Transcript edta titrations - Clayton State University

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
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
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
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)
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
[MY n 4 ]
Kf 
[M n  ][Y 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
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)
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
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 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 Y4α Y4
[Y 4 ]

[EDTA]
[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
[MY n 4 ]
Kf 
[M n  ][Y 4 ]
[Y 4 ]  α Y4 [EDTA]
[MY n 4 ]
Kf 
[M n  ]α Y 4- [EDTA]
[MY n 4 ]
K f  α Y 4  K f 
[M n  ][EDTA]
- K´f is the conditional (effective) formation constant
- Describes formation of MYn-4 at any given pH
pM
EDTA TITRATION CURVES
pM = Equivalent point
of Ca2+
log(Mn+)
Ca2+
Mg2+
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