Chemical Equilibrium aA + bB cC+dD Consider the reaction The equilibrium constant is But, incorporating activities, the correct expression for K is

Thermodynamic dissolution constants CaSO4.2H2O  Ca+2 + SO4-2 + 2H2O Account for activities, not just concentrations If solid is ‘pure’, its activity = 1; The activity of water = 1 E.g., Kdis for gypsum: Kdis = (Ca+2)(SO4-2)(H2O)2 (gypsum) Kdis = (Ca+2)(SO4-2) = Kso Kso = 2.4 x 10-5 for gypsum (~10-4.62) 1 1

Ion Activity Product (IAP) ML(s)  M+aq + L-aq Kso = 10x Gypsum  Ca+2 + SO4-2 + 2H2O Kso = 10-4.62 IAP  (M+)(L-) = Kso at equilibrium. Kso is the maximum IAP a solution will tolerate without precipitation. If IAP > Kso then precipitation occurs and if IAP < Kso then dissolution occurs

Use IAP and Kso to determine the relative saturation of a solution: IAP/Kso = 1 then soil solution is in equilibrium with the solid IAP/Kso < 1 then soil solution is undersaturated with respect to the solid phase and the solid will continue to dissolve IAP/Kso > 1 then soil solution is saturated with respect to the solid phase and secondary minerals may precipitate (depending on kinetics)

IAP/Kso is also related to the saturation index, SI: SI = log IAP/Kso When SI = 0, then soil solution is at equilibrium (IAP = Kso and IAP/Kso = 1 and log 1 = 0) When SI < 0, then solution is undersaturated (solid dissolves) When SI > 0, then solution is supersaturated (solid precipitates)

Common Ion Effect A salt is generally less soluble in a solution containing an ion which is the same as one of the constituent ions of that salt. Le Chatelier's Principle: "If a system in equilibrium is subjected to a stress the equilibrium will shift in the direction which tends to relieve that stress." (and return to equilibrium) (M+)(L-) = Kso

Gypsum dissolution in water Gypsum + water        Ca2+ + SO42- + water More Ca+2 or SO42- in solution drives eqn to the left = lower solubility

Common Ion effect on Gypsum solubility Solubility of gypsum MgSO4 CaCl2 Concentration

Ion-pairing aka ion activity or ionic strength effect Mineral solubility is enhanced or increased when different ions are added to the solution forming ion pairs and complexes Gypsum + MgCl2  Ca2+ + SO42- + Mg2+ + 2Cl- + CaCl+ + CaCl20 + MgCl+ + MgSO40... As ionic strength increases, solubility increases Dissolution reaction is driven to the right (solubility increases)

Ion-pairing or Ion activity effect on gypsum solubility MgCl2 Solubility of gypsum NaCl Concentration