Precipitation and dissolution equilibria

If a solid substance or liquid is in contact with water in which it dissolves without substantial chemical change an equilibrium state is established under the given conditions. A saturated solution is formed if a sufficient proportion of the solute is present. 

The reciprocal value of the solubility product is called a solubility constant. 

In the technology of water the solubilities of hydroxides and carbonates are of the greatest importance, these are infuenced by metal ion hydrolysis. With metal cations water forms so-caUed aqua complexes. Usually, four or six molecules of water are coordinated around the central atom (e.g. [Fe(H20)e] +, [A1(H20)6] +, [Mn(H20)e] +, etc.). 

Some reactions taking place in water and their equilibrium constants are presented in Table 3.13. 

In water treatment practice the precipitation and hydrolytic reactions play an essential role particularly in determining the optimum pH of water and residual concentration of metals. Theoretical calculation of optimum conditions of precipitation (pH, quantity of precipitant) and residual concentration of the ion to be removed are always checked by experiment. 

---

In qualitative analysis, we determine what substances are present in a mixture but not their quantities. An analysis that aims at identifying the cations present in a mixture is called qualitative cation analysis. Qualitative cation analysis provides us with many examples of precipitation (and dissolution) equilibria, acid-base equilibria, and oxidation-reduction reactions. Also, in the general chemistry laboratory it offers the challenge of unraveling a mystery—solving a qualitative analysis "unknown."... 

---