Complex Titrations - T3

Question 1

What is a complex Titration

Answer 1

Titration method based on complex formation due to reaction between metal ions (cation) and complexing agent (ligand)

Question 2

Explain complexation

Answer 2

• Metal Chelate Complexes
• Essentially, a Lewis acid-base reaction, in which an electron pair is donated from one chemical to another
• Ligand that attaches to a metal ion through more than one ligand atom
• Most chelating agents contain N or O
• Elements that contain free electron pairs that may be donated to a metal

Question 3

What are ligands

Answer 3

• Ligand (the donor species or complexing agents) is the groups bound to the central ion in the complex

Example of ligand: H2O, NH3, Cl-, Br-, EDTA

Question 4

Defibe a Chelate

Answer 4

A Chelate is produced when ligands are attached to the central ion at two or more coordinating sites to form a five-or six-member heterocyclic ring

Question 5

What are chelating agents

Answer 5

• The organic ligands that involved in the coordination
• Multidentate ligands
• Most widely used chelating agent = EDTA

Question 6

Explain the multidentatae

Answer 6

1. Unidentate– A ligand that is attached to a central ion at only one point (1 binding site)
   E.g. Ammonia
2. Bidentate– A ligand that is attached to a central ion at two points due to two lone pair electrons (2 binding sites)
   E.g. Ethylene diamine
3. Poly/ Multidentate– A ligand that has two or
   more coordinating sites (more than two binding sites)

Question 7

Expalain the Formation Constant (Kf)

Answer 7

• The equilibrium constant for the reaction between a metal ion (Mn+) and a chelating agent (L-P) is known as a formation constant or stability constant or binding constant
• It is a measure of the strength of the interaction between the reagents that come together to form the complex
• A metal ion, M reacts with a ligand, L and forming a complex, ML in complexation reaction

M + L ⇌ ML

• Complexation reactions occur in a stepwise fashion followed by additional reactions:

ML + L ⇌ ML2

ML2 + L ⇌ ML3

MLn-1 + L ⇌ MLn

Question 8

Explain polydentate ligands

Answer 8

• React more completely with cations compared to ligands with a lesser number of donor groups and tend to form 1:1 complexes
• These ligands are more satisfacory

Question 9

Explain chelate effect

Answer 9

• Multidentate complexes are more stable because of the entropy effect

dG = dH - TdS where G = free energy

H = enthalpy
S = entropy

• When M-EDTA tetradentate (4 binding sites) complex dissociates, there would be only 2 products = M+ and EDTA ligand. (low entropy).
• Whereas when a ML4 complex dissociates there will be 5 products = M + 4L (higher entropy)

Question 10

What happens as you remove a hydrogen atom and Y atom

Answer 10

Ka increases

Question 11

Explain EDTA Titration curve and its 3 regions

Answer 11

1. Reg. 1

• Before equivalence point
• Mn+ : excess, unreacted
• MYn-4 : dissociation - negligible

2. Reg 2

• At the equivalence point
• MYn-4 : dissolves some some free Mn+,[Mn+]=[EDTA]
• (MYn-4 ==> Mn+ + EDTA)

3. Reg 3

• After equivalence point.
• EDTA in excess

Question 12

What are we able to measure in EDTA Titration curves

Answer 12

• We are able to measure the concentration of free ion during titration with EDTA
• The titration of a metal ion with EDTA is similar to the titration of a strong acid (M+) with a weak base (EDTA)

Question 13

pH Limitations go EDTA Titration curves

Answer 13

• The metal-EDTA complex becomes less stable as pH decreases thus Kf decreases
• [Fe3+] = 5.4 x 10-7 at pH 2.0 ===> [Fe3+] =1.4 x 10-12 at pH 8 therefore, more uncomplexed
  iron at lower pH
• In order to get a “complete” titration (Kf ≥106), EDTA requires a certain minimum pH for the titration of each metal ion
• End Point becomes less distinct as pH is lowered, limiting the utility of EDTA as a titrant

Question 14

By adjusting the pH of an EDTA
titration…

Answer 14

One type of metal ion (e.g. Fe3+) can be
titrated without interference from others (e.g.
Ca2+)

Question 15

Explain fraction and EDTA Titrations

Answer 15

• Because EDTA has many forms, when we prepare a solution of EDTA we only know its total concentration, not the concentration of the specific form, such as Y4−
• At any pH, a mass balance of EDTA can be performed

CEDTA = [H6Y2+] + [H5Y+] + [H4Y] + [H3Y−]+[H2Y2−] + [HY3−] + [Y4-]

All 7 forms of EDTA

Question 16

Explain EDTA

Answer 16

• (Ethylenediaminetetraacetic acid)
• One of the most common chelating agents used for complexometric titrations in analytical chemistry
• EDTA has 6 nitrogens & oxygens (4 carboxyl & 2 amine groups) in its structure giving it 6 free electron pairs that it can donate to metal ions
• HEXADENTAT
• High Kf values
• 6 acid-base sites in its structure

Question 17

Explain Acid-Base Forms of EDTA Titrations

Answer 17

• EDTA exists in up to 7 different acid-base forms depending on the solution pH
• In practice, EDTA only partially ionizes therefore < 6 coordinate covalent bonds form
• In pH <= 12, amine groups remain protonated therefore do not donate electrons
• The most basic form (Y4-) is the one which primarily reacts with metal ions. – 4 donated electron pairs

Question 18

EDTA Titrations

Answer 18

• EDTA forms an octahedral complex with most 2+ metal cations, M2+, in aqueous solution
• The main reason that EDTA is used so extensively in the standardization of metal
  cation solutions is that the formation constant for most metal cation-EDTA complexes
  is very high, meaning that the equilibrium for the reaction:

M2+ + H4Y → MH2Y + 2H+
lies far to the right

• Carrying out the reaction in a basic buffer solution removes H+ as it is formed, which also favors the formation of the EDTA-metal cation complex reaction product
• For most purposes it can be considered that the formation of the metal cation-EDTA complex goes to completion, and this is chiefly why EDTA is used in titrations / standardizations of this type