Thermochemical Titrations

Abstract

“Enthalpy titrations” represent one of the newest methodological tools in the arsenal of modern instrumental analysis. In principle, they depend on differential plots of temperature versus volume of titrant, recorded in an adiabatic titration cell. The use of temperature sensitive semiconductors (called “thermistors”), wired in a Wheatstone bridge circuit as resistance thermometers, has made it possible to record automatically “enthalpograms” (= thermometric titration curves) in extremely dilute solutions (10^-4 to 10^-2 molar). The response of a thermistor bridge corresponds typically to that of a one thousand junction conventional thermocouple, permitting the measurement of temperature changes as small as 0.01 °C with a precision and accuracy of ± 0.0001 °. Applications to quantitative analysis are described, including proton transfer (acid–base), electron transfer (oxidation–reduction), precipitation and chelation reactions. The significance of enthalpy titrations is discussed as rapid and convenient means for determining heats of reaction within extremely narrow temperature intervals. Examples are presented illustrating the types of thermodynamic information which have been obtained by judicious interpretation of enthalpograms, in a wide range of temperatures (25 to 500°C) and media (water, conventional organic solvents and molten salts): The nature of solvated species and of associated ion aggregates, which are the chemical entities actually present in solution, have been elucidated. Unique potentialities of thermochemical titrations are inherent in the fact that they depend in toto on the enthalpy term in the equation:

ΔH⁰ = ΔF⁰ + TΔS⁰.

In contradistinction, most of the common procedures used in analytical chemistry are solely a function of the free energy parameter:

ΔF⁰ = -RT ln K.