Complexometric Titrations
CONTENTS
• Complexometric
Titrations
• Terminology
• Principle
involved in complexometric titrations
• Titrations
with EDTA
• Chemistry
of Indicators
• Theory
involved in indicators
• Types
of complexometric titrations
Direct
titration
Back
titration
Replacement of one complex by another
Alkalimetric titration of metals (Indirect Titration)
• Masking
& Demasking agents
OBJECTIVES
By the end of this lecture, students will be able to:
Ø Explain
the principle involved in Complexometric titrations
Ø Describe
complexing agents
Ø Explain
the method of complexometric titration with EDTA
Ø Explain the mechanism of indicators used in
complexometric titrations
Ø Explain
the role of masking and demasking agents
Ø Brief
the theory of metal ion indicators
Complexometric
Titrations
• Complexometric
titration is a type of titration based on complex formation between the
analyte (metal ions) and titrant (especially EDTA).
• Complexometric
titrations are particularly useful for determination of a mixture of different
metal ions in solution
• An
indicator with a marked color change is usually used to detect the end-point of
the titration
Terminology
• Ligand
– Electron donating species (should possess atleast one electron pair to donate)
• Central
metal ion – The central atom (a metal ion or cation) accepts an
electron pair from one or more ligands
• Chelate
– Multidentate ligands complexed to metal ions are called chelates.
Chelates always have a “chelate ring.” For example, the
zinc-8-hydroxyquinolate complex.
• Coordination
number
• Formation
constant
Any complexation reaction can in theory be applied as a
volumetric technique provided that :
• Reaction
reaches equilibrium rapidly following each addition of titrant.
• Interfering
situations do not arise such as stepwise formation of various complexes
resulting in the presence of more than one complex in solution in significant
concentration during the titration process
• A
complexometric indicator capable of locating equivalence point with fair
accuracy is available
• In
practice, the use of EDTA as a titrant is well established.
General
Principles of complexometry
Most metal ions form coordination compounds with
electron-pair donors (ligands)
Mn+ + qLm-
ßà MLqn-mq
Formation
constant,Kf = [MLqn-mq]/[Mn+][Lm-]q
The number of coordinate covalent bonds formed is called the
“coordination number” (e.g. 2,4,6)
e.g., Cu2+ has coordination number of 4
Cu2+ + 4 NH3 ßà
Cu(NH3)42+
Cu2+ + 4 Cl– ßà Cu(Cl)42-
• The
most useful complex-formation reactions for titrimetry involve chelate
formation
• A
chelate is formed when a metal ion coordinates with two of more donor groups of
a single ligand
Types of
Ligands
• Ligands
are classified regarding the number of donor groups available:
unidentate (one donor group)
Bidentate (two donor groups)
Tridendate (three donor groups)
Tetradendate (four donor groups)
Pentadentate(five donor groups)
Hexadentate(six donor groups)
• Multidentate
ligands (especially with 4 and 6 donors) are preferred for titrimetry.
– react
more completely with metal ion
– usually
react in a single step
– provide
sharper end-points
Examples for Ligands
• Unidendate
or Monodendate
Anionic ligands such as halides, SCN1-, CN1-,
OH1-, RCOO1-, S2-, C2O42-
(oxalate), etc.
Molecular ligands include water, ammonia, RNH2
(amines) C5H5N (pyridine)
• Bidendate
ligands
Glycine complexed with copper(II).
Ethylene diamine complexed with zinc ion
Structure
of EDTA
Properties
of EDTA
• Ethylenediamine
tetraacetic acid, has four carboxyl groups and two amine groups (act as
electron pair donors or Lewis bases)
• EDTA
has the ability to donate its six lone pairs of electrons for the formation of
coordinate covalent bonds to metal cations (Makes EDTA a hexadentate ligand)
In practice EDTA
is usually only partially ionized, and
• Thus
forms fewer than six coordinate covalent bonds with metal cations
• Disodium
EDTA, commonly used in the standardization of aqueous solutions of transition
metal cations
• Only
forms four coordinate covalent bonds to metal cations at pH values less than or
equal to 12 as in this range of pH values the amine
• Groups
remain protonated and thus unable to donate electrons to the formation of
coordinate covalent bonds
Complexometric
Titration with EDTA
• it
is almost always necessary to use a complexometric indicator to carry out metal
cation titrations using EDTA
• Usually
an organic dye such as Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B
or Murexide to determine when the end point has been reached
• Dyes
bind to the metal cations in solution to form colored complexes
• EDTA
binds to metal cations much more strongly than the dye used as an
• indicator
• EDTA
will displace the dye from the metal cations as it is added to the solution of
analyte
• A
color change in the solution being titrated indicates that all of the dye has
been displaced from the metal cations in solution, End point has been reached
EDTA
Titrations
• General
shape of titration curves obtained by titrating 10.0 mL of a 0.01M solution of
a metal ion M with a 0.01 M EDTA solution
• Apparent
stability constants of various metal-EDTA complexes are indicated at the
extreme right of the curves
• It
is evident that the greater the stability constant, the sharper is the end
point provided the pH is maintained constant
• In
acid-base titrations the end point is generally detected by a pH-sensitive
Indicator.
• In
the EDTA titration a metal ion sensitive indicator (metal indicator or
metal-ion indicator) is often employed to detect changes of pM
Three Regions of EDTA Titration
The curves are easily calculated by dividing
the curve up into domains:
•
The pM before equivalence.
•
The pM at equivalence.
•
The pM after equivalence.
As the pH
increases, the equilibrium shifts to the right.
Titration curves for
100 mL 0.1 M Ca2+ versus 0.1
M Na2EDTA at pH 7 and 10.
Advantage
of EDTA Titrations
• Enables
us to analyze ions in very small quantities.
• Care
should be taken on effects of pH on the titration method
• Biological
use of complexometric titration
• Application
on living cells.
Indicators
• Indicators form complexes with specific metal ions,
which differ in colour from the free indicator and produce a sudden colour
change at the equivalence point
• Contain
types of chelate groupings and generally possess resonance systems typical of
dyestuffs
• End
point of the titration can also be evaluated by other methods including
potentiometric, amperometric, and spectrophotometric techniques.
Types of complexometric titrations
• Direct
titration
• Back
titration
• Replacement
of one complex by another
• Alkalimetric
titration of metals (Indirect Titration)
Direct Titration
• Solution
containing the metal ion to be determined is buffered to the desired pH
• Titrated
directly with the standard EDTA solution
• It
may be necessary to prevent precipitation of the hydroxide of the metal (or a
basic salt)
• By
the addition of some auxiliary complexing agent, such as tartrate or citrate or
triethanolamine
• At
the equivalence point the magnitude of the concentration of the metal ion being
determined decreases abruptly
• This
is generally determined by the change in colour of a metal indicator or by
amperometric, spectrophotometric and potentiometric methods
• Example:
Magnesium sulphate directly titrated with EDTA solution using mordant black-II
as an indicator.
Back Titration
• Many
metals cannot be titrated directly for various reasons, may precipitate from
the solution in the pH range necessary for the titration or may form inert
complexes or a suitable metal indicator is not available
• In
such cases an excess of standard EDTA solution is added, the resulting solution
is buffered to the desired pH
• Excess
of the EDTA is back-titrated with a standard metal ion solution
• A
solution of zinc chloride or sulphate and magnesium chloride or sulphate is
often used for this purpose
• Example:
Determination of Mn. This cannot be directly titrated with EDTA because of
precipitation of Mn(OH)2.
An excess of known volume of EDTA added to an acidic solution of Mn salt
and then ammonia buffer is used to adjust the PH to 10. Excess EDTA is back
titrated with a standard Zn solution using Eriochrome black -T as indicator.
Replacement or
Substitution Reaction
• Substitution
titrations may be used for metal ions that do not have sharp end point.
• Metal
may be determined by the displacement of an equivalent amount from a less stable EDTA complex.
Example: Titration of
calcium
• An
excess Mg-EDTA chelate is added to ca solution. Ca quantitatively displaces Mg
form Mg-EDTA chelate. This displacement takes place because ca forms a more
stable complex with EDTA.
• Free
Mg metal is directly titrated with standard EDTA solution.
Alkalimetric
Titration
• It
is used for the determination of ions such as anions ,which donot react with
EDTA chelate
• Protons
from disodium EDTA are displaced by a heavy metal
• Liberated
protons can be titrated with a standard
solution of sodium hydroxide using an acid-base indicator or a potentiometric
end point
• Alternatively,
an iodate-iodide mixture is added as well as the EDTA solution and
• Liberated
iodine is titrated with a standard thiosulphate solution using starch solution
as indicator.
• Solution
of the metal to be determined must be accurately neutralized before titration
• It
is often a difficult to account on the hydrolysis of many salts and constitutes
a weak feature of alkalimetric titration
Metal Ion Indicators
• Success of an EDTA titration depends upon the precise determination of the end point
• Most common procedure utilises metal ion indicators
Requisites of a metal ion indicator for use in the visual detection of end points include:
(a) Colour reaction must be before the end point, when nearly all the metal ion is complexed with EDTA, the solution is strongly coloured.
(b) Colour reaction should be specific or selective.
(c) Metal-indicator complex must possess sufficient stability,Otherwise, due to dissociation, a sharp colour change is not attained
(d)Metal-indicator complex must be less stable than the metal-EDTA complex to ensure that, at the end point EDTA removes metal ions from the metal indicator-complex
(e)Change in equilibrium from the metal indicator complex to the metal-EDTA complex should be sharp and rapid
(f) Colour contrast between the free indicator and the metal-indicator complex should be readily observed
(g) Indicator must be very sensitive to metal ions (i.e. to pM) so that the colour change occurs as near to equivalence point as possible
(h)Above requirements must be fulfilled within the pH range at which the titration is performed
Theory of Metal Ion Indicators
• Use of a metal ion indicator in EDTA titration may be written as
M-In + EDTA ———–à M-EDTA + In
• This reaction will proceed if the metal-indicator complex M-In is less stable than the metal-EDTA complex M-EDTA
• Former dissociates to a limited extent, andDuring the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In To leave the free indicator (In)
• The stability of the metal-indicator complex may be expressed in terms of the formation constant (or indicator constant)
• KIn = (M-In)/(M)(In)
• Indicator color change is effected by hydrogen ion concentration
Masking and
Demasking Agents
• EDTA
is a very unselective reagent because it complexes with numerous doubly, triply
and quadruply charged cations
• When
a solution containing two cations which complex with EDTA is titrated without
the addition of a complex-forming indicator
• Then
ratio of the stability constants of the EDTA complexes of the two metals M and
N must be such that KM/KN >106 If N is not to interfere with the
titration of M
• Constants
KM and KN considered in the above expression should be
the apparent stability constants of the complexes
• If
complex-forming indicators are used, then for a similar titration error KM/KN
> 108
The following procedures will help to increase the selectivity:
(a) Suitable control of the pH of the solution
• Makes
use of the different stabilities of metal-EDTA complexes
• Bismuth
and thorium can be titrated in an acidic solution (pH = 2) with xylenol orange
or methyl thymol blue as indicator and most divalent cations do not interfere
• A
mixture of bismuth and lead ions can be successfully titrated by first
titrating the bismuth at pH 2 with xylenol orange as indicator, and then adding
hexamine to raise the pH to about 5 and then titrating the lead.
(b) Use of masking agents
• Masking
may be defined as the process in which a substance, without physical separation
of it or its reaction products, it is so transformed that it does not enter
into a particular reaction
• Demasking
is the process in which the masked substance regains its ability to enter into
a particular reaction
• By
the use of masking agents, some of the cations in a mixture can often be
‘masked’ so that they can no longer react with EDTA or with the indicator
• An
effective masking agent is the cyanide ion
• This
forms stable cyanide complexes with the cations of Cd, Zn, Hg(II), Cu, Co, Ni,
Ag and platinum metals but not with the alkaline earth metals like manganese
and lead.
(c) Selective demasking
• Cyanide
complexes of zinc and cadmium may be demasked with formaldehyde-acetic acid
solution or better with chloral hydrate
• Use
of masking and selective demasking agents permits the successive titration of many
metals
A solution containing Mg, Zn, and Cu can be titrated as
follows:
1. Add excess of standard EDTA and back-titrate with
standard Mg solution using solochrome black as indicator gives the sum of all
the metals present
2. Treat an aliquot portion with excess of KCN (Poison !)
and titrate as before
This gives Mg only
3. Add excess of chloral hydrate (or of formaldehyde-acetic
acid solution, 3:1) To the titrated solution in order to liberate the Zn from
the cyanide complex, and Titrate until the indicator turns blue. This gives the
Zn only. Cu content may then be found by difference
SUMMARY
• Complexometric
titration is a type of titration based on complex formation between the
analyte and titrant.
• Complexometric
titrations are particularly useful for determination of a mixture of different
metal ions in solution
• An
indicator with a marked color change is usually used to detect the end-point of
the titration
• In
practice, the use of EDTA as a titrant is well established
• The
most useful complex-formation reactions for titrimetry involve chelate
formation
• Ethylenediamine
tetra acetic acid, has four carboxyl groups and two amine groups that can act
as electron pair donors, or Lewis bases
• Usually
an organic dye such as Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B
or Murexide used as indicators.
• Indicators
form complexes with specific metal ions, which differ in colour from the free
indicator and produce a sudden colour change at the equivalence point
• Contain
types of chelate groupings and generally possess resonance systems typical of
dyestuffs
• Types
of complexometric titrations
Direct
titration
Back
titration
Replacement of one complex by another
Alkalimetric titration of metals
• Masking
may be defined as the process in which a substance, without physical separation
of it or its reaction products.
• Demasking
is the process in which the masked substance regains its ability to enter into
a particular reaction