Transcript CHAPTER 11

CHAPTER 15
Complexation Titrations: Taking
Advantage of Complexing Agents
Terms Defined
• Complex formation – the process whereby a
species with one or more unshared electron
pairs forms coordinate bonds with metal
ions.
• Ligand – an ion or molecule that forms a
covalent bond with a cation or a neutral
metal atom by donating a pair of electrons
that are then shared by the two.
• Chelating agent – substance with multiple
sites available for coordination bonding
with metal ions. Such bonding typically
results in the formation of five or sic
membered rings
• Dentate – (Latin) having toothlike
projections
Forming Complexes
• Metals ions are Lewis acids, because they
accept electrons from Lewis bases. When
metal cations combine with Lewis bases,
the resulting species is called a complex ion,
and the base is called a ligand.1
• The coordination number is the number of
covalent bonds that the metal cation tends to
form with the electron donor.
• For example, copper (II) has coordination number
of four. The species formed from such
coordination or complexing, can be electrically
positive, neutral or negative. Copper when
complexed with ammonia results in a cationic
complex, Cu(NH3)42+, when complexed with
glycine, a neutral complex, Cu(NH2CH2COO)2,
and when complexed with chloride, an anionic
complex, CuCl42-.
• When a metal cation is complexed to ligands
forming a neutral compound, the complex is called
coordinated compound.1
• A chelate is produced when a metal ion
coordinates with two or more donor groups of a
single ligand to form a five or six membered
heterocyclic ring. The copper complex of glycine,
is an example of a chelate:
N H2
Cu
2+
+
2
H
C
O
O
O
O
C
Cu
OH
H
O
N H2
N H2
• Some common inorganic ligands are ammonia,
water, and halides.
• A ligand that has one donor group such as
ammonia, is called unidentate. Glycine, which has
two groups available for covalent bonding, (the
carbonyl oxygen and the aminal nitrogen), is
called bidentate. As titrants, multidentate ligands,
particularly tetradentate and hexadentate chelating
agents, those having four or six donor groups,
have two advantages over their unidentate titrants.
• First, these multidentate titrants, generally
react more completely with cations, thereby
providing sharper more accurately end
points. Second, they ordinarily react with
metal ions in a single-step process, whereas
with unidentate ligands usually involves
two or more intermediate species.
• An example of a hexadendate ligand is
EDTA (Ethylenediaminetetraacetic Acid). It
has six potential sites for complex
formationthe electron pairs on the two
nitrogen atoms and the four electron-rich
carboxyl groups.
• The ligands of EDTA wrap around the metal
ion and effectively form a cage around the
ion forming a more stable metal complex.
This ability is called the chelate effect, and
EDTA is called a chelating agent. The
properties of EDTA were demonstrated in
Lab 27E-3 and 4 Determination of
Manganese and Hardness of Water.
• EDTA, H6Y2+, is a hexaprotic system; it has six
acidic hydrogens that are lost upon metal-complex
formation. The first four pK values apply to
carboxyl protons, and the last two are for the
ammonium protons (pK1 = 0.0, pK2 = 1.5, pK3 =
2.0, pK4 = 2.66, pK5 = 6.16, and pK6 = 10.24).
The pH of an EDTA solution affects the
equilibrium constant of complex formation.
Solutions of high pH used in analytical procedures
do not significantly effect the stability of a
complex.
• In order to determine the concentration of all
possible EDTA containing species in titration, we
use:
CT = [H6Y2+]+[H5Y+]+[H4Y]+[H3Y-]+[H2Y-2]+ [HY3]+[Y4-]
• If we want to determine the relative concentrations
of individual species of EDTA during titration, we
use the alpha values.
• The alpha value for a species is given by
aspecies = [species]/CT
For a given hydronium ion concentration such
as [Y ]/ C ,
4-
T
4+K
,a4=K1+K2+K3+K4/[H3O+]
+ 3
+ 2
+
1[H3O ] +K1K2[H30 ] +K1K2K3[H3O ]+K1K2K3K4
• Conditional Formation Constants can also
be computed using alpha constants, but only
at a single pH.The overall formation
constants are called beta values.
K/ = a4KMY = [MY(n-4)+]/[Mn+] a4CT
Summary Uni vs Multidentate
ligands
• In order to provide an overall comparison of
a unidentate ligand, such as ammonia, vs a
multidentate ligand such as EDTA, we can
examine the following titration curves.
• The titration concerns a reaction that has an overal equilibrium
constant of 1020. Curve A is derived for the reaction in which a metal
ion M, that has a coordinatioin number of four, reacts with a
tetradentate ligand, D, to form the complex MD, a 1:1 complex. Curve
B, depicts the reaction of M with a bidentate ligand B to form the
complex MB2, a 1:2 complex, in two steps. The formation constant for
this reactions would be 1012 and 108 respectively for the first and
second reactions. Curve C depicts the reaction of M with a unidentate
ligand, A, that forms the complex MA4, a 1:4 complex in four steps.
The formation constants for this reaction would be 108, 106, 104, 102
for the first, second, third and fourth reactions. This figure shows how
a sharper end point is obtained for a reaction that takes place in a
single step.
Examples
Page 383 15-14 a
a)
26.37 mL X 0.0741 mmol Mg(NO3)2/mLX 1mmol
EDTA/mmol Mg(NO2)2 X1mL EDTA/0.0500 mmole
EDTA = 39.1 mL EDTA
Page 384 15-22
(13.31 X 0.03560) mmol EDTA/9.76 g sample X 1mmolTI2SO4/2mmolEDTA X
0.5048g TI2SO4 / mmol TI2SO4 X 100% = 1.228% TI2SO4
Examples
Page 384 15-24
# mmol Fe3+ = (13.73 X 0.01200) mmol EDTA X 1mmol Fe2+/mmolEDTA =
0.16476 mmol
#mmol Fe2+ = (29.62 –13.73) mL EDTA X 0.01200 mmol EDTA/mL EDTA X
1mmol Fe2+/mmol EDTA = 0.19068 mmol
(0.16476 mmol Fe3+X 55.847 mg Fe3+/mmol)/50.00 mL X 10-3 L/mL = 184.0 ppm
Fe3+
(0.19068 X 55.847)/0.05000 = 213.0 ppm Fe2+