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

2. CHAPTER 13
EDTA COMPLEXES

3. 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)]

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. 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+)

6. EDTA - Ethylenediaminetetraacetic acid [CH2N(CH2CO2H)2]2
(C10H16N2O8, 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. It is hexaprotic in the form H6Y2+
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 = (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)
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. 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

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. 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 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. 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

27. 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

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. 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)

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