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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 13

ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 13. DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university. CHAPTER 13 EDTA COMPLEXES. METAL-CHELATE COMPLEXES. Ligand - An atom or group of atoms bound to metal ions to form complexes

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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 13

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  1. ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 13 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

  2. CHAPTER 13 EDTA COMPLEXES

  3. METAL-CHELATE COMPLEXES Ligand - An atom or group of atoms bound to metal ions to form complexes Monodentate Ligand - Binds to metal ions through only one ligand atom [cyanide (CN-) binds through only carbon] Multidentate (Chelating) Ligand - Binds to metal ions through more than one ligand atom [EDTA is hexadentate (binds through two N and four O atoms)]

  4. METAL-CHELATE COMPLEXES - Most transition metal ions bind to six ligands (Mn2+, Co2+, Ni2+) - Proteins act as chelating ligands for ions passing through ion channels in cell membranes (nerves) Metal chelate complexes are important in medicine - Synthetic ligands as anticancer agents - Chelation therapy is used to enhance iron excretion which reduces heart and liver diseases - Chelation therapy for mercury and lead poisoning

  5. METAL-CHELATE COMPLEXES Synthetic Aminocarboxylic Acid Chelating Ligands Ethylenediaminetetraacetic acid (EDTA) Trans-1,2-diaminocyclohexanetetraacetic acid (DCTA) Diethylenetriaminepentaacetic acid (DTPA) Bis(aminoethyl)glycolether-N,N,N´,N´-tetraacetic acid (EGTA) - Form 1:1 complexes with metal ions (but not with monodentate ions like Li+, Na+, K+)

  6. EDTA - Ethylenediaminetetraacetic acid [CH2N(CH2CO2H)2]2 (C10H16N2O8, 292.24 g/mol) Density = 0.86 g/cm3 Melting point is about 240 oC - Most widely used chelate in analytical chemistry - Colorless and water-soluble - Strong metal binding agent (chelating agent) - Forms 1:1 complexes with most metal ions which remain in solution with diminished reactivity

  7. EDTA It is hexaprotic in the form H6Y2+ HO2CH2C CH2CO2H + + HNCH2CH2NH HO2CH2C CH2CO2H

  8. EDTA - Six pKa values - First four apply to carboxyl protons (COOH) - Next two apply to ammonium protons (NH+) pKa1 = 0.0 (CO2H) pKa2 = 1.5 (CO2H) pKa3 = 2.00 (CO2H) pKa4 = 2.69 (CO2H) pKa5 = 6.13 (NH+) pKa6 = 10.37 (NH+)

  9. EDTA - Neutral EDTA is tetraprotic in the form H4Y - Protonated below pH of 10.24 - Fully protonated form H6Y2+ predominates at very low pH - Fully deprotonated form Y4- predominates at very high pH - Y4- is the ligand form that binds to metal ions - Common reagent found in labs is the disodium salt (Na2H2Y·2H2O)

  10. EDTA Synthesis - Previously formed from ethylenediamine (1,2-diaminoethane) and chloroacetic acid - Currently formed from ethelynediamine methanal (formaldehyde) and sodium cyanide

  11. EDTA Uses - Food additives (preservatives), soaps, cleaning agents, - Hardwater and wastewater treatment - Textile industry, pulp and paper industry

  12. EDTA Complexometric Titration - Titration based on complex formation Formation constant (stability constant) - Equilibrium constant for complex formation (Kf) Mn+ + Y4-↔ MYn-4 - EDTA complexes have large Kf values - Higher for more positively charged metal ions

  13. EDTA - Metal-EDTA complex is unstable at very low pH - H+ competes with metal ion for EDTA - Metal-EDTA complex is unstable at very high pH - OH- competes with EDTA for metal ion - Unreactive hydroxide complexes may form - Metal hydroxide may precipitate

  14. EDTA Use of Auxilliary Complexing Agent (ACA) - Prevents metal ion from precipitating in the hydroxide form - Forms weak complex with metal ion - Displaced by EDTA during titration Examples Ascorbate Citrate Tartrate Ammonia triethanolamine

  15. EDTA Examples - Titration of Ca2+ and Mg2+ at pH 10 Ascorbic acid (ascorbate) as ACA - Titration of Pb2+ at pH 10 Tartaric acid (tartrate) as ACA

  16. METAL ION INDICATORS - A compound that changes color upon binding to a metal ion - Binds to metal ion less strongly than EDTA - Must readily give up its metal ion to EDTA - Metal ion is said to block indicator if it is not readily given up Two Common Indicators Calmagite: from red/blue/orange to wine red Xylenol orange: from yellow/violet to red Cu2+, Ni2+, Fe3+, Al3+, Cr3+, Co2+ block calagmite

  17. EDTA TITRATIONS Direct Titration - Analyte is titrated with standard EDTA - Analyte is buffered to an appropriate pH where reaction with EDTA is complete - ACA may be required to prevent metal hydroxide precipitation in the absence of EDTA

  18. EDTA TITRATIONS Back Titration Necessary under three conditions - If analyte blocks the indicator - If analyte precipitates in the absence of EDTA - If analyte reacts too slowly with EDTA - A known excess EDTA is added to analyte - Excess EDTA is titrated with a standard solution of a metal ion (metal must not displace analyte from EDTA)

  19. EDTA TITRATIONS Displacement Titration - There is no satisfactory indicator for some metal ions - Analyte is treated with excess Mg(EDTA)2- to displace Mg2+ Mn+ + MgY2-→ MYn-4 + Mg2+ - Mg2+ is titrated with standard EDTA An example is Hg2+ For displacement to occur Kf of HgY2- must be greater than Kf of MgY2-

  20. EDTA TITRATIONS Indirect Titration - Used to analyze anions that precipitate metal ions CO32-, CrO42-, S2-, SO42- - Anion is precipitated with excess metal ion - Precipitate is filtered and washed - Excess metal ion in filtrate is titrated with EDTA

  21. EDTA TITRATIONS Indirect Titration Alternatively - Anion is precipitated with excess metal ion (SO42- with excess Ba2+ at pH 1) - Precipitate is filtered and washed - Boiled with excess EDTA at higher pH (pH 10) to bring metal ion back into solution as EDTA complex - Excess EDTA is back titrated with Mg2+

  22. EDTA TITRATIONS Masking - Masking agent protects some component of analyte from reaction with EDTA - Masks by forming complexes with the components - F- masks Al3+, Fe3+, Ti4+, Be2+ - HF may form and is extremely hazardous [Al3+ with F- forms AlF63- complex]

  23. EDTA TITRATIONS Masking - CN- masks Hg2+, Zn2+, Ag+, Co2+, Cu+, Fe2+/3+, Ni2+ but not Pb2+, Mn2+, Mg2+, Ca2+ - Gaseous HCN may form at pH below 11 and is very toxic - Triethanolamine masks Al3+, Fe3+, Mn2+ - 2,3-dimercaptopropanol masks Bi3+, Cu2+, Hg2+, Pb2+, Cd2+

  24. WATER HARDNESS - Total concentration of alkaline earth ions in water - Concntration of Ca2+ and Mg2+ are usually much greater than the rest - Hardness is [Ca2+] + [Mg2+] - Often expressed as milligrams of CaCO3 per liter (ppm) If [Ca2+] + [Mg2+] = 1.00 mM = 1.00 mmol/L ~ 100 mg CaCO3 = 1.00 mmol CaCO3 Implies hardness is 100 mg CaCO3 per liter (100 ppm)

  25. WATER HARDNESS To Measure Hardness - Treat water with ascorbic acid to reduce Fe3+ to Fe2+ - Treat water with CN- to mask Fe2+, Cu+, and other metal ions - Titrate with EDTA in ammonia buffer at pH 10 - Determine [Ca2+] + [Mg2+] OR - Titrate with EDTA at pH 13 without ammonia - Mg(OH)2 precipitates at pH 13 and is not accessible to EDTA - [Ca2+] is determined separately in this case

  26. WATER HARDNESS Titration of Ca2+ and Mg2+ with EDTA - Add small amount of calmagite indicator to solution - Red MgIn/CaIn complex is formed - Titrate with EDTA until color changes to blue

  27. WATER HARDNESS Titration of Ca2+ and Mg2+ with EDTA - Mg2+/Ca2+ in solution is used up as EDTA is added - Just before equivalence point the last EDTA displaces indicator from MgIn - Unbound In is blue and indicates end point MgIn + EDTA → MgEDTA + In

  28. WATER HARDNESS - Hard water does not lather with soap - Reacts with soap to form insoluble curds - Much soap must be used to consume Ca2+ and Mg2+ before becoming useful

  29. WATER HARDNESS - Hard water is good for irrigation - Metal ions flocculate colloidal particles in soil - Increase permeability of soil to water

  30. WATER HARDNESS Soft Water - Hardness is less than 60 mg CaCO3 per liter (60 ppm) Temporary Hardness - Insoluble carbonate react with CO2 to produce bicarbonate CaCO3(s) + CO2 + H2O → Ca(HCO3)2(aq) - CaCO3 precipitates on heating - The reason why boiler pipes clog Permanent Hardness - Hardness caused by other salts (mostly CaSO4) - Soluble and cannot be removed by heating

  31. FRACTIONAL COMPOSITION OF EDTA Fraction of EDTA in the form Y4- [EDTA] = total concentration of all free EDTA species (EDTA not bound to metal ions) [EDTA] = [H6Y2+] + [H5Y+] + [H4Y] + [H3Y-] + [H2Y2-] + [HY3-] + [Y4-]

  32. FRACTIONAL COMPOSITION OF EDTA [H6Y2+] = [H+]6 [H5Y+] = [H+]5K1 [H4Y] = [H+]4K1K2 [H3Y-] = [H+]3K1K2K3 [H2Y2-] = [H+]2K1K2K3K4 [HY3-] = [H+]K1K2K3K4K5 [Y4-] = K1K2K3K4K5K6

  33. CONDITIONAL FORMATION CONSTANT - K´f is the conditional (effective) formation constant - Describes formation of MYn-4 at any given pH

  34. EDTA TITRATION CURVES Ca2+ pM = - log(Mn+) pM Mg2+ Equivalent point of Ca2+ Equivalent point of Mg2+ Volume of EDTA added (mL)

  35. EDTA TITRATION CURVES The steepest part of the titration curve - Greater for Ca2+ than for Mg2+ - Kf for CaY2- is greater than Kf for MgY2- - End point is more distinct at high pH - pH should not be too high for metal hydroxides to precipitate

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