Precipitation, superimposition

The superimposition of precipitation and/or complexation equilibria to the redox equilibrium may lead to the phenomena described in Sect. 14.2.1, which are of analytical interest. 

Frequently, a metallic ion forms a series of complexes with the same ligand, one of which is poorly soluble. This is the case with some hydroxo and chloro complexes. We shall see that a new sort of equilibrium constant permits us to investigate the superimposition of complexation and precipitation phenomena. 

Dependence of the Solubility on the Solution s Ionic Strength and on the Presence of Common Ions Superimposition of Several Precipitation Equilibria... 

The case of the superimposition of two precipitation equilibria is of practical importance in analysis since it occurs when the separation of two ions by precipitation is carried out. More precisely, the operation consists of preferentially precipitating one of the present ions in the admixture by adding an appropriate reagent. Of course, in this process, the markedly weaker solubility of one of the ions in the presence of the reagent is taken into account. Simple calculations based on the consideration of solubility products permit us to predict the maximum extent of the separation. 

From the quantitative standpoint, the superimposition of complexation and precipitation phenomena is difficult to tackle rigorously. Indeed, numerous relations... 

The formation of silver oxide and its dissolution in the presence of ammonia is slightly more complicated than the preceding case. The ligand ammonia NH3 is basic. Not only does the superimposition of precipitation and complexation phenomena occur,... 

We have seen that solubility exhibits a minimum value for pCl 3. Then it is about Iq-5.8 jnoi/L. For pCl > 3, the solubility increases when pCl increases. For pCl < 3, the solubility increases when pCl decreases. In this example, we are faced with the superimposition of two sorts of phenomena precipitation and complexation. From studying this example, we see that perfect knowledge of the precipitation reaction and of all the chemical phenomena occurring simultaneously with it is absolutely necessary to control the losses by making the precipitate soluble. A parameter that in some cases may be of great interest to limit losses is the pH value. For example, let s investigate the precipitation of calcium phosphate Ca3(P04)2. Its solubility depends on the following four chemical reactions ... 

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