Learning objectives Introduction Complexing agents Complexing Titration using EDTA Need for Maintenance of pH pH Indicators used in complexometric Titrations Types of EDTA Titration Factors Influencing EDTA reaction Masking and demasking agents Conclusion Reference

1. COMPLEXOMETRIC TITRATIONS
G. Nikitha, M.Pharmacy
Assistant Professor
Department of Pharmaceutical Chemistry
Sree Dattha Institute of Pharmacy
Hyderabad
Subject: Pharmaceutical Inorganic Chemistry
Year: Pharm-D 1st Year

2. CONTENTS
 Learning objectives
 Introduction
 Complexing agents
 Complexing Titration using EDTA
 Need for Maintenance of pH
 pH Indicators used in complexometric Titrations
 Types of EDTA Titration
 Factors Influencing EDTA reaction
 Masking and demasking agents
 Conclusion
 Reference

3. LEARNING OBJECTIVES
In these topic we will discuss about
 Introduction of Complexometric Titration
 Complexing agents
 Complexing Titration using EDTA
 Need for Maintenance of pH
 pH Indicators used in complexometric Titrations
 Types of EDTA Titration
 Factors Influencing EDTA reaction
 Masking and demasking agents

4. INTRODUCTION
 Complexometric titration is a form of volumetric titration in which the
formation of a colored complex is used to indicate the end point of a titration.
 The complexes are formed by the reaction of a metal ion ( an acceptor, a central
atom or a cation) with an anion, a neutral molecule or very rarely a positive ion.
 Comlexometric Titrations have been similar to acid base titrations. In acid base
titration acid has been specified which donates the protons or accepts the
electrons.
 In complexometric titration metal ion accept the electrons and the species which
donates them is termed as ligand

5. COMPLEXING AGENTS
The complexing agents are of different type which depends up on the functional
group present.
1. They can be neutral molecules with lone pair of electron. Such electron donors
such as S, O, N, halogens, groups like COOH, NH2, CN-
2. Groups with easily replaceable proton- COOH, Phenolic and enolic OH.

6. COMPLEXING TITRATION USING EDTA
Structure of EDTA: Ethylenediamine tetra-acetic acid
EDTA is the most commonly used complexing agent which forms the complex
with divalent, trivalent or tetravalent ions to form complexes. Irrespective of
the valency of the ion it forms 1:1 complex with ions. The number of rings
formed depends upon the valency of the ion.

7. NEED FOR MAINTENANCE OF PH
pH maintenance is required for two reasons.
1. pH affects the stability as well as the formation of the complex with metal ion.
2. Up on pH 10 pM increases with pH. The colour of the indicator metal ion
complex depends upon the pM which in turn depends on pH.

8. PH INDICATORS USED IN COMPLEXOMETRIC
TITRATIONS OR METAL ION INDICATORS
Metal ions under test are titrated with a standard solution of EDTA disodium salt as
a titrant. In order to locate the end point of a reaction, indicator are used.
Indicators which are used in complexometric titration are called metal
indicators or metallochrome indicators. They give once colour in presence of
metal ions and a different colour in the absence of metal ions or in a free state.
Solochrome black T, modrant black T, Variamine blue, murexide etc. are some
indicators which are used in titartions
The most commonly used indicators are
1. Mordant Black
2. Xylenol orange
3. Murexide
4. Calcon mixture

9. 1. Mordant Black:
Free indicator is blue in colour (pH 10). On complexation with metal ions a pink
colour is formed. Below pH 6.3 and above 11.5, it is reddish in colour. Hence it
is used at about pH 10. It is used in the estimation of metal ions like calcium,
magnesium, zinc, cadmium, manganese, lead and mercury.
2. Xylenol orange:
It has yellow colour in acid solution and red colour in alkaline solution. The metal
complexes are red in colour. It is used in the estimation of aluminum hydroxide,
titanium dioxide.

10. 3. Murexide:
It is used in the estimation of calcium in the presence of magnesium. This is
because magnesium murexide complex is less stable than calcium murexide
complex.
4. Calcon mixture:
It is sodium 2-hydroxide-(2-hydroxy-1-naphthylazo) napthalene-4-sulponate. It is
otherwise called as solochrome dark blue. It is used in the assay of calcium
carbonate and calcium chloride.

11. TYPES OF EDTA TITRATION
Types of EDTA Titrations are
1. Direct Titration
2. Back Titration
3. Replacement of one complex by another
4. Alkalimetric titration of metal

12. 1. Direct Titration:
To a weight amount of the substance to be estimated, measured volume of suitable
buffer solution 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 and indicator. The volume of edetate
consumeed in blank titration is subtracted from that obtained in the original
estimation.
Examples of such estimation:
Bismuth- Nitrate, carbonate, oxynitrate, subnitrate
Calcium- Chloride, gluconate, lactone
Magnesium- carbonate, oxide, stearate, sulphate, trisilicate
Zinc- sulphate, oxide

13. Preparation and standardization of 0.05 M EDTA :
18.62g of disodium EDTA is accurately weighed and dissolved in sufficient water
to make 1 litre.
About 200mg of calcium carbonate is first of all accurately weighed. It is then
transferred to a suitable container and sufficient water and dil HCl are added to
dissolve calcium carbonate. It is diluted with water to 50ml. To this 15ml of
NaOH and 300mg of hydroxy napthol blue as indicator are added and titrated
with disodium EDTA solution until the solution becomes deep blue in colour

14. 2. Back Titration:
Back Titration is necessary in the following cases.
 Metals which are precipitated as hydroxides in the pH used.
 For insoluble substances like lead sulphate, calcium oxalate.
 Those react slowly with disodium acetate
Method
In these method a known excess of disodium edetate, buffer solution and few drops
of indicator is added. The complexation is favoured by heating then cooled and
the excess edetate is back titrated against magnesium sulphate or zinc sulphate.
Example:
Aluminum hydroxide gel, calcium Phosphate

15. 3. Replacement of one complex by another:
When sharp end point is not obtained by direct or back titration, this method is
followed. In these method the metal ion determined by displacement of an
equivalent amount of magnesium or zinc from less stable edetate complex. The
liberated ions are titrated against edetate.
Example:
Cadminum, lead and mercury can be determined by these method.
4. Alkalimetric titration of metal:
When a polyvalent ion is added to a complex protons are liberated. The acidity is
titrated with an alkali.

16. FACTORS INFLUENCING EDTA REACTION
The following factors are known to influence the complexation of the metal ion and
EDTA:
1. Activity of Metal ion
2. The pH at which reaction is performed
3. Solvents in which reaction is performed. Organic solvents are known are known
to influence the stability of complex.

17. MASKING AND DEMASKING AGENTS
Masking and demasking agents are those which precipitate or complex ions are
selectively in order to estimate a specific ion.
Need for masking or demasking agents
1.EDTA as well as its salt disodium edetate forms complexes with several metal
ions. During estimation of specific ion other ionic impurities are also estimated.
This gives false values.
2. Sometimes when two or more ions are to be estimated in a mixture each ion has
to be selectively titrated.
For above reasons Masking and demasking agents are required.
This can be done by using any one of the three methods
1. Addition of Precipitants
2. Addition of Complexing agents
3. pH control.

18. CONCLUSION
In these topic we have discussed that
 Introduction of Complexometric Titration
 Complexing agents
 Complexing Titration using EDTA
 Need for Maintenance of pH
 pH Indicators used in complexometric Titrations
 Types of EDTA Titration
 Factors Influencing EDTA reaction
 Masking and demasking agents

19. REFERENCE
 Pharmaceutical Chemistry -Inorganic Volume-1 by G. R. Chatwal.
 Essentials of Inorganic Chemistry by Katja A. Strohfeldt.
 Indian Pharmacopoeia.
 M.L Schroff, Inorganic Pharmaceutical Chemistry.
 P. GunduRao, Inorganic Pharmaceutical Chemistry, 3rd Edition
 A.I. Vogel, Text Book of Quantitative Inorganic analysis.
 Bentley and Driver's Textbook of Pharmaceutical Chemistry.

20. THANK YOU