Potentiometric titrimetry has been the predominant automated titrimetric technique since the 1970s, so it is worthwhile considering the basic differences between it and thermometric titrimetry.
Potentiometrically-sensed titrations rely on a free energy change in the reaction system. Measurement of a free energy dependent term is necessary.
: ΔG = -RT lnK (1)
Where:
: ΔG = change on free energy
: R = universal gas constant
: T = temperature in kelvins (K) or degrees Rankine (°R)
: K = equilibrium constant at temperature T
: ln is the natural logarithm function
In order for a reaction to be amenable to potentiometric titrimetry, the free energy change must be sufficient for an appropriate sensor to respond with a significant inflection (or "kink") in the titration curve where sensor response is plotted against the amount of titrant delivered.
However, free energy is just one of three related parameters in describing any chemical reaction:
: ΔH = ΔG + TΔS (2)
where:
: ΔH = change in enthalpy
: ΔG = change in free energy
: ΔS = change in entropy
: T = temperature in K
For any reaction where the free energy is not opposed by the entropy change, the enthalpy change will be significantly greater than the free energy. Thus a titration based on a change in temperature (which permits observation of the enthalpy change) will show a greater inflection than will curves obtained from sensors reacting to free energy changes alone.