Hg þ S ¼ HgS

reverse to Eq. (4). Some precipitates can be optionally considered as the species of AaBb or AaBbCc type. For example, the solubility product for MgHPO4 can be written as Ksp = [Mg+2] [HPO4 –2 ] or Ksp1 = [Mg+2][H+1][PO4 –3 ] = KspK3P.

The ferrocyanide ion Fe(CN)6 –<sup>4</sup> (with evaluated stability constant K6 ca. 1037) can be considered as practically undissociated, i.e., Fe(CN)6 –<sup>4</sup> is kinetically inert [28], and then it does not give Fe+2 and CN–<sup>1</sup> ions. The solubility product of Zn2Fe(CN)6 is Ksp = [Zn+2] 2 [Fe(CN)6 –4 ]. Therefore, consideration of Zn2Fe(CN)6 as a ternary salt with Ksp1 = [Zn+2] 2 [Fe2+][CN–<sup>1</sup> ] <sup>6</sup> = Ksp/ K6 is not acceptable.

In principle, the solubility product values are formulated for stoichiometric compounds, and specified as such in the related tables. However, some precipitates obtained in laboratory have nonstoichiometric composition, e.g., dolomite Ca1+xMg1-x(CO3)2 [22, 23], FexS [29]. In particular, Fex<sup>S</sup> can be rewritten as Fe+2pFe+3qS; from the relations: 2<sup>p</sup> + 3<sup>q</sup> � 2 = 0 and <sup>p</sup> <sup>+</sup> <sup>q</sup> <sup>=</sup> <sup>x</sup>, we get q/p = 2(1 � x)/(3x � 2).

In this context, some remark needs a formulation of Ksp for some hydroxyoxides (e.g., FeOOH) and oxides (e.g., Ag2O). The related solubility products are formulated after completion of the corresponding reactions with water, e.g., FeOOH + H2O = Fe(OH)3, Fe2O3∙xH2O + (3 � x)H2O = <sup>2</sup>Fe(OH)3 ) Fe(OH)3 = Fe+3 + 3OH–<sup>1</sup> ) <sup>K</sup>sp = [Fe+3][OH–<sup>1</sup> ] 3 ; Ag2O + H2O=2AgOH ) AgOH = Ag+1 + OH–<sup>1</sup> ) <sup>K</sup>sp = [Ag+1][OH–<sup>1</sup> ], see it in the context with gcd(a,b) = 1.

The solubility product can be involved not only with dissociation reaction. For example, the dissolution reaction Ca(OH)2 + 2H+1 = Ca+2 + 2H2O [30], characterized by Ksp1 = [Ca+2]/[H+1] 2 , is involved with Ksp = [Ca+2][OH–<sup>1</sup> ] <sup>2</sup> in the relation Ksp1 = Ksp/K<sup>w</sup> 2 . In Ref. [31], the solubility product is associated with formation (not dissociation) of a precipitate.
