Download Chapter 12 - Winona State University

Chapter 12
EDTA Titrations
Overview
12-1 Metal-Chelate Complexes
12-2 EDTA
12-3 EDTA Titration Curves
12-4 Do It with a Spreadsheet
12-5 Auxiliary Complexing Agents
12-6 Metal Ion Indicators
12-7 EDTA Titration Techniques
12-2: EDTA
• Ethylenediaminetetraacetic acid (EDTA) is a
compound that forms strong 1:1 complexes
with most metal ions.
• Used in industrial processes and products
such as detergents, cleaning agents, and
food additives that prevent metal-catalyzed
oxidation of food.
• Metal-EDTA complexes find their way into the
environment because they can pass through
wastewater treatment plants unscathed.
12-1: Metal-Chelate Complexes
• Metal ions are Lewis acids that accept electron pairs
from electron-donating ligands (Lewis bases).
• Monodentate ligands bind to a metal ion through only
one atom (e.g., CN- through the C atom).
• A ligand that attaches to a metal ion through more than
one ligand atom is said to be multidentate, or a chelating
agent.
12-1: Metal-Chelate Complexes
• Ethylenediamine is a simple chelating
agent, which is bidentate.
• Chelate effect – the ability of multidentate
ligands to form more stable metal
complexes than those formed by similar
monodentate ligands.
12-1: Metal-Chelate Complexes
The reaction of
with two molecules of
ethylenediamine is more favorable than its reaction
with four molecules of methylamine, a
monodentate ligand:
12-1: Metal-Chelate Complexes
Adenosine triphosphate (ATP) is an important
tetradentate ligand, which binds divalent metal
ions (Mg2+, Mn2+, Co2+, and Ni2+) through four of
their six coordination positions. The fifth and sixth
positions are occupied by water molecules.
12-1: Analytically Useful Chelating
Agents
12-2: EDTA
• A titration based on complex formation is called
a complexometric titration.
• EDTA is the most widely used chelator in
analytical chemistry.
• By direct titration or through an indirect series of
reactions, virtually every element of the periodic
table can be measured with EDTA.
12-2: EDTA: Acid-Base Properties
EDTA is a hexaprotic system, designated H6Y2+.
The first four pK values apply to carboxyl protons; the
last two are for the ammonium protons.
The neutral acid is tetraprotic, with the formula H4Y,
which can be dried at 140oC for 2 h and used as a
primary standard.
12-2: EDTA: Acid-Base Properties
The fraction of EDTA in each of its protonated forms
is plotted in the figure:
12-2: EDTA: Acid-Base Properties
We can define a for each species as the fraction of
EDTA in that form. For example, aY4- is defined as:
where [EDTA] is the total concentration of all free (not
complexed to metals) EDTA species in the solution.
Therefore, aY4- is given by:
12-1: Example: What Does aY4Mean?
12-2: EDTA Complexes
The equilibrium constant for the reaction of a metal
with a ligand is called the formation constant, Kf, or
the stability constant:
Kf for EDTA is defined in terms of the species Y4reacting with the metal ion.
12-2: EDTA Complexes
The table shows that formation constants for most
EDTA complexes are large and tend to be larger or
more positively charged cations.
12-2: Conditional Formation Constant
• Most EDTA is not Y4- below pH 10.37. The species
HY3- and H2Y2-, and so on, predominate at lower pH.
• If the pH is fixed by a buffer, then aY4- is a constant
that can be combined with Kf:
• This is called the conditional formation constant and it
describes the formation of MYn-4 at any particular pH.
12-2: Conditional Formation Constant
12-3: EDTA Titration Curves
• We can calculate the n+
concentration of free M
during its titration with
EDTA:
• If K f' is large, we can
consider the reaction to
be complete at each
point in the titration.
• The titration curve is a
plot of pM versus the
volume of EDTA added
and has three natural
regions.
12-3: EDTA Titration Curves
Region 1: Before the Equivalence Point
• The concentration of free metal is equal to the
concentration of excess, unreacted Mn+.
Region 2: At the Equivalence Point
• [Mn+] = [EDTA]
Region 3: After the Equivalence Point
• The concentration of free EDTA can be equated to
the concentration of excess EDTA added after the
equivalence point.
12-3: Titration Calculations
Consider the reaction of 50.0 mL of 0.040 0 M Ca2+
(buffered to pH 10.00) with 0.080 0 M EDTA.
Region 1: Before the Equivalence Point
• After the addition of 5.0 mL of EDTA
12-3 Titration Calculations
Region 2: At the Equivalence Point
• Virtually all the metal is in the form CaY2-. [CaY2-] is
equal to the original [Ca2+] with a correction for
dilution.
• The concentration of free Ca2+ is small and unknown.
12-3 Titration Calculations
12-3 Titration Calculations
Region 3: After the Equivalence Point
• Virtually all of the metal is in the form CaY2-, and
there is excess, unreacted EDTA.
• After the addition of 26.0 mL of EDTA, there is 1.0
mL of excess EDTA.
12-3 Titration Calculations
The concentration of
Ca2+ is governed by:
12-4: Do It with a Spreadsheet
Consider the titration of metal ion M (initial
concentration = CM, volume VM) with a solution of
ligand L (concentration = CL, volume added = VL) to
form a 1:1 complex:
The mass balances for metal and ligand are:
12-4: Do It with a Spreadsheet
Substituting Kf[M][L] from Equation 12-8 for [ML] in the
mass balance:
Now substitute the expression for [L] from Equation 12-10
back into Equation 12-9, then solve for the fraction of
titration, .
12-4: Do It with a Spreadsheet
The equation for titrating L with M:
12-5 Auxiliary Complexing Agents
• For many metals to be titrated
in alkaline solutions with
EDTA, we use an auxiliary
complexing agent to prevent
metal hydroxide precipitation.
• Reagents are ligands such as
ammonia, tartarate, citrate, or
triethanolamine
• They must bind strongly
enough to prevent metal
hydroxide from precipitating,
but weakly enough to give up
the metal when EDTA is
added.
12-5: Metal-Ligand Equilibria
• Consider a metal ion that forms two complexes
with the auxiliary complexing ligand L:
• b are the cumulative formation constants.
• The fraction of metal ion in the uncomplexed state,
M, is:
• Where Mtot is the total concentration of all forms of
M (M, ML, and ML2).
12-5: Metal-Ligand Equilibria
• The mass balance is: Mtot = [M] + [ML] + [ML2]
• Equations 12-13 and 12-14 allow us to say
[ML] = b1[M][L] and [ML2] = b2[M][L]2
• Therefore,
Mtot = [M] + b1[M][L] + b2[M][L]2
= [M]{1 + b1[L] + b2[L]2}
• Substituting into Equation 12-15:
12-5: Metal-Ligand Equilibria
12-5: EDTA Titration in the Presence of
Ammonia
• Consider the titration of Zn2+ by EDTA in the
presence of NH3.
• We now need a new conditional formation
constant to account for the fact that only some of
the EDTA is in the form Y4- and only some of the
zinc not bound to EDTA is in the form Zn2+:
12-5: EDTA Titration in the Presence of
Ammonia
12-5: EDTA Titration in the Presence of
Ammonia
12-5: EDTA Titration in the Presence of
Ammonia
12-6: Metal Ion Indicators
End-point detection methods:
1. Metal ion indicators (most common)
2. Mercury electrode
3. Ion-selective electrode
4. Glass (pH) electrode
• Metal ion indicators are compounds that change
color when they bind to a metal ion – must bind
metal less strongly than does EDTA.
• Example: the reaction of Mg2+ with EDTA at pH 10
with Calmagite indicator.
12-6: Metal Ion Indicators
12-6: Metal Ion Indicators
12-7: EDTA Titration Techniques
1. Direct titration: Analyte is titrated with standard
EDTA. Analyte is buffered to a pH where K f' for
the metal-EDTA complex is large and the color of
the free indicator is different than that of the
metal-indicator complex.
2. Back titration: A known excess of EDTA is
added to the analyte. Excess EDTA is then
titrated with a standard solution of a second metal
ion.
12-7: EDTA Titration Techniques
12-7: EDTA Titration Techniques
3. Displacement Titration: Use when the analyte,
such as Hg2+, does not have a satisfactory indicator.
• Hg2+ is treated with excess Mg(EDTA)2- to displace
Mg2+, which is titrated with standard EDTA.
There is also no suitable indicator for Ag+, however
Ag+ will displace Ni2+ from tetracyanonickelate(II) ion:
2Ag+ + Ni(CN)42- → 2Ag(CN)2- + Ni2+
The Ni2+ liberated can be titrated with EDTA to
determine the amount of Ag+ added.
12-7: EDTA Titration Techniques
4. Indirect Titration: Anions that precipitate with certain metal ions
can be analyzed with EDTA using an indirect titration.
Example: Sulfate can be analyzed by precipitation with excess Ba2+
at pH 1.
• BaSO4 (s) is washed then boiled with excess standard EDTA at
pH 10 to bring Ba2+ back into solution as Ba(EDTA)2-.
• Excess EDTA is back-titrated with Mg2+.
5. Masking: A masking agent is a reagent that protects a component
of the analyte from reaction with EDTA.
Example: Al3+ in a mixture of Mg2+ and Al3+ can be measured by
masking the Ag+ with F-, leaving only the Mg2+ to react with EDTA.