Complexometric Titrations – Pharmaceutical Analysis 1 B. Pharma 1st semester

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)

       KI= (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

Leave a comment