EDTA Titrations v2

  • EDTA Titrations

  • Chelation in Biochemistry

  • Metal-Chelate Complexes

  • Metals are Lewis acids that accept electron pairs from donating ligands that act as Lewis bases

–CN- is a common monodentate ligand, binding to a metal ion through one atom (C)

–Metals can bind to multiple ligands (usually 6)

  • A ligand that can attach to a metal by more than one atom is multidentate or a chelating ligand

  • Chelating agents can be used for titration of metals to form complex ions (complexometric titration)

  • Chelating Agents in Analytical Chemistry

  • Ethylenediamenetetraacetic acid (EDTA)

  • Acid/Base Properties of EDTA

  • EDTA is a hexaprotic system (H6Y2+) with 4 carboxylic acids and 2 ammoniums:

  • We usually express the equilibrium for the formation of complex ion in terms of the Y4- form (all six protons dissociated). You should not take this to mean that only the Y4- form reacts

  • Fraction of EDTA in Y4- Form

  • Similar to acids and bases, we can define fractional compositions, α, defined as the fraction of “free” EDTA in a particular form.

–“Free” means uncomplexed EDTA

–So, for Y4-:

  • EDTA Complexes

  • The equilibrium constant for a reaction of metal with EDTA is called the formation constant, Kf, or the stability constant:

  • Again, Kf could have been defined for any form of EDTA, it should not be understood that only the Y4- reacts to form complex ion.

  • pH Dependence of αY4-

  • Formation Constants for M-EDTA Complexes

  • Some Metals Form 7 or 8 Coordinate Complexes

  • Conditional Formation Constant

  • We saw from the fraction plot that most of the EDTA is not in the form of Y4- below a pH ~10.

  • We can derive a more useful equilibrium equation by rearranging the fraction relationship:

  • If we fix the pH of the titration with a buffer, then αY4- is a constant that can be combined with Kf

  • Example

  • Calculate the concentration of free Ca2+ in a solution of 0.10 M CaY2- at pH 10 and pH 6. Kf for CaY2- is 4.9×1010 (Table 13-2)

  • At low pH, the metal-complex is less stable

  • Calcium/EDTA Titration Curve

  • Generic Titration Curve

  • Before the Equivalence Point

  • What’s pCa2+ when we have added 5.0 mL of EDTA?

  • At the Equivalence Point

  • What’s pCa2+ when we have added 25.0 mL of EDTA?

–At the equivalence point almost all the metal is in the form CaY2-

–Free Calcium is small and can be found w/ algebra

  • After the Equivalence Point

  • What’s pCa2+ when we have added 26.0 mL of EDTA?

–We have 1.0 mL excess EDTA

  • Auxiliary Complexing Agents

  • In aqueous solution, metal-hydroxide complexes or precipitates can form, especially at alkaline pH

  • We often have to use an auxiliary complexing agent

–This is a ligand that binds strongly enough to the metal to prevent hydroxide precipitation, but weak enough to be displaced by EDTA

  • Ammonia is a common auxiliary complex for transition metals like zinc

  • Metal Ion Indicators

  • To detect the end point of EDTA titrations, we usually use a metal ion indicator or an ion-selective electrode (Ch. 15)

  • Metal ion indicators change color when the metal ion is bound to EDTA:

–Eriochrome black T is an organic ion

  • The indicator must bind less strongly than EDTA

  • Metal Ion Indicator Compounds

  • EDTA Titration Techniques

  • Direct titration: analyte is titrated with standard EDTA with solution buffered at a pH where Kf is large

  • Back titration: known excess of EDTA is added to analyte. Excess EDTA is titrated with 2nd metal ion.

  • EDTA Titration Techniques (2)

  • Displacement titration: For metals without a good indicator ion, the analyte can be treated with excess Mg(EDTA)2-. The analyte displaces Mg, and than Mg can be titrated with standard EDTA

  • Indirect titration: Anions can be analyzed by precipitation with excess metal ion and then titration of the metal in the dissolved precipitate with EDTA.

  • Example Titration

  • 25.0 mL of an unknown Ni2+ solution was treated with 25.00 mL of 0.05283 M Na2EDTA. The pH of the solution was buffered to 5.5 and than back-titrated with 17.61 mL of 0.02299 M Zn2+. What was the unknown Ni2+ M?

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