Complexometric Titration Concepts
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Questions and Answers

What type of ligand is EDTA classified as?

  • Tridentate
  • Hexadentate (correct)
  • Unidentate
  • Bidentate

    • The coordination number refers to the number of ligands attached to a central metal ion.

    True

    Name one factor that affects the stability of complexes.

    Acidity

    In complexometric titration, the metal ion acts as the _______ and the ligand acts as the _______.

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

    Match the following ligands with their type:

    <p>H2O = Unidentate EDTA = Hexadentate Ethylene Diamine = Bidentate Acetate = Unidentate</p> Signup and view all the answers

    Which of the following ions has a higher basicity?

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

    The size of the central metal ion does not influence the stability of a complex.

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

    What is a common application of complexmetric titration?

    <p>Determining metal ion concentrations</p> Signup and view all the answers

    Which of the following is NOT a reason why EDTA is considered the best titrant in complexometric titration?

    <p>It reacts with only certain metals.</p> Signup and view all the answers

    The pH at which Mg2+ is titrated with EDTA is 10.

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

    What is the stability constant (Kf) used in the titration example?

    <p>5.0 x 10^8</p> Signup and view all the answers

    At the equivalence point, the pMg of the solution is __________.

    <p>5</p> Signup and view all the answers

    What happens to the concentration of Mg2+ when 45.0 mL of EDTA is added?

    <p>It decreases to 0.0053 M.</p> Signup and view all the answers

    Match the pMg values with the corresponding volumes of EDTA added:

    <p>10.0 mL = 1.18 30.0 mL = 1.6 45.0 mL = 2.28 0.0 mL = 1.0</p> Signup and view all the answers

    What is the pH when H4Y reacts with water to form H3Y-?

    <p>2.7</p> Signup and view all the answers

    The titration curve does not alter with the value of the stability constant.

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

    Which metal ion forms a more stable complex with EDTA when compared to magnesium?

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

    At pH 10, EDTA forms complexes with both calcium and magnesium ions.

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

    What is the purpose of adjusting the pH when analyzing metal ion mixtures?

    <p>To control the stability of various metal-EDTA complexes.</p> Signup and view all the answers

    Ascorbic acid is used as a __________ to reduce Fe3+ to Fe2+ during metal ion analysis.

    <p>reducing agent</p> Signup and view all the answers

    Match the metal ion with its corresponding group in pH titration:

    <p>Ca2+ = Third group: pH=10 using ammonia buffer Cu2+ = Second group: pH 4-6 using acetate buffer Fe3+ = First group: pH 1-3 using conc.HNO3 Pb2+ = Second group: pH 4-6 using acetate buffer</p> Signup and view all the answers

    What color change occurs when titrating aluminum ions with Zn2+ using EBT indicator?

    <p>Blue to wine red</p> Signup and view all the answers

    Masking agents are used to enhance the selectivity of EDTA during complexometric titration.

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

    Which indicator is used to titrate Bi3+ ions at pH 2?

    <p>Xylenol orange</p> Signup and view all the answers

    What is the stability constant (kf) for the reaction involving Mg2+ and Y4-?

    <p>5.0 x 10^8</p> Signup and view all the answers

    The pH range for Eriochrome black T indicator is from 5.3 to 12.5.

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

    Name one characteristic that a metal ion indicator must have.

    <p>The indicator must be very sensitive to metal ions.</p> Signup and view all the answers

    In direct titration, the solution is buffered to the desired pH using ___ and ___.

    <p>NH4Cl, NH4OH</p> Signup and view all the answers

    What color does murexide exhibit at a pH of 10?

    <p>Yellow-orange</p> Signup and view all the answers

    Match the following types of EDTA titrations with their descriptions.

    <p>Direct titration = Metal ion solution buffered and titrated directly with EDTA Back-titration = Excess EDTA is added followed by titration to find unreacted EDTA Replacement titration = Metal ion replaces Mg2+ from the complex Indirect titration = Metal is determined by titrating a related species</p> Signup and view all the answers

    The color change from colorless to colored indicates the endpoint in complexometric titration.

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

    What is one requirement for a metal ion indicator in relation to the metal-EDTA complex?

    <p>The metal-indicator complex must be less stable than the metal-EDTA complex.</p> Signup and view all the answers

    Which of the following metal ions do not react with EDTA?

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

    Masking agents are used to enhance the reaction of metal ions with EDTA.

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

    What is the primary purpose of a demasking agent?

    <p>To release a metal ion from a masking agent.</p> Signup and view all the answers

    A solution is titrated to a blue color after adding ____ mL of 0.01 M EDTA solution in a sample fixed at pH 10.

    <p>30</p> Signup and view all the answers

    Match the following actions with their corresponding definitions:

    <p>Masking agents = Prevent certain ions from reacting with EDTA Demasking agents = Release masked metal ions pH adjustment = Control acidity for titration Complexation = Formation of stable metal complexes</p> Signup and view all the answers

    In the titration of a water sample containing Ca and Mg, what volume of EDTA is needed to titrate only Ca when the pH is fixed at 12?

    <p>28 mL</p> Signup and view all the answers

    The total hardness of water can be calculated based only on the volume of EDTA used in the titration.

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

    How would you calculate the concentration of Ni2+ in a paint sample using EDTA?

    <p>By subtracting the volume of EDTA consumed by Mg2+ from the total volume added to the sample and relating it to the concentration of Ni2+.</p> Signup and view all the answers

    Study Notes

    Complexometric Titration

    • Complexometric titration involves the formation of soluble complexes between metal ions (acceptors) and ligands (donors) to produce coordination compounds (complexes).
    • The metal ion is known as the central metal atom (M).
    • The anion or neutral molecule is known as the ligand (L).
    • The central metals must have small size, vacant orbitals, and increasing ability with increasing oxidation state..
    • Ligands (complexing agents) require high basicity and must contain at least one atom with lone pairs or a negative charge.
    • The number of coordination bonds around the central metal ion is called the coordination number.

    Types of Complexing Agents (Ligands)

    • Unidentate: These ligands (H₂O, NH₃, CN⁻, Cl⁻, I⁻, Br⁻) attach to the metal ion at one site.
    • Bidentate: These ligands (e.g., ethylenediamine) attach to the metal ion at two sites
    • Tridentate, tetradentate, etc.: These ligands attach to the metal ion at three or more sites, respectively. Chelating agents are examples of these types of ligands. (examples of chelating agents include EDTA, diethylene triamine, triethylenetetramine)

    Factors Affecting Complex Stability

    • Central Metal Ion: Ionic size (metal radius), ionic charge (metal charge), and acidity influence the stability.
    • Ligand: Basic character (N > O > S > I > Br > Cl > F−) enhances stability. The degree of chelation and steric effect also affect stability.

    EDTA Titration

    • EDTA (ethylenediaminetetraacetic acid) is a hexadentate ligand with 4 oxygen and 2 nitrogen donors.
    • EDTA is not a selective chelating agent
    • Titration involving EDTA is known as complexometric titration.
    • EDTA forms stoichiometric 1:1 complexes with all metals
    • EDTA has very stable complexes with all metals and has a sharp abrupt reaction.
    • The disodium salt of EDTA (Na₂H₂Y.2H₂O) is a primary standard.
    • The end point of the titration can be easily determined using metal ion indicators.

    Titration Curve

    • The titration curve changes with the stability constant as the titration is performed at a suitable pH.
    • The concentration of metal ions is calculated via pMg = −log([Mg]).

    Detection of End Point: Use of Metal Ion Indicators

    • The color reaction must be sensitive to metal ions and occur before the endpoint.
    • The metal-indicator complex must be less stable than the metal-EDTA complex.
    • There should be high color contrast between the free and complexed indicators.
    • The color reaction should be specific or at least selective.
    • The pH range at which the titration takes place determines the indicator's effectiveness.
    • Examples of metal ion indicators include Eriochrome Black T (H₂In⁻ to In³⁻), pH 5-7.3, red to blue. and Murexide.

    Types of EDTA Titration

    • Direct Titration: The metal ion solution is buffered to a desired pH and titrated directly in the presence of an indicator.
    • Back Titration: An excess of EDTA is added to the metal ion solution, and the unreacted EDTA is then titrated with a standard solution.
    • Replacement or Substitution Titration: A metal ion in a complex is replaced by another metal ion, which is then titrated.
    • Indirect Titration: The metal ion is reacted with another substance that is then titrated to determine the original metal ion.

    Examples

    • Water Hardness: EDTA is used to determine water hardness due to the presence of Ca²⁺ & Mg²⁺ salts
    • Aluminum Determination: Al³⁺ is heated with excess standard EDTA and then adjusted to pH 10 with an ammonia buffer. The residual EDTA is determined via titration with a standard Zn₂⁺ solution using EBT indicator.
    • Analysis of Metal Ion Mixtures (Selectivity Improvements): Selectivity is improved via pH control, adjusting oxidation numbers, masking and demasking agents, and selective metal indicators.
    • Analysis of Metal Ion Mixtures (Control of pH of the medium): Different metal ions are titrated in various pH ranges using various buffers (examples provided include different metal ions and relevant pH ranges)
    • Analysis of Metal Ion Mixtures (Masking and Demasking Agents): Masking agents prevent reaction of certain metal ions with EDTA, and demasking agents release the metal ion from the masking agent. (examples included titration of Ca and Mg in the presence of Zn or Cd)

    Additional Considerations

    • The provided notes outline a variety of complexometric titration applications and considerations for adjusting pH, controlling the reaction, determining the concentration of metal ions, and conducting analyses on mixtures of multiple metals. Using appropriate pH control with indicators, and understanding the various types of titrations, are important for determining endpoint and assessing accuracy.

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    Lecture 8 Analytical PDF

    Description

    Explore the essential concepts of complexometric titration, focusing on the formation of soluble complexes and the role of ligands. Understand the classification of different types of ligands and their coordination with metal ions. This quiz will test your knowledge of the principles behind these chemical interactions.

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