Transcript Quantitative Chemical Analysis 7e

CHEMISTRY 59-320
ANALYTICAL CHEMISTRY
Fall - 2010
Lecture 16
Chapter 12: EDTA Titrations
12-1 Metal-Chelate complexes
• Metal ions are Lewis acids, accepting electrons pairs
from electron-donating ligands that are Lewis bases.
• Monodentate ligand: binds to a metal ion through
only one atom.
• Multidentate ligand: attaches to a metal ion through
more than one ligand atom, also known as chelating
ligand.
• Chelate effect: the ability of multidentate ligands to
form more stable metal complexes than those formed
by similar monodentate ligands.
• A titration based on complex formation is
called a complexometric titration.
• Following are structures of analytically useful
chelating agents:
12-2 EDTA
• EDTA is an abbreviation for ethylenediaminetetraacetic
acid, a compound that forms strong 1:1 complexes with
most metal ions.
•
EDTA is a hexaprotic system, designated H6Y2+. The
highlighted, acidic hydrogen atoms are the ones that
are lost upon metal-complex formation.
• The neutral acid of EDTA is tetraprotic,
with the formula H4Y. A commonly used
reagent is the disodium salt, Na2H2Y ·
2H2O
• We can define α for each species as
the fraction of EDTA in that form. For
example,
is defined above.
Fractional composition diagram
for EDTA
• The equilibrium constant for the reaction of a metal
with a ligand is called the formation constant, Kf, or
the stability constant
• The number
is called the conditional formation
constant, or the effective formation constant. It describes the
formation of MYn−4 at any particular pH.
Conditional formation constant
• pH affects the titration of
Ca2+ with EDTA. Below pH ≈
8, the end point is not sharp
enough to allow accurate
determination.
• The conditional formation
constant for CaY2− is just too
small for “complete” reaction
at low pH.
12-3 EDTA Titration Curves
•
The titration curve is a graph of pM
versus the volume of added EDTA.
• The right side figure illustrates for
reaction of 50.0 mL of 0.050 0 M
Mn+ with 0.050 0 M EDTA,
assuming Kf’= 1.15 × 1016,
where the concentration of free
Mn+ decreases as the titration
proceeds.
• There are three regions in an
EDTA titration curve:
(a) Before the equivalence point.
(b) At the equivalence point.
(c) After the equivalence point.
• Since at equivalence point,
there is exactly as much
EDTA as metal in the
solution, we can treat the
solution as if it had been
made by dissolving pure
MYn−4. Some free Mn+ is
generated by the slight
dissociation of MYn−4:
• The Ca2+ end point is more
distinct than the Sr2+ end
point because the
conditional formation
constant, for CaY2− is greater
than that of SrY2−
EDTA titration calculations
• Problem 12-7: Consider the titration of 25.0 mL of 0.0200
M MnSO4 with 0.0100 M EDTA in a solution buffered to
pH 8.00. Calculate pMn2+ at the following volumes of
added EDTA and sketch the titration curve: (a) 0 ml, (b)
20.0 ml, (c) 40.0 ml, (d) 49.0 ml, (e) 49.9 ml, (f) 50.0 ml,
(g) 50.1 ml, (h) 55.0 ml, (i) 60.0 ml.
• Solution: from Table 12.2, we get log(Kf) = 13.89. At pH
= 8.00, α(Y4-) = 4.2 x10-3.
The above calculation will be carried out based on
whether the titration is before or after the equivalence
point. Here Ve*0.0100 M = 25.0*0.0200 M; Ve = 50.0 ml
• (a) to (e) are before the equivalence point and will follow the same
calculation equation:
[Mn2+] * (0.025 + V) = 0.025 *0.02 – 0.01*V
• (f) this is the equivalence point
Kf’ = 4.2 x10-3 x 1013.89 = [MnY2-]/([Mn2+][EDTA])
= (0.02*0.025/0.075)/ ([Mn2+])2
[Mn2+] =8.179 x10-7.
pMn2+ =
• Calculations (g) to (i) are beyond the equivalence point, where
[EDTA] can be obtained from the following
[EDTA]*(0.025 + V) = 0.01*V – 0.02*0.025
then Kf’ = 4.2 x10-3 x 1013.89 = [MnY2-]/([Mn2+][EDTA])
A beaker containing 50.0 mL of 0.300 M Ca2+ at pH 9 is titrated
with 0.150 M EDTA.
The pCa at the equivalence point is:
(a) 4.97.
(b) 5.13.
(c) 5.84.
At the equivalent point, all Ca2+ ions have formed complex
and free Ca2+ come from the dissociation of complex:
CaY2Ca2+ + EDTA
For Ca2+ Kf = 1010.65 ; α = 0.041 at pH = 9
2


CaY
'

Kf 
 1.8  109
2
Ca  EDTA
2


CaY
0.1
2

Ca  

1.8  109 1.8  109
2
Ca 2   7.45 106
pCa= 5.13
12-5 Auxiliary Complexing Agents
• It is a ligand that binds the metal strong
enough to prevent metal hydroxide from
precipitating, but weakly enough to give
up the metal when EDTA is added.
• Consider a metal ion that forms two
complexes with the auxiliary agent.
• The fraction of metal ion in the uncomplexed state M,
can be calculated as
• Ammonia complexes zinc in four different forms
• EDTA titration in the presence of auxiliary complexing agent
ammonia
Example: EDTA titration in the presence of ammonia
(b) When 50.0 ml EDTA is added, the system reaches equivalence
point.
(c) When 60.0 ml EDTA is added, the system is beyond equivalence
point.
Metal Ion Indicators
• The most common technique
to detect the end point in
EDTA titrations is to use a
metal ion indicator.
• are compounds whose color
changes when they bind to a
metal ion. Useful indicators
must bind metal less strongly
than EDTA does.
• Masking agent:
In a direct titration, analyte
is titrated with standard EDTA.
The analyte is buffered to a pH
at which the conditional formation
constant for the metal-EDTA complex
is large and the color of the free
indicator is distinctly different
from that of the metal-indicator complex.
12-7 EDTA titration techniques
•
•
•
•
•
Direct titration:
Back titration:
Displacement titration:
Indirect titration:
Masking agent: