Physicochemical Analysis

a method of investigating physicochemical systems that makes possible a determination of the nature of the interactions between the components of a system through a study of the relations between the system’s physical properties and composition. The principles of physicochemical analysis were established in the late 19th century by J. Gibbs, D. I. Mendeleev, and J. van’t Hoff. The analytical method received its development in the research of H. Le Châtelier, G. Tammann, H. Roozeboom, and, in particular, N. S. Kurnakov and his school.

Physicochemical analysis involves the measurement of various physical properties of systems, most often phase transition temperatures and other thermal properties (thermal conductivity, heat capacity, thermal expansion), electrical properties (conductivity, dielectric permittivity), and optical properties (refractive index, rotation of the plane of polarization of light). Also measured are the density, viscosity, and hardness, as well as the dependence of the rate of the transformations occurring in a system on the system’s composition. X-ray diffraction analysis and techniques of microscopic metallography are extensively used in physicochemical analysis.

Physicochemical analyses are made by constructing and geometrically analyzing composition-properties diagrams and phase diagrams (composition-temperature, composition-pressure). Since the analytical expressions describing phase equilibria are unwieldy and only approximately determine the regions of phase existence, a geometric analysis of the diagrams is the most common procedure for judging the composition and boundaries of phase existence of a system without separating the phases from the mixture and subjecting them to ordinary chemical analysis. For this reason, physicochemical analysis is an important method of studying systems made up of two, three, or more components, for example, alloys, minerals, solutions, carbides, oxides, semiconductors, superconductors, and systems formed by organic compounds.

Physicochemical analysis is based on the phase rule and on the principles of continuity and correspondence, which were introduced by N. S. Kurnakov. The continuity principle holds that during continuous changes in the parameters of a state the properties of a system also undergo continuous change (provided that the number of phases in the system remains constant); when the number of phases changes, certain properties change abruptly (continuity being broken). According to the correspondence principle, each phase or group of phases of a system corresponds to a certain geometric shape (point, line, surface, volume) on the composition-properties diagram. Thus, the onset of phase crystallization corresponds to the liquidus curves (or surfaces), above which is located the region of existence of one liquid phase (solution or melt); the end of crystallization corresponds to the solidus lines (or surfaces), below which only solid phases exist.

During a continuous change in the composition of a system, the components of the system may form a chemical compound. If the compound does not undergo dissociation and has an unvarying composition (daltonide), a singular point is observed on the composition-properties diagrams. The formation of a chemical compound of variable composition (berthollide) corresponds to the flat maximum on phase diagrams, at which the liquidus and solidus lines (or surfaces) touch; in this case, no singular point is present on the composition-properties diagram.