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221-chapter-12 - EDTA Titrations Introduction 1 Metal...

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EDTA Titrations Introduction 1.) Metal Chelate Complexes Any reagent which reacts with an analyte in a known ratio and with a large equilibrium constant can potentially be used in a titration. Complexation Titrations are based on the reaction of a metal ion with a chemical agent to form a metal-ligand complex. Metal Ligand Metal-Ligand Complex Metal Lewis Acid or Electron-pair acceptor Ligand – Lewis Base or Electron-pair donor Note: multiple atoms from EDTA are binding Mn 2+
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EDTA Titrations Introduction 1.) Metal Chelate Complexes Complexation Titrations are essentially a Lewis acid-base reaction, in which an electron pair is donated from one chemical to another The ligands used in complexometric titrations are also known as chelating agents. - 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 Fe-DTPA Complex
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EDTA Titrations Metal Chelation in Nature 1.) Potassium Ion Channels in Cell Membranes Electrical signals are essential for life Electrical signals are highly controlled by the selective passage of ions across cellular membranes - Ion channels control this function - Potassium ion channels are the largest and most diverse group - Used in brain, heart and nervous system Current Opinion in Structural Biology 2001, 11:408–414 Opening of potassium channel allows K + to exit cell and change the electrical potential across membrane K channel spans membrane channel contains pore that only allows + + http://www.bimcore.emory.edu/home/molmod/Wthiel/Kchannel.html
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EDTA Titrations Metal –Chelate Complexes K f = 8x10 9 K f = 4x10 9 2 ethylenediamine molecules binds tighter than 4 methylamine molecules
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EDTA Titrations Metal –Chelate Complexes 2.) Chelate Effect Usually chelating agents with more than one electron pair to donate will form stronger complexes with metal ions than chelating agents with only one electron pair. - Typically more than one O or N - Larger K f values Multidentate ligand : a chelating agent with more than one free electron pair - Stoichiometry is 1:1 regardless of the ion charge Monodentate ligand : a chelating agent with only one pair of free electrons Multidentate ligand that binds radioactive metal attached to monoclonal antibody (mAb). mAb is a protein that binds to a specific feature on a tumor cell delivering toxic dose of radiation.
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EDTA Titrations EDTA 1.) EDTA (E thylened iaminet etraa cetic acid) One of the most common chelating agents used for complexometric titrations in analytical chemistry. EDTA has 6 nitrogens & oxygens in its structure giving it 6 free electron pairs that it can donate to metal ions. - High K f values - 6 acid-base sites in its structure
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EDTA Titrations EDTA 2.) Acid-Base Forms EDTA exists in up to 7 different acid-base forms depending on the solution pH.
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