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

2. 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 (~ ) 1 1

3. Ion Activity Product (IAP)
ML(s)  M+aq + L-aq Kso = 10x
Gypsum  Ca+2 + SO H2O Kso =
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

4. 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)

5. 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)

6. 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

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

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

9. 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)

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