COPLEXOMETRIC TITRATION
A complexing agent is an electron donating ion or molecule,called a ligand, which by it's ability to form one or more covalent bonds with the metal ion, produces a complex, which has different properties from those of the metal ion.thus the metal may not be precipitated from the complex by the usual metal ion precipitants. The stability of the complex ion varies and greater the stability, more marked will be the differences in properties, from of the original cations.
Bonding in Complexes :
The bonds are either ordinary covalent bonds, in which both the metal and the ligand contribute one electron each or co-ordinate bonds in which both electrons are contributed by the ligand.
Chelating agents :
Complexes involving simple ligands, that is those forming only one bond, are described as 'co-ordinate compounds'. Ligands having more than one electron donating group are called 'chelating agents'.
Many organic compounds will chelate metals if they contain groups with an easily replaceable proton ( - COOH, phenolic and enolic-OH) or neutral groups offering a lone pair of electrons ( NH2, CO and alcoholic OH) and the structure of the molecules are such as to permit the formation of stable rings. The greater the number of rings which can be formed; the chelate is likely to be more stable. Most rings formed in chelates involve the highest valency state of a metal, since these are more stable than those involving lower valency states.
HOOC-CH2
CH2COOH
Types of EDTA titrations -
a) Direct titration -
The solution containing the metal ion to be determined is buffered to the desired pH (usually pH-I0) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal ion by the addition of some auxiliary complexing agent such as tartrate, citrate or triethanolamine.
b) Back titraton -
Many metals cannot be titrated directly as they may precipitate from the solution in the necessary pH range or they may form inert complexes or a suitable metal indicator is not available. In such cases, an excess of standard EDTA is added, the resultant solution is buffered to the desired pH and the excess of EDTA is back titrated with a standard metal ion solution e.g. a solution of Zinc chloride / sulphate or Magnesium chloride / sulphate is often used for this purpose. The end point is detected with the aid of metal indicator, which responds to the Zinc or Magnesium ions.
c) Replacement or substitution titration -
Substitution titration may be used for metal ions, that do not react (or react unsatisfactorily) with a metal indicator. The metal cation to be determined may be treated with the Magnesium complex of EDTA, when Magnesium ion is set free equivalent to the cation present and can be titrated with a standard solution of EDTA and a suitable metal indicator.
Metal ion indicators -
Bonding in Complexes :
The bonds are either ordinary covalent bonds, in which both the metal and the ligand contribute one electron each or co-ordinate bonds in which both electrons are contributed by the ligand.
Chelating agents :
Complexes involving simple ligands, that is those forming only one bond, are described as 'co-ordinate compounds'. Ligands having more than one electron donating group are called 'chelating agents'.
Many organic compounds will chelate metals if they contain groups with an easily replaceable proton ( - COOH, phenolic and enolic-OH) or neutral groups offering a lone pair of electrons ( NH2, CO and alcoholic OH) and the structure of the molecules are such as to permit the formation of stable rings. The greater the number of rings which can be formed; the chelate is likely to be more stable. Most rings formed in chelates involve the highest valency state of a metal, since these are more stable than those involving lower valency states.
The solubility of
metal chelates in water depends upon the presence of hydrophilic groups such as
COOH, SO3 H,NH2 and OH. When both acidic and basic groups
are present, the complex will be soluble over a wide range of pH. When
hydrophilic groups are absent, the solubilities of both the chelating agent and
the metal chelate, will be low; but they will be soluble in organic solvents.
The term 'sequestering agent', is generally applied to chelating agents, which
form water soluble complexes with bi or polyvalent metal ions. Thus, although
the metal remains in solution, they fail to give normal ionic reactions.
Ethylenediamine tetra-acetic acid (EDTA) is a typical sequestering agent and
can be represented as.
HOOC-CH2 CH2COOH
N-CH2-CH2-N
Types of EDTA titrations -
a) Direct titration -
The solution containing the metal ion to be determined is buffered to the desired pH (usually pH-I0) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal ion by the addition of some auxiliary complexing agent such as tartrate, citrate or triethanolamine.
b) Back titraton -
Many metals cannot be titrated directly as they may precipitate from the solution in the necessary pH range or they may form inert complexes or a suitable metal indicator is not available. In such cases, an excess of standard EDTA is added, the resultant solution is buffered to the desired pH and the excess of EDTA is back titrated with a standard metal ion solution e.g. a solution of Zinc chloride / sulphate or Magnesium chloride / sulphate is often used for this purpose. The end point is detected with the aid of metal indicator, which responds to the Zinc or Magnesium ions.
c) Replacement or substitution titration -
Substitution titration may be used for metal ions, that do not react (or react unsatisfactorily) with a metal indicator. The metal cation to be determined may be treated with the Magnesium complex of EDTA, when Magnesium ion is set free equivalent to the cation present and can be titrated with a standard solution of EDTA and a suitable metal indicator.
Metal ion indicators -
The success of EDTA
titration depends upon the precise determination of the end point. The
requisites of metal ion indicators for use in visual detection of end point
include
a) The colour
reaction must be such that before the end point, when nearly all metal ion is
complexed with EDTA, the solution is strongly coloured.
b) The colour
reaction should be specific or at least selective.
c) The metal
indicator complex must possess sufficient stability, otherwise because of
dissociation, a sharp colour change is not obtained. The metal indicator
complex must however be less stable then the metal-EDTA complex to ensure that
at the end point EDTA removes metal ion from the metal indicator complex. The
change in equilibrium from the metal indicator complex to the metal-EDTA
complex, should be sharp and rapid.
d) The colour
contrast between the free indicator and the metal-indicator complex, should be
such as to be readily observed.
e) The
indicator must be very sensitive to metal ions, so that the colour change
occurs as near to the equivalence point as possible.
f) The above
requirements must be fulfilled within the pH range at which the titration is
performed.
Some examples of metal ion indicators -
a) Murexide -
This
is the Ammonium salt of purpuric acid and it was probably the first metal ion
indicator, to be employed in the EDTA titration. Murexide solutions are reddish
violet upto pH = 9, violet from pH -9 to 11 and blue above pH - 11. These
colour changes are due to progressive displacement of protons from imido
groups.
Murexide forms
complexes with many metal ions, only those with Cu, Ni, Co, and Ca are sufficiently
stable to find application in analysis. Their colour in alkaline solution are
orange (Cu), yellow (Ni & Co) and red (Ca); the colours vary some what with
the pH of the solution.
Aqueous solutions of Murexide are unstable and
must be prepared every day. Normally, it is better to prepare a mixture of the
indicator with pure Sodium chloride in the ratio (1:500) and employ 0.2 to 0.4
g in each titration.
b) Solochrome
black ( eriochrome black T ) - This is the most commonly used metal
indicator for EDTA titration and the colour can be observed with the ions of
Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni and Pt metals. To maintain the
pH constant at the value 10, a buffer mixture is added and most of the above
metals must be kept in solution with the aid of a weak complexing agent such as
Ammonia or Tartrate. The cations of Cu, Co, Ni, Al, Fe (III), Ti (IV) and Pt,
form such stable indicator complexes, that the dyestuff can no longer be
liberated by adding EDTA, making direct titration with EDTA impracticable.
However, with Cu, Co, Ni, Al, a back titration can be carried out, for the rate
of reaction of the EDTA complexes with the indicator is slow and it is possible
to titrate the excess of EDTA with standard Zinc or Magnesium ions.
Cu,
Ni, Co, Cr, Fe or Al even in traces, must be absent when conducting a direct
titration with other metals; if the metal ion to be titrated does not react
with cyanide ion or with triethanolamine; these substances can be used as
masking reagents. The addition of 0.5 to 1 ml of 0.001 M o-phenanthroline prior
to the EDTA titration eliminates the blocking effect of these metals with
Solochrome black and also with Xylenol orange.
c) Patton
and Reeder's indicator - Its main use is in the direct titration of Calcium,
particularly in the presence of Magnesium. A sharp colour change from wine red
to pure blue is obtained, when Calcium ions are titrated with EDTA at pH values
between 12 and 14.
The
dyestuff is thoroughly mixed with 100 times its weight of Sodium sulphate
and 1 g of the mixture is used in each
titration. The indicator is not very stable in alkaline solution.
d) Solochrome dark blue or Calcon -
This is sometimes referred as Eriochrome blue black RC and is used in the
titrations of Calcium in the presence of Magnesium, which must be carried out
at a pH 12.3, in order to avoid the interference with Magnesium. Magnesium is
precipitated as Magnesium hydroxide and the colour change for Calcium is from
pink to pure blue.
e) Xylenol
orange - Direct EDTA titrations of Bi, Th, Zn, Cd, Pb, Co, etc. are readily
carried out and the colour change is sharp. By appropriate pH adjustment
certain pairs of metal may be titrated, in a single sample solution. Thus
Bismuth may be titrated at pH = 1 to 2 and Zinc or lead after adjustment to
pH-5 by addition of Hexamine .
f) Calmagite
-An important advantage of this indicator is that its aqueous solution is
very stable and the colour change is somewhat clearer and sharper.
g) Fast
sulphon black F -This indicator is virtually specific for Cu ions. The
presence of Ammonia or pyridine is required for colour complex with Copper to
form.
h) Bromopyrogallol
red -This indicator possesses acid-base indicator properties and is
resistant to oxidation. It forms coloured complex with many cations e.g.
Bismuth at pH- 2 to 3 in Nitric acid solution, has a colour change from blue to
claret red.
i) Thymolphthalein
complexone (Thymolphthalexone) -This indicator contains a stable lactone ring and reacts only in
an alkaline medium. The indicator may be used for Calcium, where the colour
change is from blue to colourless. Manganese and Nickel may be determined by
adding an excess of standard Calcium chloride solution; the colour change is
from very pale blue to deep blue.
j) Zincon -This
indicator is specific for Zinc at pH 9 to 10. It can also be used as an indicator for the titration of
Calcium in the presence of Magnesium.
Some practical considerations -The following points must be borne in mind, while carrying out
complexometric titrations.
a) Adjustment
of pH - For many EDTA titrations, the pH of the solution is extremely
critical; often limits of + 1 unit of pH must be achieved for a
successful titration. Whenever a buffer solution is added, it must be ensured
that the requisite buffering action is in fact achieved. Hence, it is necessary
to make certain that the original solution has first been made almost neutral
by cautious addition of Sodium hydroxide or Ammonium hydroxide. When acid
solution containing the metallic ion is neutralised by the addition of alkali,
care must be taken to ensure that the metal hydroxide is not precipitated.
b) Concentration
of metal ion to be titrated - Most titrations are successful with 0.25
millimole of the metal ion in a volume of 50 to 150 ml of solution. If the
concentration of metal ion is too high, then the end point may be difficult to
judge.
c) Amount
of indicator - The addition of too much indicator is a fault which must be
guarded against; as end point anticipation, which is of great practical value;
may be virtually lost if too much of indicator is added; as the colour is too
intense. In general a satisfactory colour is obtained by the use of 30 to 50 mg
of a solid mixture.
d) Attainment
of the end point - In many EDTA titrations, the colour change in the
neighbourhood of the end point is very slow; hence cautious addition of the
titrant, coupled with continuous stirring of the solution is recommended.
e) Detection
of colour change - The detection of the end point of the titration is
dependent upon the recognition of a specific change in colour and for many
observers affected by colour blindness, this may be difficult. This difficulty
may be overcome by replacing the human element with a colourimeter or a
spectrometer.
