Common anions formation

Figure 7.1. Solubility of simple salts as a function of the common anion concentration (Example 7.2). The cations and anions of these salts do not protolyze in the neutral pH range. The equilibrium solubility is given by the metal-ion concentration. At high anion or cation concentration, complex formation or ion-pair binding becomes possible (dashed lines). If the salt is dissolved in pure water (or in an inert electrolyte), the solubility is defined by the electroneutrality z[Me" J = /i[anion ]. If z = n (e.g., BaS04), the solubility is given by the intersection (-I-). If z the electroneutrality condition is fulfilled at a point slightly displaced from the intersection (t). The insert exemplifies the solubility equilibrium for Cap2 ( o = 10" ) and lists the domains of over- and undersaturation.

As it can be seen from the tables, binary compounds are formed only in systems with a common anion. The compounds originate in the systems Li+, Rb+Z/X" and Li+, Cs+//X , where X = F, Cl, Br, I. On the other hand, phase diagrams with continuous solid solutions can be observed predominantly in systems with soft cations. These facts are connected with the value of the repulsion energy of evenly charged ions (see Eq. (2.7)). The higher the repulsion energy, the higher is the probability of binary compound formation. 

It seems to me that further information on this subject might be obtainable from studies of the kinetics of the reaction in the presence of added salts with a common anion (e.g., KCl for the case of catalysis by CujCL), or by the use of an inert solvent of low dielectric constant, which would. tend to encourage the formation of ion-pairs. 

When the polymerizations of cyclic sulfides are carried out with anionic initiators, many side reactions can occur. On the other hand, common anionic initiators, like KOH, yield optically active polymers from optically active propylene sulfide. An example of a side reaction is formation of propylene and sodium sulfide in sodium naphthalene initiated polymerizations. 

Separations at high pH common anions, silicate, borate, cyanide, formate, acetate... 

Preferential adsorption of one type of ions onto the particle surface coupled with the formation of a diffuse layer of the counterions (ions of opposite charge) leads to electrostatic stabilization due to repulsion between the double layers (Chapter 4). The valence and radius of the counterions can modify the repulsion between the particles and so can influence the stability of the suspension. Counterions with higher valence are more effective for causing flocculation Schulze-Hardy rule), while for ions of the same valence, the smaller ions are more effective. For monovalent cations, the effectiveness of flocculation is in the order Li > Na > K+ > NH4, while for divalent cations, Mg " > Ca " > Sr " > Ba. This sequence is known as the Hofmeister series. For common anions, the effectiveness of flocculation is in the order 804 > Cl > NOJ. 

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