Chemical Equilibrium
Consider the reaction
aA + bB
cC+dD

C  D
K
a
b
A B
c
The equilibrium constant is
d
But, incorporating activities, the correct expression for K is
K
c
C  D
d
 A B
b
a

C   D 

b
A  B 
c
c
C
a a
A
d
d
D
b
B
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:
1
Kdis = (Ca+2)(SO4-2)(H2O)2
(gypsum)
1
Kdis = (Ca+2)(SO4-2) = Kso
Kso = 2.4 x 10-5 for gypsum (~10-4.62)
Ion Activity Product (IAP)
ML(s)  M+aq + L-aq
Gypsum  Ca+2 + SO4-2 + 2H2O
Kso = 10x
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
Solubility of
gypsum
Common Ion effect on Gypsum
solubility
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
Solubility of
gypsum
MgCl2
NaCl
Concentration