Thermometric Titration - Advantages And Applications

A thermometric titration utilizes the enthalpy change of the reaction involved to locate the end point. It has been defined as “a titration in an adiabatic system yielding a plot of temperature vs. volume of titrant.” The procedure consists of delivering the titrant from a thermostated buret into a solution contained within a thermally insulated vessel. and observing the temperature change of the solution either upon continuous addition, or after each successive incremental addition, of titrant.

Advantages

These temperature-volume plots resemble the corresponding graphs obtained from conductometric, photometric, and amperometric titrations. Yet while each of the latter three methods is severely limited to specific kinds of systems - e.g., conductometric titration requires electrolytic solutes in solvents of high dielectric constant, almost all reactions exhibit detectable enthalpy changes (positive or negative). This wide applicability, coupled with simplicity, suggests a potential increase in the use of thermometric titrations in analytical chemistry, particularly in those media in which electrometric and photometric methods are inapplicable.

The analytical sensitivity of the thermometric titration method is linearly related to concentration, in contrast to the logarithmic relation to concentration which exists for many other analytical methods, e.g., potentiometric methods. A linear relation is an advantage when very dilute solutions or solutions with high concentrations of interfering ions are being analyzed. For example, a pH titration of a solution containing pyridine at a concentration below 0.05 M will give a poorly defined end point, while the end point of a thermometric titration is well defined.

Applications

Neutralization

Weak acids and bases have been studied by several workers. Bell and Cowell recommended the use of thermometric titration for the preparation of neutral solutions of ammonium citrate. Linde, Rogers, and Hume titrated both weak and strong acids and bases, showing that clear end points were obtainable even in emulsions and thick slurries, and that a mixture of sodium hydroxide and sodium carbonate could be determined with good accuracy. Jordan and coworkers show that, unlike potentiometric titration which is dependent upon free-energy changes, thermometric titration works very well even for extremely weak acids. End points are precise and accurate for acids as weak as boric acid, because the enthalpy change of neutralization is not very different from that of a strong acid.

Complexation

The greatest number of papers on thermometric titration deal with investigations of complex formation, all of which are of analytical importance either directly or indirectly. For EDTA titrations, an accuracy within 3% is possible with cation concentrations as low as 0.0005 M.

Thermometric Titration - Curves

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.)