ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 13

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 13
DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 13 EDTA COMPLEXES
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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)
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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
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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-tetra
acetic acid (EGTA) - Form 11 complexes with
metal ions (but not with monodentate ions like
Li, Na, K)
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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 11
complexes with most metal ions which remain in
solution with diminished reactivity
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EDTA
It is hexaprotic in the form H6Y2
HO2CH2C
CH2CO2H


HNCH2CH2NH
HO2CH2C
CH2CO2H
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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)
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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)
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EDTA
Synthesis - Previously formed from
ethylenediamine (1,2-diaminoethane) and
chloroacetic acid - Currently formed
from ethelynediamine methanal (formaldehyde) and
sodium cyanide
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EDTA
Uses - Food additives (preservatives), soaps,
cleaning agents, - Hardwater and wastewater
treatment - Textile industry, pulp and paper
industry
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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
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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
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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 triethanolamin
e
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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
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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
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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
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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)
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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-
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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WATER HARDNESS
- Hard water is good for irrigation - Metal
ions flocculate colloidal particles in soil -
Increase permeability of soil to water
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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
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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-
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FRACTIONAL COMPOSITION OF EDTA
H6Y2 H6 H5Y H5K1 H4Y
H4K1K2 H3Y- H3K1K2K3 H2Y2-
H2K1K2K3K4 HY3- HK1K2K3K4K5 Y4-
K1K2K3K4K5K6
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CONDITIONAL FORMATION CONSTANT
- Kf is the conditional (effective) formation
constant - Describes formation of MYn-4 at any
given pH
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EDTA TITRATION CURVES
Ca2
pM - log(Mn)
pM
Mg2
Equivalent point of Ca2
Equivalent point of Mg2
Volume of EDTA added (mL)
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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