Speciation Any element exists in a solution, solid, or gas as 1 to n ions, molecules, or solids Example: Ca 2+ can exist in solution as: Ca ++ CaCl + CaNO.

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Presentation transcript:

Speciation Any element exists in a solution, solid, or gas as 1 to n ions, molecules, or solids Example: Ca 2+ can exist in solution as: Ca ++ CaCl + CaNO 3 + Ca(H 3 SiO 4 ) 2 CaF + CaOH + Ca(O-phth) CaH 2 SiO 4 CaPO 4 - CaB(OH) 4 + CaH 3 SiO 4 + CaSO 4 CaCH 3 COO + CaHCO 3 + CaHPO 4 0 CaCO 3 0 Plus more species  gases and minerals!!

How do we know about all those species?? Based on complexation  how any ion interacts with another ion to form a molecule, or complex (many of these are still in solution) Yet we do not measure how much CaNO 3 +, CaF +, or CaPO 4 - there is in a particular water sample We measure Ca 2+  But is that Ca 2+ really how the Ca exists in a water??

Aqueous Complexes Why do we care?? 1.Complexation of an ion also occurring in a mineral increases solubility 2.Some elements occur as complexes more commonly than as free ions 3.Adsorption of elements greatly determined by the complex it resides in 4.Toxicity/ bioavailability of elements depends on the complexation

Defining Complexes Use equilibrium expressions: cC + lHL  CL + lH+ Where B is just like K eq !

Mass Action & Mass Balance mCa 2+ =mCa 2+ +MCaCl + + mCaCl CaCL CaHCO CaCO CaF + + CaSO CaHSO CaOH + +… Final equation to solve the problem sees the mass action for each complex substituted into the mass balance equation

Mineral dissolution/precipitation To determine whether or not a water is saturated with an aluminosilicate such as K-feldspar, we could write a dissolution reaction such as: KAlSi 3 O 8 + 4H + + 4H 2 O  K + + Al H 4 SiO 4 0 We could then determine the equilibrium constant: from Gibbs free energies of formation. The IAP could then be determined from a water analysis, and the saturation index calculated.