213 PHC
9th lecture
(1) Gary D. Christian, Analytical Chemistry, 6th edition.
By the end of the lecture the student
should be able to:
Explain the different stages of comlexometric
reactions titration curves.
 Select a suitable indicator to detect the end
point.

A titration is performed by adding the
chelating agent to the sample.
e.g. titration of Ca
2+
with EDTA
Ca2+ + H4Y  CaY2-
the eq. point, the Ca2+ conc. Is
nearly equal to the unchelated Ca2+ .
 Before
 At
the eq. point and beyond, pCa is
determined from the dissociation of the
chelate at the given pH.
 The
more stable the chelate (Kf large),
the farther to the right will be the
equilibrium of the reaction, and the
larger will be the end point break.
The highly stable chelates can be
titrated at low pH values.
This is important because it allows
the titration of some metals in the
presence of others whose EDTA
chelates are too weak to titrate at
the lower pH.
Indicators used are themselves
chelating agent.
Eriochrome Black T:
o
o
o
o
o
It contains 3 ionizable protons.
Used for the titration of Mg2+ with EDTA.
It forms a red complex with part of the Mg2+.
The color of the uncomplexed indicator is
blue.
As soon as all the free Mg2+ is titrated, the
EDTA displaces the indicator from the
magnesium, causing a change in the color.
MgIn- + H2Y2- → MgY2- + In- + 2H+
red
colorless
colorless
blue
The metal-indicator complex must be less stable
than the metal-EDTA complex, or the EDTA will
not displace it from the metal.
Also, it must not be too weak or the EDTA will
start replacing it at the beginning of the
titration.
The metal-indicator complex should be 10 to 100
times less stable than the metal-titrant
complex.
 Complexometric
titration curves.
 Types of indicators used.