7.complexometric titrations

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 I norganic Chemistry Year: Pharm -D 1 st 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, NH 2 , 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 .

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