Free Ions Versus Complexed Ions In discussing the F– ion-selective electrode, we
noted that the membrane potential is influenced by the concentration of F–, but not
the concentration of HF. An analysis for fluoride, therefore, is pH-dependent.
Below a pH of approximately 4, fluoride is present predominantly as HF, and a
quantitative analysis for total fluoride is impossible. If the pH is increased to greater
than 4, however, the equilibrium
HF(aq) + H2O(l) t H3O+(aq) + F–(aq)
shifts to the right, and a quantitative analysis for total fluoride is possible.
Most potentiometric electrodes are selective for only the free, uncomplexed
analyte and do not respond to complexed forms of the analyte. Solution conditions,
therefore, must be carefully controlled if the purpose of the analysis is to determine
the analyte’s total concentration. On the other hand, this selectivity provides
a significant advantage over other quantitative methods of analysis when it is
necessary to determine the concentration of free ions. For example, calcium is
present in urine both as free Ca2+ ions and as protein-bound Ca2+ ions. If a urine
sample is analyzed by atomic absorption spectroscopy, the signal is proportional to
the total concentration of Ca2+, since both free and bound calcium are atomized.
Analysis with a Ca2+ ISE, however, gives a signal that is a function of only free Ca2+
ions since the protein-bound ions cannot interact with the electrode’s membrane.