ANALYTICAL CHEMISTRY CHEM 3811

CHAPTER 13

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciencesClayton state university

CHAPTER 13

EDTA 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 throughion channels in cell membranes (nerves)

Metal chelate complexes are important in medicine- Synthetic ligands as anticancer agents- Chelation therapy is used to enhance iron excretionwhich reduces heart and liver diseases- Chelation therapy for mercury and lead poisoningMETAL-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+) METAL-CHELATE COMPLEXES

- 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 ionswhich remain in solution with diminished reactivityEDTA

It is hexaprotic in the form H6Y2+HNCH2CH2NHHO2CH2CHO2CH2CCH2CO2HCH2CO2H++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(Na2H2Y2H2O)EDTA

Synthesis

- Previously formed from ethylenediamine (1,2-diaminoethane) and chloroacetic acid

- Currently formed fromethelynediamine methanal (formaldehyde)andsodium cyanideEDTA

Uses

- Food additives (preservatives), soaps, cleaning agents,

- Hardwater and wastewater treatment

- Textile industry, pulp and paper industryEDTA

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 precipitateEDTA

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

ExamplesAscorbateCitrateTartrateAmmoniatriethanolamineEDTA

Examples

- Titration of Ca2+ and Mg2+ at pH 10Ascorbic acid (ascorbate) as ACA

- Titration of Pb2+ at pH 10Tartaric acid (tartrate) as ACAEDTA

- 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 IndicatorsCalmagite: from red/blue/orange to wine redXylenol orange: from yellow/violet to red

Cu2+, Ni2+, Fe3+, Al3+, Cr3+, Co2+ block calagmiteMETAL ION INDICATORS

Direct Titration

- Analyte is titrated with standard EDTA

- Analyte is buffered to an appropriate pH where reaction withEDTA is complete

- ACA may be required to prevent metal hydroxide precipitation in the absence of EDTAEDTA 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 solutionof a metal ion(metal must not displace analyte from EDTA)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 occurKf 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 EDTAEDTA 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 analytefrom 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+EDTA TITRATIONS

- Total concentration of alkaline earth ions in water

- Concntration of Ca2+ and Mg2+ are usually much greaterthan 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 CaCO3Implies 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 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 blueWATER 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 displacesindicator from MgIn

- Unbound In is blue and indicates end point

MgIn + EDTA MgEDTA + InWATER 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 usefulWATER 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 bicarbonateCaCO3(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 heatingWATER HARDNESS

FRACTIONAL COMPOSITION OF EDTAFraction 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- Kf is the conditional (effective) formation constant- Describes formation of MYn-4 at any given pH

EDTA TITRATION CURVESVolume of EDTA added (mL)pMpM = - log(Mn+)Mg2+Ca2+Equivalent pointof Ca2+Equivalent pointof Mg2+

EDTA TITRATION CURVESThe 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