Thermometric enthalpimetric titrations (TET) are characterized by the continuous addition of the titrant to the sample under
effectively adiabatic conditions. The total amount of heat evolved (if the reaction is exothermic) or
absorbed (if the reaction is endothermic) is monitored using the unbalance potential of a Wheatstone
bridge circuit, incorporating a temperature-sensitive semiconductor (thermistor) as one arm of the
bridge. Simple styrofoam-insulated reaction
cells will maintain pseudoadiabatic conditions for the short period
of a titration.
The heat capacity of the system will remain essentially
constant if the change in volume of the solution is minimized and if the titrant and titrate are
initially at the same
temperature (usually room temperature). The TET
ethalpogram (a thermometric
titration curve), shown in
the figure below. illustrates
an exothermic titration reaction. The base line AB represents the temperature–time
blank, recorded prior to
the start of the actual titration. B corresponds to the beginning of the addition of the titrant, C is
the end point, and CD is the excess reagent line. In order to minimize variations in heat
capacity during titrations, it is customary to use a titrant that is 50 to 100 times more concentrated than the
specimen being titrated. Thus the volume of the titrate solutions is maintained virtually constant, but the titrant is diluted appreciably.
Correction for the latter is conveniently made by linear back-extrapolation CB’. Under these conditions, the extrapolated ordinate
height, BB’, represents a measure
of the change of temperature due to the titration reaction.
Typical thermometric titration curves for an exothermic
process: (a) idealized
curve, and (b) actual
curve, illustrating extrapolation correction for curvature due to
incompleteness of reaction. AB—temperature–time blank, slope due to
heat leakage into or out of titration cell; B—start of titration; BC—titration branch; C′—end
point; CD—excess reagent
line, slope due to heat leakage and
temperature difference between reagent and solution being titrated; ΔT— corrected temperature change;
ΔV—end-point volume. (From
L. Meites,
ed., Handbook of Analytical Chemistry, McGraw-Hill, New York, 1963.)
