Solubility Dependence on pH

1
Solubility Dependence on pH
Salts of basic ions (F-, OH-, S2-, CO32-, C2O42-,
PO43-) have higher solubility at low pH because
of connected equilibria.
2
pH Effect on Solubility
pH can also have an effect on the solubility of
minerals. Metals such as aluminum or lead can be
freed from insoluble forms with a low enough pH.
3
Example-Barium Oxalate

• Find the composition of a solution containing
  barium oxalate.
• Step 1Write pertinent reactions
• Step 2 Charge balance
• Step 3 Mass balance

4
Example-Barium Oxalate (cont)

• Step 4 Equilibrium constants
• Step 5 Count equations and unknowns
• Equations Charge balance, mass balance, 4 K
  equations
• Unknowns H, Ba2, C2O42-, HC2O4-,
  H2C2O4, OH-
• Step 6 Solve
• 6 equations/6 unknowns
• Or, if youre going to fix the pH at a particular
  value, then you should express all of the
  chemical constituents as a function of H or OH-

5
Example-Barium Oxalate (cont)
6
pH Effect on Barium Oxalate
7
Solving With a Fixed pH

• Step 1 Write equations systematically as normal
• Step 2 Express concentration of each protonated
  form HnA of anion in terms of A
• Step 3 Substitute expressions for HnA into the
  mass balance equation to give a relation between
  M and A
• Step 4 Put the M/A relation into Ksp to
  solve for M
• Step 5 With M and OH- solve for other anions

8
Keep in Mind

• In solving the equilibrium system, we are limited
  to the reactions we have knowledge of.
• In the previous example Ba2 reacts with OH- to
  form BaOH in solution
• At best then, our answers are an approximation if
  we dont have the equilibrium information for
  other reactions