Russian Chemical Bulletin, International Edition, Vol. 61, No. 4, pp. 741—751, April, 2012 741 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 0737—0747, April, 2012. 1066-5285/12/6104-0741 © 2012 Springer Science+Business Media, Inc. Chemical equilibrium in a heterogeneous fluid phase system: thermodynamic regularities and topology of phase diagrams A. M. Toikka, M. A. Toikka, and M. A. Trofimova Department of Chemical Thermodynamics and Kinetics, Saint Petersburg State University, 26 Universitetsky prosp., 198504 Saint Petersburg, Russian Federation. E-mail: alexander.toikka@chem.spbu.ru Singularities of the systems with simultaneous phase and chemical equilibria were exam- ined. The thermodynamic regularities were revealed and topology of phase diagrams was pre- sented for multicomponent heterogeneous systems including two or several equilibrium liquid phases (solution layering). The thermodynamic conditions for displacement of phase and chem- ical equilibrium are considered. Key words: chemical and phase equilibrium, topology of diagrams, thermodynamics. Real physicochemical systems are sophisticated and affected by a combination of various internal and external factors. The systems in which the phase and chemical processes occur simultaneously are among the most im- portant and frequently studied objects. Interest in study- ing similar objects, i.e., heterogeneous systems with inter- action, is not purely academic. Simultaneously occurring phase and chemical processes are especially interesting for such areas as technology of heavy organic synthesis or separation of substances. A combination of the chemical reaction with simultaneous phase separation makes it pos- sible to optimize the technological process towards energy and resource economy and to enhance ecological safety of the technology. Similar combined processes represent a type of reactive mass-transfer processes. For example, reactive distillation (or distillation rectification), when chemical synthesis is accompanied by the vapor—liquid phase transition, is practically significant. 1—5 Such reac- tive mass-transfer processes as reactive extraction or chem- ical synthesis in a membrane reactor are also technologi- cally significant. At the same time, the phase transition occurring during the chemical reaction can play a nega- tive role in technology. For example, when an initially homogeneous solution is layered during the reaction, the synthesis process is impeded. On developing the technol- ogy of organic synthesis, the choice of initial reactant con- centrations are often dictated by the need to avoid unde- sirable phase transformations due to which the reaction mixture becomes heterogeneous. Therefore, information on the structure of phase diagrams of the reaction system is very important. Basic research into combined reactive mass-transfer processes and equilibrium states with simul- taneous chemical and phase equilibrium allows one to establish new thermodynamic and kinetic regularities. In particular, the following new singularities were observed when studying the vapor—liquid phase transitions in the systems with chemical reactions: reactive azeotrope (or chemiazeotrope), 6—8 kinetic azeotrope, 9 and others. The full characterization of the topological structure of phase diagrams of reactive systems can include such singulari- ties as the shape of reactive (or stiochiometric) lines, ar- rangement of varieties of chemical equilibria, and con- stancy of chemical affinity (isoaffinic varieties). 10—12 In this respect, the vapor—liquid systems in which chem- ical reactions occur are among the most studied fluid mix- tures. 1,4,13,14 In this review, we consider the main singularities of the topological structure of phase diagrams of liquid phase heterogeneous systems (i.e., the systems including two or more liquid phases) related to chemical reactions. In ad- dition, the thermodynamic regularities due to the simulta- neous attainment of phase and chemical equilibria and their displacement in these systems were examined. The review includes our results and analysis of literature data on the liquid phase systems in chemical reactions compli- cated by layering. Varieties of chemical equilibrium and reactive lines We can focus now on the main concepts on elements of phase diagrams of multicomponent systems related to the occurrence of the chemical reaction. Reactive or stio- chiometric lines reflect the displacement of the reaction mixture composition during the reaction. Their arrange- ment in the concentration area is uniquely specified by the stoichiometric equation of the chemical reaction. Figure 1 presents two variants of arrangement of reactive lines in the case of reactions in the system of three substances: