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

Multidentate (Chelating) Ligand 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)]

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

Synthetic Aminocarboxylic Acid Chelating Ligands 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+)

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

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

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+)

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)

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

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

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

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

Use of Auxilliary Complexing Agent (ACA) 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

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

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

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

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)

Displacement Titration 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-

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

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+

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]

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+

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)

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

Titration of Ca2+ and Mg2+ with EDTA 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

Titration of Ca2+ and Mg2+ with EDTA 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

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

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

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

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-]

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

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

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

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