ANALYTICAL CHEMISTRY CHEM 3811

CHAPTER 13

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton 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 poisoning

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+)

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 reactivity

EDTA

It is hexaprotic in the form H6Y2+

HNCH2CH2NH

HO2CH2C

HO2CH2C

CH2CO2H

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 fromethelynediamine

methanal (formaldehyde)and

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

]][Y[M][MYK 4n

4n

f

- 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

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

ExamplesAscorbate

CitrateTartrate

Ammoniatriethanolamine

EDTA

Examples

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

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

EDTA

- 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 red

Xylenol orange: from yellow/violet to red

Cu2+, Ni2+, Fe3+, Al3+, Cr3+, Co2+ block calagmite

METAL 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 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 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-, CrO4

2-, 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(SO4

2- 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 AlF6

3- 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 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

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

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 displacesindicator 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

WATER HARDNESS

FRACTIONAL COMPOSITION OF EDTA

[EDTA]][Yα

4

Y4

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

]][Y[M][MYK 4n

4n

f

[EDTA]α][Y 4Y4

[EDTA]]α[M][MYK

-4Yn

4n

f

][EDTA][M][MYKαK n

4n

fYf 4

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

EDTA TITRATION CURVES

Volume of EDTA added (mL)

pM

pM = - log(Mn+)

Mg2+

Ca2+

Equivalent pointof Ca2+

Equivalent pointof Mg2+

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