The solubility product constant Ksp is a useful parameter for calculating the aqueous solubility of sparingly soluble compounds under various conditions. It may be determined by direct measurement or calculated from the standard Gibbs energies of formation ∆fG° of the species involved at their standard states. Thus, if Ksp = [M+]m·[A–]n is the equilibrium constant for the reaction
MmAn (s) ⇌ mM+(aq) + nA– (aq)
where MmAn is the slightly soluble substance and M+ and A- are the ions produced in solution by the dissociation of MmAn , then the Gibbs energy change is
∆G° = m ∆fG° (M+,aq) + n ∆fG° (A–,aq) –∆fG° (MmAn, s)
The solubility product constant is calculated from the equation
ln Ksp = –∆G°/RT
The table gives selected values of Ksp at 25 °C. Many of these have been calculated from standard state thermodynamic data in Refs. 1 and 2; other values are taken from publications of the IUPAC Solubility Data Project (Refs. 3 to 7).
The above formulation is not convenient for treating sulfides because the S-2 ion is usually not present in significant concentrations (see Ref. 8). This is due to the hydrolysis reaction
S-2 + H2O ⇌ HS– + OH–
which is strongly shifted to the right except in very basic solutions. Furthermore, the equilibrium constant for this reaction, which depends on the second ionization constant of H2S, is poorly known. Therefore, it is more useful in the case of sulfides to define a different solubility product Kspa based on the reaction
MmSn (s) + 2H+ ⇌ mM+ + nH2S (aq)
Column definitions for the table are as follows.
Column heading | Definition |
Name | Name of inorganic salt; listed alphabetically |
Mol. form. | Molecular formula of inorganic salt |
Ksp | Solubility product constant at 25 °C |
Kspa | Solubility product constant for sulfides at 25 °C (see discussion above) |
Values of Kspa taken from Ref. 8 are given for several sulfides in the last column. Additional discussion of sulfide equilibria may be found in Refs. 7 and 9.
Name | Mol. form. | CAS Reg. No. | Mol. Wt. | Ksp | Kspa |
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Aluminum phosphate | AlPO4 | 7784-30-7 | 121.953 | 9.84·10-21 | |
Barium bromate | Ba(BrO3)2 | 13967-90-3 | 393.131 | 2.43·10-4 | |
Barium carbonate | BaCO3 | 513-77-9 | 197.336 | 2.58·10-9 | |
Barium chromate(VI) | BaCrO4 | 10294-40-3 | 253.321 | 1.17·10-10 | |
Barium fluoride | BaF2 | 7787-32-8 | 175.324 | 1.84·10-7 | |
Barium hydroxide octahydrate | Ba(OH)2·8H2O | 12230-71-6 | 315.464 | 2.55·10-4 | |
Barium iodate | Ba(IO3)2 | 10567-69-8 | 487.132 | 4.01·10-9 | |
Barium iodate monohydrate | Ba(IO3)2·H2O | 7787-34-0 | 505.148 | 1.67·10-9 | |
Barium molybdate | BaMoO4 | 7787-37-3 | 297.27 | 3.54·10-8 | |
Barium selenate | BaSeO4 | 7787-41-9 | 280.29 | 3.40·10-8 | |
Barium sulfate | BaSO4 | 7727-43-7 | 233.390 | 1.08·10-10 | |
Barium sulfite | BaSO3 | 7787-39-5 | 217.390 | 5.0·10-10 | |
Beryllium hydroxide (α) | Be(OH)2 | 13327-32-7 | 43.027 | 6.92·10-22 | |
Bismuth arsenate | BiAsO4 | 13702-38-0 | 347.900 | 4.43·10-10 | |
Bismuth triiodide | BiI3 | 7787-64-6 | 589.693 | 7.71·10-19 | |
Cadmium arsenate | Cd3(AsO4)2 | 25160-35-4 | 615.071 | 2.2·10-33 | |
Cadmium carbonate | CdCO3 | 513-78-0 | 172.420 | 1.0·10-12 | |
Cadmium fluoride | CdF2 | 7790-79-6 | 150.408 | 6.44·10-3 | |
Cadmium hydroxide | Cd(OH)2 | 21041-95-2 | 146.426 | 7.2·10-15 |