The procedure to study the phase equilibrium lines “solid - liquid” was developed for the binary systems with using a coefficient of Bjerrum Guggenheim. The universality of this procedure to solve a mathematical problem of phase diagrams and its efficiency to find the behavioristic characteristics of a solvent and the dissolved components in the equilibrium phases were demonstrated. Two types of the generalized mathematical expression as the modified Le Chatelier-Shreder equation were proposed to describe analytically the liquidus and solidus lines of phase diagrams of the whole class of systems