Complexometric Reactions and Titration
Recall that titration is the quantitative measurement of an analyte in solution by reacting
it completely with a standardized reagent. Complexes form in a fixed stoichiometry so a
standard solution of a ligand can be used to titrate a metal ion in solution. Similarly, a
standard solution of a metal ion can serve as the titrant for a species that acts as a ligand.
Many metal ions form slightly dissociated complexes with various ligands (complexing
agents).
 Complexes used to mask undesired reactions.
 Its formation can serve as the basis of accurate and convenient titrations for metal
ions.
 Complexometric titrations are useful for the determination of a large number of
metals.
 Most complexing agents are weak acids or weak bases whose equilibria
influenced by the pH.
Nomenclature
Ligand
Neutral or anionic species with unpaired electrons that can bond to a metal ion. Common
ligands are CN-, NH3, OH-, and halides.
Complex
An association of a central metal ion and surrounding ligands in solution - also called
coordination complex.
Coordination number
The number of bonds with the central metal ion, usually 2, 4, or 6.
Chelate
A specific type of complex in which at least one ligand contains more than one atom with
unpaired electrons so it can make multiple bonds with the central metal ion. Ligands that
make two bonds are called bidendate, ligand that make three bonds are tridendate, and so
on.
Complexes; formation constant:
Many cations will form complexes in solution with a variety of substances that have a
pair of unshared electrons (e.g. N, O, S) capable of satisfying the coordination number of
the metal.
The number of molecules of complexing agent (ligand) will depend on the coordination
number of the metal and on the number of complexing groups on the ligand molecule.
Cu2+ + NH3
[CuNH3]2+
Cu2+ is a Lewis acid (electron pair acceptor) and NH3+ is a Lewis base (electron pair
donor).
When a complex is formed, the solvating molecules (H2O) are replaced one by one by the
new ligands (in the above example: NH3). The complex formation takes place step by
step where every step corresponds to the substitution of one H2O by NH3 .
Every step is characterized by an equilibrium constant Ki , called the stepwise stability
constant (or stepwise formation constant).
Example:
Cu2+ + NH3
[CuNH3]2+ + NH3
[CuNH3]2+
[Cu(NH3)2]2+
[Cu(NH3)2]2+ + NH3
[Cu(NH3)3]2+
[Cu(NH3)3]2+ + NH3
[Cu(NH3)4]2+
K1, K2, K3 and K4 are the stepwise stability constants. These equilibrium constants are
described by a general expression
Chelates:
A Chelating agent (the Ligand) is an organic agent that has tow or more groups which are
capable of complexing with a metal ion. The complex formed is called a Chelate. The
titration with a chelating agent is called a Chelometric titration a type of Complexometric
titration.
The most generally useful titrating agent is amino carboxylic acids, in which nitrogen and
carboxylate groups serve as ligands. The amino nitrogens are more basic and are
protonated (NH3+) more strongly than carboxylate groups. When these groups bind o
the metal atoms, they will lose their protons. The metal complexes formed with these
multidentate complexing agents are often 1:1 regardless of the charge on the metal ions,
because there are sufficient complexing groups on one molecule to satisfy the
coordination sites of the metal ions.
The most widely used chelating agent in titration is Ethylene Diamine Tetra Acetic acid –
EDTA.
H4Y + H2O
H3Y- + H3O+
H3Y- + H2O
H2Y2- + H3O+
H2Y2- + H2O
HY3- + H3O+
HY3- + H2O
4-
+
Y
+ H3O
Ethylenediaminetetraacetic
acid (EDTA)
A pair of unshared electrons capable of complexing with a metal ion is contained on each
of two nitrogens and each of the four COOH groups. Thus, there are six complexing
groups in EDTA. EDTA can be represented by the symbol H4Y since it is a tetraprotic
acid and the H in H4Y refer to the four ionisable hydrogen.
EDTA equilibria:
We can represent EDTA as having four Ka values corresponding to the stepwise
dissociation of the four protons:
Ka1= 1.0x10-2 H4Y + H2O
H3Y- + H3O+
Ka2= 2.2x10-3 H3Y + H2O
H2Y2- + H3O+
2Ka3=6.9x10-7 H2Y + H2O
HY3- + H3O+
3Ka4=5.5x10-11 HY + H2O
Y4- + H3O+
Formation constant:
Consider the formation of the EDTA chelate of Ca+2. This can be represented by
Ca+2 + y-4 ↔ CaY2
The effect of pH on EDTA equilibria:
The equilibrium is shifted to the hydrogen ion concentration is increased due to
competition for the chelating anion by the hydrogen ions.
Detection of the end point:
Indicators:
Indicators used for Chelometric titrations are themselves chelating agents, they are
usually dyes of the O,O- dihydroxy azotype.
Erichrome Black T (EBT):
Is a typical indicator, it contains 3ionizable protons, so it can be represented by H3In.
EBT can be used for the titration of Mg+2 with EDTA.
MgIn- + H2Y2 → MgY2 + HIn2- +H+
red
colorless
colorless
blue
Eriochrome black T
OH
-
O3S
O2N
OH
N N
pK1=6.3
pK1=11.6
The metal indicator complex should be 10-100 times less stable than the metal-titrant
complex so metal titrant will displace the indicator from the metal giving good and sharp
end point.
EBT can not be used to indicate the direct titration of Ca2+ alone with EDTA, however,
because the indicator forms too weak complex with Ca2+ to give a sharp end point.
The Ca2+ titrates first since its EDTA chelate is more stable. When Mg-EDTA is added to
the sample, the Ca2+ in the sample displaces the EDAT from Mg2+ so that the Mg2+ is free
to react with the indicator. At the end point, an equivalent amount of EDTA displaces the
indicator from Mg+2 (Ca2+ Titer) causing the color change, and no correction is required
for the added Mg-EDTA.
Ca2+ + EDTA → Ca-EDTA
(x mole) Ca2+ + (y mole) EDTA-Mg
x mole > y mole
K(EDTA-Mg) = 4.9 x 108
K(EDTA-Ca) = 5.0 x 1010
(y mole) Ca-EDTA + (y mole) Mg2+ + (x-y mole) Ca2+
EDTA
(y mole) Ca-EDTA + (y mole) Mg-EDTA + (x-y mole) Ca-EDTA
mole EDTA= C x V = (y mole) + (x-y mole) = x mole
Since EBT and other indicators are weak acids, their color will depend some what on he
ph, because the different ionized species of the indicator have different colors.
For example with EBT,
H2In- is red (pH <6)
HIn-2 is blue (pH 6-12)
In-3 is yellow orange (pH >12)
An indicator is useful for indication of titrations of only those metals that form a more
stable complex with titrant than with the indicator at the given pH.
Calmagite indicator gives a some what improves end point over EBT for the titration of
Ca+2 and Mg+2 with EDTA; it also has a longer shelf life.
Chelating agents such as EDTA are used in the treatment of heavy metal poisoning, for
example, when children ingest chipped paint that contains lead. Heavy metals such as
lead form more stable EDTA chelates than Ca+2 does. Chelated lead is excreted via
kidneys.