In this field, V. G. Khlopin and his colleagues and students (M. S. Merkulova, V. I. Grebenshchikov and others) developed a methodology for studying the process of isomorphous coprecipitation of microcomponents and ways to achieve equilibrium in the solid phase-solution system, - the influence of many factors on this process was established and the hypothesis of V. G. Khlopin (1924) about the subordination of the process of fractional crystallization to the law of substance distribution between two immiscible phases was proved (Khlopin's law). The possibility of using the method of isomorphic co-crystallization not only for the isolation of radioactive elements, but also for the study of their state in liquid and solid phases - for determining their valence was shown. V. G. Khlopin and A. G. Samartseva established the existence of compounds of divalent and hexavalent polonium by this method. The process of adsorption of crystalline precipitates by the surface was also studied, - the distribution between the gas phase and crystalline precipitate, as well as between the salt melt and the solid phase.
Thus, in this section, V. G. Khlopin's studies address the following key issues: <br>
1. conditions for achieving true (thermodynamic) microcomponent equilibrium between the crystalline solid phase and solution; <br>
2. the use of radioelements as indicators in determining the mechanism of isomorphic substitution of dissociated ions; <br>
3. application of general laws of isomorphous substitution for development of a method for fixation of chemical compounds present in extremely small proportions and unstable in the solid phase, establishment of their valence and chemical type, - for revealing new chemical equilibria both in the solid phase and in solution; <br>
4. conditions of adsorption equilibrium between solid crystalline phase and solution.