Complexometric TITRATION FOR PG IST SEM

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COMPLEXOMETRIC TITRATIONS
Dr .C.Sridhar
Head of the dept.
Pharmaceutical analysis

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•INTRODUCTION:
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.

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 Definition:
 Complexometric titration is the
volumetric method of analysis in which
the simple metal ion is converted into
complex by addition of reagent known as
ligand or complexing agent and is used to
estimate polyvalent ions i.e., divalent,
trivalent.
 E.g.:-

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Some of the basic considerations
 Complexometric titrations are particularly useful for the
determination of a mixture of different metal ions in solutions.
 If Complex formed is water soluble and stable those Water
soluble complexes are called sequestering agent.
 If a complexing agent can form more than one bond with
polyvalent ion, then it is considered as polydentate and called
as chelating agent.
 The difference between sequestering agent and chelating agent
is that the sequestering agent remains in the solution in the
chelated condition where as chelating agent forms an insoluble
complex with the metal ion.
 In all EDTA Complexometric reactions the ratio of EDTA to
the metal ion is 1:1
 E.g.; dimethylglyoxime and salicylaldoxime are e.g. for
chelating agent where as EDTA is an e.g. for sequestering
agent.

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TYPES OF COMPLEXOMETRIC
TITRATIONS
1. Direct titration
2. Back titration
3. Replacement of one complex
by another
4. Alkalimetric titration of metals

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Direct titration
A standard solution is added directly to the solution of the
substance being determined are referred to as a direct titration.
 The solution containing the metal ion to be determined is
buffered to the desired pH and few drops of indicator is added.
 The contents are titrated against standardized disodium edetate
solution till the end point shown by the colour change.
 A blank titration is carried out omitting the substance to be
determined, but contains all the other solutions like buffer &
indicator.
 The volume of edetate consumed in blank titration is
substracted from that obtained in the original estimation
Examples of such estimation:
Bismuth - Nitrate, carbonate, oxynitrate, sub nitrate
Calcium - Chloride, gluconate, lactate, carbonate,
Magnesium - Carbonate, oxide, stearate, sulphate,

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1. Assay of magnesium sulphate
Principle:
 Magnesium sulphate forms stable complex with disodium edetate in presence of
strong ammonia-ammonium chloride buffer.
 It is assayed by direct titration method.
 Mordant black mixture is used as indicator for detection of end point {red to blue}
and standard 0.05 M disodium edetate is employed as titrant.
Mg2+ + EDTA → Mg EDTA
Materials required:
 Standard disodium edetate 0.05 M, Magnesium sulphate sample (0.4 g),
 Ammonia-ammonium chloride buffer (10ml), Mordant black mixture as indicator.

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 Assay:
 Weigh accurately about 0.3 g magnesium sulphate and
dissolve in 50 ml of water. Add 10 ml of strong ammonia-
ammonium chloride solution, and titrate with 0.05M disodium
ethylenediaminetetraacetate using 0.1 g of mordant black II
mixture as indicator, until the pink colour is discharged from
the blue.
Equivalent factor:
0.006019g Mg SO4 ≡ 1 ml 0.05M disodium EDTA

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 2. Assay of calcium carbonate:
 Principle:
 This is based on the principle of complexometric
titration. Disodium edetate is used as sequestering
agent. Dil HCl is used in order to dissolve calcium
carbonate in water because as such it is insoluble in
water.
 Acid will generate Co2 which is formed as follows.
 2HCl + CaCo3 → CaCl2 + H2CO3
 H2CO3 → H2O +CO2
 Ca CO3+H20+CO2→ Ca (HCO3)2
Insoluble Soluble

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PROCEDURE
 Weigh accurately about 0.1g and dissolve in 3ml of
dilute hydrochloric acid and 10 ml of water. Boil for
ten minutes, cool, dilute to 50 ml with water. Titrate
with 0.05 M disodium ethylenediamine tetra acetate
to within a few ml of the expected end-point. Add
NaOH and a mixture of calcon and anhydrous sodium
sulphate as indicator and continue the titration until
the colour changes to a full blue colour.
 Each ml of 0.05M disodium ethylenediamine tetra
acetate is equivalent to 0.005004 g of CaC03.

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Back titration
Determinations involving back titrations consist
in the addition of excess of a standard
volumetric solution to a weighed amount of
sample , and determination of the excess not
required by the sample hence the amount of
volumetric solution used by the substance is
determined.

13.  In general this method is used for :
1. Volatile substances eg: ammonia
2. Insoluble substances eg: calcium
carbonate
3. Substances which need excess of reagent
eg: lactic acid
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Example
 Alum: KAl (S04)2. 12H20
 Aluminum Hydroxide gel,
dried- aluminum hydroxide gel
 Calcium phosphate

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PROCEDURE:
 Weigh accurately about 80mg of sample.
 To the sample add 45ml of water,0.2g of
sodium chloride and 20ml ethanol.
 Heat to boiling and add 0.05M lead nitrate,
intially dropwise and then more rapidly, with
constant stirring.
 Heat to coagulate the precipitate, cool to room
temperature.
PROCEDURE FOR ASSAY OF ALUM

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 Filter and wash the residue with small
volumes of alcohol.
 To the filtrate and washings, add
hexamine(1g) and titrate the excess lead
nitrate with 0.05M disodium edetate using
xylenol orange solution as indicator.
 The end point is yellow.

17. Assay of aluminium hydroxide gel
Procedure:
 Weigh accurately about 5g of the substance in
a 100ml of conical flask, add 3ml of 6M HCL .
 Dissolve the suspension by a slight warming
on a water bath.
 Cool to room temperature and transfer to a
100ml volumetric flask and make up to the
mark with water.
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18.  Pipette out 25ml of this solution in to a 250ml
conical flask and add 40ml of the standard
0.05MEDTA solution .
 Dilute with 80ml of water and add 2-3 drops of
methyl red indicator
 Add 1M NaOH to neutralise the mixture as will
be recognised by change of colour of the
indicator from red to yellow.
 This solution is warmed on a water bath for
30mns then add 3g of hexamine and xylenol
orange as indicator,the titration mixture becomes
yellow in colour. 18

19.  Now the mixture is titrated with a
standard 0.05M lead nitrate solution.
 End point is violet colour.
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APPLICATIONS
 Determination of water hardness
 Determination of metal ions
 Complexometric titrations are widely
used in the medical industry.
 The traces of zinc in water can be
determined with complexometric titration

21. REFERENCES
 Pharmaceutical Analysis by P C Kamboj
 Practical pharmaceutical chemistry by
A.H.Beckett,J.B.Stenlake
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