In this paper such experimental methods of research of heterogeneous systems "solid phase - multicomponent solution" as the solubility method and the residual concentration method are examined. The use of the predictive thermodynamic method for analyzing the behavior of poorly soluble compounds is also surveyed. In the presence of a complete set of thermodynamic functions of all possible reactions it becomes possible to estimate the amount of precipitate depending on the initial concentrations of the ions concentrations in the mixture and the pH of the solution. By calculating the temperature coefficient of the residual concentration of one of the salt components, it is also possible to determine the influence of the temperature on the precipitation degree of the less soluble compound. The method of investigating residual concentrations can be extended to more complex systems, where additional reactions with a complexing agent result in the formation of polynuclear, protonated or mixed species.