In the broadest sense, the category of "dissociation" used in physicochemical terminology determines the nature of the decay of complex compounds into elements that make up these compounds. Electrolytic dissociation, which is understood as the process of decomposition of complex compounds into ions, under the influence of solvent molecules, is especially distinguished. And another, quite independent in its properties, type of dissociation is the dissociation of complex compounds.
The peculiarity of this process is due to the fact that the spheres of complex compounds are extremely different among themselves in the degree of stability of the elements. This means, first of all, the mismatch of the external and internal spheres of matter. Its particles, which are located in the outer sphere, are very weakly bound to the complex ion because their bonding is ensured only by electrostatic forces. As a result, they easily detach from the main substance in an aqueous solution.
Such dissociation of complex compounds is called primary. It is distinguished by some features. The main one is that it proceeds in the external sphere and ends almost completely, and this is similar to the process, which is the electrolytic dissociation of complex compounds. There is another variant of its course. For example, if we observe a reversible process in which the decomposition of the inner sphere occurs, then this process is called the secondary dissociation of complex compounds.
A characteristic property of secondary dissociation is that an equilibrium state develops between the complex element of the substance, ligands, and the central ion. An example is such a reaction. Take a solution that contains the complex ion [Ag (NH3) 2] +. If a drop of any chloride acts on it, then we will not find the expected precipitate. The fact is that, as a rule, during the interaction of chlorides with ordinary silver compounds, a precipitate appears in the form of silver chloride. It becomes clear that in this case, the amount of ions that is contained in the ammonia solution is too small. It is such that even if excess chloride ions are introduced into the solution, it does not allow to reach the solubility level of silver. However, if then potassium ions are added to the resulting solution, then in the precipitate we get silver iodide. This fact indicates that silver ions, although in small quantities, are still present in this solution. A precipitate precipitates, the presence of which indicates that the concentration of the solution is quite sufficient for the formation of a precipitate. This situation is explained by the fact that the solubility level of silver iodide is much lower than that of silver chloride.
According to this example, we can conclude that the dissociation of complex compounds in solutions is based on the laws of action of the electron masses of elements, and therefore can be described by a certain equilibrium constant, reflecting the degree of instability of the ion. These constants are very different for different ions of complex compounds. The reason for this diversity is due to the fact that constant expressions include concentrated ions and molecules. The degrees of this concentration can be very different. Therefore, they determine the variety of ion instability constants .
A phenomenon characteristic of the dissociation of complex compounds is that the lower the concentration level of the decomposition products obtained during the reactions, the more stable the complex compound appears, and therefore the ion instability will be lower. Particles that exhibit higher stability in solutions have lower instability constants.
As a rule, in real solutions the so-called stepwise dissociation of the complex takes place, because the ratios of the complexes present in the solution are different. In this case, the value of the total instability constant is calculated by multiplying the constants of all the complexes represented in this solution.