Complexometric Titration and Ligands

Complexometric Titration

• Complexometry is the formation of slightly ionized complexes in solution
• A complex is formed by a reaction of a metal ion (electron acceptor) and a ligand (electron donor)
• Chelating agents form strong 1:1 complexes with metal ions
• A chelating agent is used to titrate metal ions in solution, known as complexometric titration.
• Different titration types use different electron donors and acceptors.

Ligands

• Ligands are molecules that can form coordinate bonds with metal ions
• Unidentate ligands have one lone pair
  • Examples include NH3, Cl-, Br-
• Bidentate ligands have two lone pairs
  • Examples include ethylene diamine
• Multidentate ligands have more than two lone pairs
  • Examples include EDTA (ethylenediaminetetraacetic acid)
• Multidentate ligands create more stable complexes.

Multidentate Ligands

• Stability increases with the use of multidentate ligands
• Ethylenediamine forms a stable complex with cadmium with a very large formation constant
• Other multidentate ligands such as methylamine also form complexes with various metals.

Ethylenediaminetetraacetic Acid (EDTA)

• EDTA forms very stable complexes with many metal ions. This is a "cage" around the metal ions
• It's a hexadentate ligand, meaning it has six donor atoms that can coordinate with metal ions.
• The donor atoms are from two amine groups and four carboxylate groups.
• EDTA can exist in different protonated forms, and its ability to form complexes is greatly affected by the pH

EDTA (Properties of pK's)

• EDTA is a hexaprotic weak acid
• Different forms of EDTA exist in solutions
  • Various protonated and deprotonated forms, i.e., Y4-, HY3-, H2Y2-, etc.
  • The fraction of each form depends on the pH
• Specific pKa values for the different protonations of EDTA

Formation Constant (Kf)

• Kf is the equilibrium constant for the formation of a metal-EDTA complex.
• The higher the value of Kf, the more stable the complex.
• For a +1 cation—Ag⁺ + Y^4- → AgY^3-
• For a +2 cation—Hg²⁺ + Y^4- → HgY^2-
• For a +3 cation—Fe³⁺ + Y^4- → FeY^-
• For a +n cation—Mnⁿ⁺ + Y^4- → MY^n-4
  • Kf= [MY^n-4]/[Mⁿ⁺][Y^4-]

Table of Formation Constants

• A table showing log K_f values of different metal ions with EDTA
  • These values represent the stability of the complexes

Conditional Formation Constant

• Most EDTA is not in Y^4- form below pH 10.37
• K_f treats free EDTA in one form for complex formation
• K_f' = K_f α_Y4- = [MY^n-4]/[Mⁿ⁺][EDTA]
• α_Y4- accounts for the different forms of EDTA at varying pH values.

Important Features of EDTA

• Formed complexes are stable and water-soluble
• Instantaneous complex formation with metals in 1:1 ratio.
• Equivalence point is easily measured.
• Complex formation or dissociation is affected by the pH of the solution.

Metal Indicators

• Organic dyes that form chelates with metal ions
• Free form of the indicator and metal-indicator complex have different colors
• The reaction between the metal ion and indicator is reversible
• Indicator complex is less stable than the metal-EDTA complex
• The color change depends on the pH

Eriochrome Black T (EBT)

• EBT is blue in free form, and forms a wine-red complex with metals at neutral pH
• Commonly used for Mg²⁺, Pb²⁺, and Zn²⁺
• Can't be used to determine Cu²⁺, Fe²⁺, Al³⁺, Co²⁺, or Ni²⁺

Direct Titration

• Similar to acid-base titrations
• M-EDTA complex must be more stable than the M-indicator complex
• Reaction must happen in a buffered medium
• Metal precipitation must be catalyzed for a rapid reaction. Auxiliary reagents (ammonia, citric or tartaric acid) are used to prevent precipitation

Back Titration

• Known excess of EDTA is added
• The excess EDTA is then titrated with a standard solution of a second metal ion.
• Three situations for back titration:
  • Analyte precipitates in the absence of EDTA
  • Analyte reacts too slowly with EDTA
  • Analyte blocks the indicator

EDTA Titration of Mixtures

• EDTA is not selective
• EDTA selectivity can be increased by controlling the pH and the oxidation state of the metal ions, by using masking and demasking agents.

pH Effect

• Log K_f values for metal-EDTA complexes vary with the pH.
  • The minimum pH for various metals to form complexes needs to be considered.

Control pH of Medium

• Trivalent / tetravalent cations (Fe³⁺, Th⁴⁺, Bi³⁺) and Hg²⁺ form stable complexes at low pH (1-3), while divalent metals (Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺) form stable complexes at moderate pH (4-6) and alkaline earth metals (Ba²⁺, Ca²⁺, Sr²⁺) and Mg²⁺ are suitable at pH 10 using ammonia buffer.

Adjust Oxidation Number of Metal Ion

• Interference between metal ions of the same pH group is solved by adjusting the oxidation number of different metal ions. Ascorbic acid can be used as a reducing agent to reduce these metal ions.

Masking and Demasking Agents

• Masking agents are used to prevent interference by other metal ions
• They strongly form metal complexes
• Demasking agents break down complexes formed by masking agents. Cyanide complexes of Cd, Cu, Fe, Zn can be broken by formaldehyde in acetic acid

Masking Agent Table

• A table showing masking agents and the species they can mask.

This quiz explores the principles of complexometric titration, focusing on the formation of complexes between metal ions and ligands. It covers the types of ligands including unidentate, bidentate, and multidentate, as well as their roles in stabilizing metal complexes. Test your understanding of these concepts and their applications in analytical chemistry.