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Lecture 17 - Open Systems (Solid/Liquid Equilibrium). Exam #3 (Dec 5 @ 1:30pm) Precipitation & Dissolution Solubility product (Ks0) Estimating solubility From Ks0 - ignoring complex formation Effects of complexation pC-pH diagram (estimate max/min solubility).

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lecture 17 open systems solid liquid equilibrium
Lecture 17 - Open Systems (Solid/Liquid Equilibrium)
  • Exam #3 (Dec 5 @ 1:30pm)
  • Precipitation & Dissolution
      • Solubility product (Ks0)
        • Estimating solubility
      • From Ks0 - ignoring complex formation
      • Effects of complexation
      • pC-pH diagram (estimate max/min solubility)
example ferric iron solubility
Example - Ferric iron solubility

Fe(OH)3 = Fe3+ + 3OH- log Kso = -38.8

metal solubility effect of complexes
Metal Solubility - Effect of Complexes
  • Metal cations exist in solution as species other than the aquo ion
  • These complexes impact metal solubility substantially and must be accounted for

Example - Ferric Iron

fe iii hydrolysis
Fe(III) Hydrolysis*

(A) Fe3+ + OH- = FeOH2+ logK1 = 11.81

(B) Fe3+ + 2OH- = Fe(OH)2+ logb2 = 23.4

(C) Fe3+ + 4OH- = Fe(OH)4- logb4 = 34.4

OR

(A*) Fe3+ + H2O = FeOH2+ + H+ logK1* = -2.19

(B*) Fe3+ + 2H2O = Fe(OH)2+ + 2H+ logb2* = -4.60

(C*) Fe3+ + 4H2O = Fe(OH)4- + 4H+ logb4* = -21.6

system for fe iii solubility hydroxo complexes
System for Fe(III) solubility & hydroxo complexes
  • System: Open
  • Components: Fe3+ & H2O
  • Species: Fe3+, FeOH2+, Fe(OH)2+, Fe(OH)3(s), Fe(OH)4-, H2O, OH-, H+
  • Equilibria:

H2O = H+ + OH- Kw = [H+][OH-] = 10-14

(plus 4 reactions for Fe-hydoxo & Fe-solubility)

  • Mass Balance:

Don't need!

  • Charge Balance:

Don't need!

combine solubility and complex formation equilibria
Combine solubility and complex formation equilibria
  • Fe3+/Fe(OH)3(s)

Fe(OH)3 + 3H+ = Fe3+ + 3H2O log Ks = 3.20

  • FeOH2+/Fe(OH)3(s)

Fe(OH)3 + 2H+ = FeOH2++ 2H2O logKs1 = 1.01

  • Fe(OH)2+/Fe(OH)3(s)

Fe(OH)3 + H+ = Fe(OH)2++ H2O logKs2 = -1.40

  • Fe(OH)4-/Fe(OH)3(s)

Fe(OH)3 + H2O = Fe(OH)4- + H+ logKs4 = -18.40

fe iii solubility
Fe(III) - solubility

Fe(OH)3(s)

FeOH2+

Fe(OH)4-

Fe3+

Fe(OH)2+

so what
So What?
  • Complexes increase mineral solubility
  • Diagram gives pH of min or max solubility
    • What are these for ferric iron?
  • Many other ligands form solids (e.g., CO32-)
  • For systems dominated by a particular solid we can estimate total metal concentration and dominant species.
carbonate system mixed open system
Carbonate System - Mixed Open System
  • System: Open
  • Components: Ca2+, CO2(g) & H2O
  • Species: H2CO3, HCO3-,CO32-, CO2(g), H2O, OH-, H+, CaCO3, Ca2+
  • Equilibria:

H2O = H+ + OH- Kw = [H+][OH-] = 10-14

H2CO3 = H+ + HCO3- KA1 = [H+][HCO3-]/[H2CO3] = 10-6.35

HCO3- = H+ + CO32- KA2 = [H+][CO32-]/[HCO3-] = 10-10.33

CO2(g) = CO2(l) KH = [CO2(l)]/PCO2 = 10-1.5 (mol/L-atm)

CaCO3 = Ca2+ + CO32- Kso = 10-8.48

  • Mass Balance: CT = [H2CO3] + [HCO3-] +[CO32-]

CaT = [Ca2+]

  • Charge Balance: 2[CO32-] + [HCO3-] + [OH-] = [H+] + 2[Ca+]
  • Proton Balance: Invalid because CO2 is proton active!!
what do we know
What do we know?
  • Our carbonate species in open systems
  • Total Ca based on solid/liquid
final equation
Final Equation
  • Substitute in charge balance equation
  • Assume water pH near neutral, thus [H+], [OH-] & [CO32-] are negligible.
  • Solve for [H+] to get pH 8.24
importance of mineral dissolution precipitation
Importance of mineral dissolution/precipitation
  • Natural Waters
    • Dictate major cation concentrations (e.g., Ca2+, Mg2+)
    • Dictate carbonate system (particularly in g.w.)
  • Treatment systems
    • Hardness removal
    • Iron removal by aeration
    • Phosphate removal
  • Polluted waters
    • Acid mine drainage results from pyrite dissolution
importance of mineral dissolution precipitation16
Importance of mineral dissolution/precipitation
  • Typically non-equilibrium processes (due to slow kinetics), but we can use equilibrium approaches because
    • Often one solid phase controls solution composition
    • The optimum pH for mineral precipitation dictated by equilibrium solubility constant
solubility
Solubility
  • Described like all other reactions

AaBb(s) = aA + bB Ks

  • Ks termed solubility product
    • For uncomplexed ligand we get Ks

CaCO3(s) = Ca2+ + CO32- Kso

    • For complexed ligand we use Ks

CaCO3(s) + H+ = Ca2+ + HCO3- Ks