine. It is suggested that charge-balance modeling based on basic physical chemistry may reveal non-trivial ontological details of the system under study.