Equilibrium constants, such as solubility product constants (Ksp), are essential for understanding ionic compounds' solubility behavior in aqueous systems. These constants provide critical insights into the concentration of dissociated ions at equilibrium, which is crucial for processes in fields such as analytical chemistry, materials science, and environmental engineering. Alfa Chemistry provides the equilibrium constants Ksp of some aqueous solutions of ionic compounds at 25 ℃ for reference.
| Ionic Compounds | Formula | Ksp | Kspa |
| Aluminium hydroxide | Al(OH)3 | 3*10-34 | |
| Aluminium phosphate | AlPO4 | 9.84*10-21 | |
| Barium bromate | Ba(BrO3)2 | 2.43*10-4 | |
| Barium carbonate | BaCO3 | 2.58*10 | |
| Barium chromate(VI) | BaCrO4 | 1.17*10-10 | |
| Barium fluoride | BaF2 | 1.84*10-7 | |
| Barium hydroxide octahydrate | Ba(OH)2 ·8H2O | 2.55*10-4 | |
| Barium iodate | Ba(IO3)2 | 4.01 *10 | |
| Barium iodate monohydrate | Ba(IO3)2·H2O | 1.67 *10 | |
| Barium molybdate | BaMoO4 | 3.54*10-8 | |
| Barium nitrate | Ba(NO3)2 | 4.64*10-3 | |
| Barium selenate | BaSeO4 | 3.40*10-8 | |
| Barium sulfate | BaSO4 | 1.08*10-10 | |
| Barium sulfite | BaSO3 | 5.0*10-10 | |
| Beryllium hydroxide | Be(OH)2 | 6.92*10-22 | |
| Bismuth arsenate | BiAsO4 | 4.43*10-10 | |
| Bismuth iodide | BiI | 7.71*10-19 | |
| Bismuth triiodite | BiI3 | 7.71*10-19 | |
| Cadmium arsenate | Cd3(AsO4)2 | 2.2*10-33 | |
| Cadmium carbonate | CdCO3 | 1.0*10-12 | |
| Cadmium fluoride | CdF2 | 6.44 *10-3 | |
| Cadmium hydroxide | Cd(OH)2 | 7.2*10-15 | |
| Cadmium iodate | Cd(IO3)2 | 2.5*10-8 | |
| Cadmium oxalate trihydrate | CdC2O4·3H2O | 1.42*10-8 | |
| Cadmium phosphate | Cd3(PO4)2 | 2.53*10-33 | |
| Cadmium sulfide | CdS | 8*10-7 | |
| Calcium carbonate (aragonite) | CaCO3 | 6.0*10 | |
| Calcium carbonate (calcite) | CaCO3 | 3.36*10 | |
| Calcium fluoride | CaF2 | 3.45*10-11 | |
| Calcium hydroxide | Ca(OH)2 | 5.02*10-6 | |
| Calcium iodate | Ca(IO3)2 | 6.47*10-6 | |
| Calcium iodate hexahydrate | Ca(IO3)2·6H2O | 7.10 *10-7 | |
| Calcium molybdate | CaMoO4 | 1.46*10-8 | |
| Calcium oxalate monohydrate | CaC2O4 ·H2O | 2.32*10 | |
| Calcium phosphate | Ca3 (PO4)2 | 2.07*10-33 | |
| Calcium sulfate | CaSO4 | 4.93*10-5 | |
| Calcium sulfate dihydrate | CaSO4 ·2H2O | 3.14 *10-5 | |
| Calcium sulfite hemihydrate | CaSO3 ·0.5H2O | 3.1 *10-7 | |
| Cesium perchlorate | CsClO4 | 3.95*10-3 | |
| Cesium periodate | CsIO4 | 5.16 *10-6 | |
| Chromium(II)hydroxide | Cr(OH)2 | 1*10-17 | |
| Chromium(III)hydroxide | Cr(OH)3 | 1*10-30 | |
| Cobalt(II) arsenate | Co3 (AsO4)2 | 6.80 *10-29 | |
| Cobalt(II) carbonate | CoCO3 | 1.0 *10-10 | |
| Cobalt(II) hydroxide | Co(OH)2 | 5.92*10-15 | |
| Cobalt(II) iodate dihydrate | Co(IO3)2·2H2O | 1.21*10-2 | |
| Cobalt(II) phosphate | Co3 (PO4)2 | 2.05*10-35 | |
| Copper(I) bromide | CuBr | 6.27*10 | |
| Copper(I) chloride | CuCl | 1.72*10-7 | |
| Copper(I) cyanide | CuCN | 3.47 *10-20 | |
| Copper(I) iodide | CuI | 1.27*10-12 | |
| Copper(I) thiocyanate | CuSCN | 1.77*10-13 | |
| Copper(II) arsenate | Cu3 (AsO4)2 | 7.95*10-36 | |
| Copper(II) hydroxide | Cu(OH)2 | 1*10-20 | |
| Copper(II) iodate monohydrate | Cu(IO3)2·H2O | 6.94 *10-8 | |
| Copper(II) oxalate | CuC2O4 | 4.43*10-10 | |
| Copper(II) phosphate | Cu3(PO4)2 | 1.40*10-37 | |
| Copper(II) sulfide | CuS | 6*10-16 | |
| Europium(III) hydroxide | Eu(OH)3 | 9.38 *10-27 | |
| Gallium(III) hydroxide | Ga(OH)3 | 7.38 *10-36 | |
| Iron(II) carbonate | FeCO3 | 3.13 *10-11 | |
| Iron(II) fluoride | FeF2 | 2.36*10-6 | |
| Iron(II) hydroxide | Fe(OH)2 | 4.87*10-17 | |
| Iron(II) sulfide | FeS | 6*102 | |
| Iron(III) hydroxide | Fe(OH)3 | 2.79*10-39 | |
| Iron(III) phosphate dihydrate | FePO4 ·2H2O | 9.91*10-16 | |
| Lanthanum iodate | La(IO3)3 | 7.50*10-12 | |
| Lead oxalate | PbC2O4 | 8.5*10 | |
| Lead(II) bromide | PbBr2 | 6.60*10-6 | |
| Lead(II) carbonate | PbCO3 | 7.40*10-14 | |
| Lead(II) chloride | PbCl2 | 1.70*10-5 | |
| Lead(II) chromate | PbCrO4 | 3*10-13 | |
| Lead(II) fluoride | PbF2 | 3.3*10-8 | |
| Lead(II) hydroxide | Pb(OH)2 | 1.43*10-20 | |
| Lead(II) iodate | Pb(IO3)2 | 3.69*10-13 | |
| Lead(II) iodide | PbI2 | 9.8*10 | |
| Lead(II) selenate | PbSeO4 | 1.37*10-7 | |
| Lead(II) sulfate | PbSO4 | 2.53*10-8 | |
| Lead(II) sulfide | PbS | 3*10-7 | |
| Lithium carbonate | Li2CO3 | 8.15*10-4 | |
| Lithium fluoride | LiF | 1.84*10-3 | |
| Lithium phosphate | Li3PO4 | 2.37*10-11 | |
| Magnesium ammonium phosphate | MgNH4PO4 | 3*10-13 | |
| Magnesium carbonate | MgCO3 | 6.82*10-6 | |
| Magnesium carbonate pentahydrate | MgCO3·5H2O | 3.79*10-6 | |
| Magnesium carbonate trihydrate | MgCO3·3H2O | 2.38*10-6 | |
| Magnesium fluoride | MgF2 | 5.16*10-11 | |
| Magnesium hydroxide | Mg(OH)2 | 5.61*10-12 | |
| Magnesium oxalate dihydrate | MgC2O4 ·2H2O | 4.83*10-6 | |
| Magnesium phosphate | Mg3 (PO4)2 | 1.04*10-24 | |
| Manganese(II) carbonate | MnCO3 | 2.24*10-11 | |
| Manganese(II) iodate | Mn(IO3)2 | 4.37*10-7 | |
| Manganese(II) oxalate dihydrate | MnC2O4 ·2H2O | 1.70*10-7 | |
| Manganese(II) sulfide (a-form) | MnS | 3*107 | |
| Mercury(I) bromide | Hg2Br2 | 6.40*10-23 | |
| Mercury(I) carbonate | Hg2CO3 | 3.6*10-17 | |
| Mercury(I) chloride | Hg2Cl2 | 1.43*10-18 | |
| Mercury(I) fluoride | Hg2F2 | 3.10*10-6 | |
| Mercury(I) iodide | Hg2I2 | 5.2*10-29 | |
| Mercury(I) oxalate | Hg2C2O4 | 1.75*10-13 | |
| Mercury(I) sulfate | Hg2SO4 | 6.5*10-7 | |
| Mercury(I) thiocyanate | Hg2(SCN)2 | 3.2*10-20 | |
| Mercury(I)hydroxide | Hg2(OH)2 | 2*10-24 | |
| Mercury(II) bromide | HgBr3 | 6.2*10-20 | |
| Mercury(II) hydroxide | Hg(OH)2 | 4*10-26 | |
| Mercury(II) iodide (red) | HgI2 | 2.9*10-29 | |
| Mercury(II) sulfide (black) | Hg2S | 2*10-32 | |
| Mercury(II) sulfide (red) | Hg2S | 4*10-33 | |
| Neodymium carbonate | Nd2(CO3)3 | 1.08*10-33 | |
| Nickel(II) carbonate | NiCO3 | 1.42*10-7 | |
| Nickel(II) hydroxide | Ni(OH)2 | 5.48*10-16 | |
| Nickel(II) iodate | Ni(IO3)2 | 4.71*10-5 | |
| Nickel(II) phosphate | Ni3(PO4)2 | 4.74*10-32 | |
| Palladium(II) thiocyanate | Pd(SCN)2 | 4.39*10-23 | |
| Potassium hexachloroplatinate | K2PtCl6 | 7.48*10-6 | |
| Potassium perchlorate | KClO4 | 1.05*10-2 | |
| Potassium periodate | KIO4 | 3.71*10-4 | |
| Praseodymium(III) hydroxide | Pr(OH)3 | 3.39*10-24 | |
| Radium iodate | Ra(IO3)2 | 1.16*10 | |
| Radium sulfate | RaSO4 | 3.66*10-11 | |
| Rubidium perchlorate | RbClO4 | 3.00*10-3 | |
| Scandium fluoride | ScF3 | 5.81*10-24 | |
| Scandium hydroxide | Sc(OH)3 | 2.22*10-31 | |
| Silver(I) acetate | AgC2H3O2 | 1.94*10-3 | |
| Silver(I) arsenate | Ag3AsO4 | 1.03*10-22 | |
| Silver(I) bromate | AgBrO3 | 5.38*10-5 | |
| Silver(I) bromide | AgBr | 5.35*10-13 | |
| Silver(I) carbonate | Ag2CO3 | 8.46*10-12 | |
| Silver(I) chloride | AgCl | 1.77*10-10 | |
| Silver(I) chromate | Ag2CrO4 | 1.12*10-12 | |
| Silver(I) cyanide | AgCN | 5.97*10-17 | |
| Silver(I) hydroxide | AgOH | 2*10-8 | |
| Silver(I) iodate | AgIO3 | 3.17*10-8 | |
| Silver(I) iodide | AgI | 8.52*10-17 | |
| Silver(I) oxalate | Ag2C2O4 | 5.40*10-12 | |
| Silver(I) phosphate | Ag3PO4 | 8.89*10-17 | |
| Silver(I) sulfate | Ag2SO4 | 1.20*10-5 | |
| Silver(I) sulfide | Ag2S | 6*10-30 | |
| Silver(I) sulfite | Ag2SO3 | 1.50*10-14 | |
| Silver(I) thiocyanate | AgSCN | 1.03*10-12 | |
| Strontium arsenate | Sr3(AsO4) | 4.29*10-19 | |
| Strontium carbonate | SrCO3 | 5.60*10-10 | |
| Strontium fluoride | SrF2 | 4.33*10 | |
| Zinc selenide | ZnSe | 3.6*10-26 | |
| Zinc selenite monohydrate | ZnSeO3 ·H2O | 1.59*10-7 | |
| Zinc sulfide (sphalerite) | ZnS | 2*10-4 | |
| Zinc sulfide (wurtzite) | ZnS | 3*10-2 |
Definition and Calculation of Ksp
For ionic compounds with limited solubility, the Ksp is defined based on the dissociation of the solid into its constituent ions in water. The general dissociation reaction is represented as:
MmAn(s) ⇌ mMn+(aq)＋nAm-(aq)
The solubility product constant is given by:
Ksp ⇌ [Mn+]m＋[Am-]n
Here:
MmAn is the sparingly soluble ionic compound.
Mn+ and Am- are the dissociated cation and anion, respectively.
[Mn+] and [Am-] are their equilibrium molar concentrations.
The calculation of Ksp from standard thermodynamic data involves the Gibbs free energy (ΔG°) of formation for the ionic species and the solid compound. The relationship is expressed as:
ΔG°=mΔfG°(Mn+, aq)＋nΔfG°(Am-, aq)－ΔfG°(MmAn, s)
Where:
ΔfG° is the standard Gibbs free energy of formation.
R is the universal gas constant.
T is the absolute temperature (in Kelvin).
This thermodynamic approach ensures precise and reproducible Ksp values for various ionic substances.
Ksp vs. Kspa
Solubility Product Constant (Ksp)
Ksp is a thermodynamic constant that describes the product of ionic concentrations of insoluble salts in saturated solution. It is constant at a given temperature, independent of whether or not equilibrium is reached in the solution.Ksp denotes the dynamic equilibrium of a poorly soluble salt as it dissolves, e.g., for AB↔A+＋B-, the expression for Ksp is:
Ksp=[A+][B-]
The value of Ksp is independent of the effects of temperature and other components of the solution, and reflects the limit of a solid's ability to dissolve.
Apparent Solubility Product Constant (Kspa)
Kspa is the solubility product constant for the actual system of a solution, which is affected by other factors (e.g., co-existing ions, complexation, pH, ionic strength, etc.) The expression for Kspa is similar to that for Ksp, but the value reflects the true state of solubility equilibrium for a given system, rather than a purely thermodynamic limit. For example, if the presence of a complexing agent in the system increases the solubility of the metal ion, the value of Kspa will be significantly greater than the theoretical Ksp.
Summary of Differences
| Characteristics | Ksp | Kspa |
| Definition | Solubility product under ideal conditions | Solubility product taking into account the effect of actual conditions |
| Dependent factors | Inherent properties (temperature dependent only) | Environmental conditions (e.g. pH, ionic strength) |
| Reflects the system | Thermodynamic equilibrium state at the limit of theory | Dissolution equilibria under practical conditions |
| Practical applications | Used to predict the solubility of salts under standard conditions and to perform chemical equilibrium calculations. | Used for the analysis of the dissolution behavior of complex systems in real environments, such as the study of the migration behavior of heavy metals in wastewater treatment. |
