Complexometry in Quantitative Analysis
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Questions and Answers

What is the characteristic of EDTA as a chelating agent?

  • Tetradentate and non-selective
  • Selective
  • Hexadentate and selective
  • Non-selective (correct)

    • What is the ratio of EDTA to metal ions in a complex?

  • 1:1 (correct)
  • 1:2
  • 2:1
  • 3:1

    • What happens to the stability of metal-EDTA complex in acidic medium?

  • It remains the same
  • It increases
  • It decreases (correct)
  • It is unaffected

    • What is the characteristic of metal indicators in complexation reactions?

    <p>Reversible reaction</p> Signup and view all the answers

    What is the condition for the reaction to shift forward in EDTA complexation?

    <p>Slightly alkaline solution</p> Signup and view all the answers

    What type of ligand is EDTA?

    <p>Hexadentate</p> Signup and view all the answers

    What is the purpose of metal indicators in EDTA complexation?

    <p>To determine the endpoint of the reaction</p> Signup and view all the answers

    What happens to the metal in strong alkaline medium?

    <p>It precipitates as hydroxide</p> Signup and view all the answers

    What is the characteristic of the complex formed between metal ions and metal indicators?

    <p>Lower stability than metal-EDTA complex</p> Signup and view all the answers

    What is the color of the free form of murexide in a pH range of 9-11?

    <p>Violet</p> Signup and view all the answers

    Study Notes

    Complexometry

    • A type of quantitative analysis where a compound forms a slightly ionized complex with a metal ion (Mn+) and an anion or neutral molecule (L)
    • The metal ion is the central metal atom, acting as a Lewis acid (electron acceptor), while the anion or neutral molecule is the ligand (L), acting as a Lewis base (electron donor)
    • The bond between Mn+ and L is a coordinate bond or dative bond

    Coordinate Bond

    • Similar to a covalent bond, but with the electrons donated from one atom to another
    • Represented by an arrow (→) from the donor to the acceptor

    Coordination Number

    • The number of coordinate bonds formed between the central metal and ligand ion
    • Characteristics:
      • Always an even number (2, 4, 6, ...)
      • Usually double the charge of the metal ion (e.g., Ag+ forms 2 coordinate bonds, Ni2+, Cu2+ form 4 coordinate bonds)
      • Higher valence metal ions form more stable complexes

    Ligands

    • Unidentate Ligands: attached to the metal at one site (e.g., I-, CN-, F-, H2O, NH3)
    • Polydentate Ligands:
      • Bidentate Ligands: attached to the metal at two sites (e.g., ethylene diamine)
      • Tridentate Ligands: attached to the metal at three sites (e.g., diethylene triamine)
      • Tetradentate Ligands: attached to the metal at four sites (e.g., triethylene tetramine)

    Chelation

    • A type of complexation where a ligand possesses two or more donor groups (polydentate)
    • Forms a ring structure compound called a metal chelate
    • Polydentate ligands are preferred as titrants due to their advantages:
      • Formation of more stable complexes due to multiple points of attachment
      • Complete reaction in one step, leading to a sharp end point

    Factors Affecting Stability of the Complex

    Metal Factors

    • Type of metal: metals with incomplete subshells (e.g., Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+) are strongest in attracting ligands and forming stable complexes
    • Ionic radius: smaller ionic radius increases the stability of the complex
    • Electric charge: higher electric charge carried by the metal ion produces stable complexes (e.g., Fe3+ > Fe2+)

    Ligand Factors

    • Basic character (ability of electron donation) of the ligand increases the stability of the complex
    • Ionic radius: larger ionic radius of the ligand increases the stability of the complex

    EDTA (Ethylene Diamine Tetracetic Acid)

    • An amino polycarboxylic acid compound used as a ligand, complexing agent, or titrant for many metal ions
    • Hexadentate ligand containing 4 oxygen and 2 nitrogen donors
    • Reacts with metal ions in a 1:1 ratio
    • Disodium salt of EDTA is freely soluble in water
    • Non-selective chelating agent
    • Stability of metal-EDTA complex affected by pH:
      • Decreases in acidic medium
      • Increases in slightly alkaline solution
      • Metal precipitates as hydroxide in strong alkaline medium

    Detection of End Point

    • Methods:
      • Metal indicators (metallochromic indicators)
      • Acid-base indicators
      • Specific indicators
      • Turbidity indicators
      • Instrumental methods

    Metal Indicators (metallochromic indicators)

    • Organic dyes that form colored complexes with metal ions
    • Characteristics:
      • Complexation reaction between metal ions and indicators is reversible
      • Color of free form differs from the color of the complexed form
      • Stability of metal-indicator complex is lower than metal-EDTA complex
    • Examples:
      • Murexide
      • Eriochrome Black T (EBT)

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    Description

    Complexometry is a type of quantitative analysis where a metal ion reacts with an anion or neutral molecule to form a complex. Learn about the role of metal ions, Ligands, and Lewis acids and bases in this process.

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