2. Complexometric Titration
EDTA as a Chelating Agent
Role of Metallochromic Indicator
Mechanism of Titration with EBT Indicator
Requirements for visual Detection
Types of EDTA Titration
Methods of Increasing Selectivity for
Complexation
Analysis of a Metals Mixture
References
3. ..
Type of volumetric analysis.
A colored complex formed between metal
and ligand.
Ligands are used for complexations.
Indicator is used to indicate the end point
of a titration.
M+ + L M-L
5. EDTA forms stable complex with various metal ions
i.e. high Kf values.
The complexation occurs in single step.
sharp change in the metal ion conc. At the
equivalence point.
The M-EDTA complexes are water soluble.
The Stoichiometry for all metal ions is same 1:1
irrespective of its charge.
6. ..
Hydrolysis of certain metal ions occurs at high
pH.
At low pH EDTA loses its ability to donate its lone
pairs.
EDTA lacks selectivity i.e. Forms stable
complexes with various metals.
12. The requirement of a metal ion indicator
for use in the visual detection of end points
include:
The color change must be sharp.
The pH of the solution should not very high.
The MIn complex must possess sufficient stability.
The MIn complex must be less stable than the M-
EDTA complex.
13. Types Of EDTA titrations
Direct
Titrations
Back
Titrations
Indirect
Titrations
Replacement
Titrations
14. • It is the simplest method in which the standard
solution of EDTA is slowly added to the metal ion
solution till the end point.
Mmols of std.EDTA
added
Mmols of metal in
the sample
15. Back titration
• This method is suitable when;
• The reaction of metal ion with EDTA is slow ,
• The metal ion precipitates.
• No suitable indicator is available
Std.EDTA added in excess
Exc.EDTA+ std.Metal =mmols of exc
EDTA
Total EDTA –Exc.EDTA=mmols of
Metal in sample
16. • When direct or back titrations do not give sharp end points.
• when there is no suitable indicator for the analyte.
Mg-EDTA
Ca+2 sample
EDTA
Ca+2 ions
Mg-EDTA
Mg+2 ions
EDTA
Mmols of EDTA= mmols of Ca+2
17. • Certain anions that form precipitate with metal
cations and do not react with EDTA can be
analyzed indirectly. SO4
-2, PO4
-3, can be
determined by this method.
• e.g. SO4
-2 can be determined by adding excess
of Ba+2 to precipitate as BaSO4
. The precipitate
is filtered and washed .
• The excess Ba+2 in the filtrate is then titrated
with EDTA.
18. pH control
Use of masking and
demasking agents
Classical separation
Kinetic masking
19. • The formation of a metal chelate complex
dependent on the pH of the medium.
• In weakly acid solution, the chelates of many
metals such as alkaline earth metals are
completely dissociated, whereas
• chelates of Bi+3, Fe2+ or Cr+2 are readily
formed at this pH.
20.
21. ..
Masking agents:
• They prevent interfering ion from reaction
without physical separation.
• These reagents form complexes with interfering
ions.
• They form more stable complexes then ind. &
EDTA
Example; fluorides, cyanides etc.
22. Demasking agent : are reagents which
regain the ability of masked ion to enter the
reaction with ind. and EDTA.
Example:
• The masking by CN– can be removed
by:
• Mixture of formaldehyde – acetic acid
23. These are attempted only be applied if they are
not tedious; further only those precipitates may
be used for separations in which, after being re-
dissolved, the cations can be determined
complexometrically. Some of the examples are
CaC2O4, nickel dimethylglyoximate, and CuSCN
24. Kinetic masking
The metal ion does not form a complex due
to its kinetic inertness. Or it reacts slow.
For example, the reaction of chromium (III)
with EDTA is quite slow.
It is, therefore, possible to titrate other metal
ions which react rapidly without interference
from Cr (III).
26. ..
Mmols of Mn+2=mmols of EDTA for both metals-mmols of Mg+2
F- + Mg+2=Mg(F)2 release EDTA+Mn+2= mmols of Mg+2
Mn+2,Mg+2+EDTA= mmols of EDTA for both metals