| 2 NH3 + | AgCl | ⇄ 2 NH3 + Ag+ + Cl−. |
| ⇅ | ||
| AgCl ↓ |
(2)
It is clear, from a comparison of this series of equations with equations (1), that they represent the same reversible reactions in a somewhat different order. Since equilibrium conditions for any reversible reaction (e.g. for A + B ⇄ C + D) are independent of the order in which the components (e.g. A, B, C, D) are brought into the system (p. [94]), the difference of order, indicated by equations (1) and (2), cannot affect the final condition of equilibrium in the system under discussion. For instance, since we again have silver chloride in contact with its saturated solution, we again must have [Ag+] × [Cl−] = KAgCl, and the experimental confirmation of this relation (and similarly of the relation [Ag+] × [NH3]2 / [Ag(NH3)2+] = K) agrees as well with this second path of the action as with the first. Conversely, since the path, by which the equilibrium is reached, does not affect the condition of equilibrium, it is perfectly legitimate to draw conclusions from equilibrium constants, without assuming to know anything at all about the path by which the condition is reached.[475]
For analytical work, the vital point is the ultimate condition of equilibrium, which determines whether a precipitate may exist and be formed in a given system or not. The instability constants of the complex ions and the solubility-product constants of precipitates are the constant factors involved in the ultimate conditions for equilibrium, and they do not depend on the path by which equilibrium is reached. Consequently, we find that the application of [p237] the theory of complex ions to analytical problems of precipitation has been in no wise invalidated by the problems presented by Haber. The theory forms now, as before, in fact, the best quantitative basis for the expression of the experimental results. The stability constants, and the concentrations of the components used, determine the limiting concentrations in which a given metal ion is capable of continued existence, and determine, therefore, the question whether an ionogen, of a given solubility, is capable of existence as a solid phase in a given system.
The Structure of Complex Ions.
| Ag─N═ | C──C | ═N─K. |
| ╲ ╱ | ||
| C | ||
| ║ | ||
| N | ||
| | | ||
| K |
The other complex cyanide ions, ferrocyanide, ferricyanide, cobalticyanide, etc., are considered to have structures entirely similar to those given to the argenticyanide ions.[479]
Complex Halide, Sulphide, Oxide and Oxonium Ions.
Complex Ions of Organic Oxygen Derivatives.
These relations clearly recall the characteristic behavior of ammoniacal and cyanide solutions, in which complex ions are formed, and the interference of the organic compounds with precipitation is of a similar nature—complex ions are formed by metal ions with these organic compounds, and the complexes are, in many instances, sufficiently stable to reduce the concentrations of the metal ions to the point, where only very difficultly soluble salts can be precipitated.