Complexometric titrations by Gunja Chaturvedi

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this articles gives entire concepts of complexometry
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   Complexometric titrations OBJECTIVES: 1.   Definition2.   Requirements for carrying out a complexometric titration3.   Ligands:- Types-   Characteristics of ligands-   Chelating agents-   Sequestering agents4. Complexometric titrations:-   Principle-   Factors affecting the end point determination-   pM (metal ions buffer)-   Methods of end point detection-   Visual indicators  –  examples and mechanism of indicatoraction-   Types of complexometric titrations-   Titration selectivity, Masking and Demasking agents5. Applications of complexometric titrations6.   References  2 DEFINITION It is basically the titrations using complexation reactions with a metal ion which involvesthe replacement of one or more of the coordinated solvent molecules by other nucleophilicgroups. And these groups bound to the central ions are called as LIGANDS. REQUIREMENTS FOR CARRYING OUT A COMPLEXOMETRICTITRATIONSLIGANDS: The molecules or ions which displace the solvent molecules are called Ligands And themaximum number of groups that can be bound to an ion is its coordinationnumber .Ligands or complexing agents or chelating agents can be any electron donatingentity, which has the ability to bind to the metal ion and produce a complex ion. Anexample of a complexation reaction between Cu (II) ion and four ammonium molecules inan aqueous solution may be expressed by the following equation: BONDING IN COMPLEXES: The bonds are either ordinary covalent bonds in which the metal and the ligand contributeone electron each, or co-ordinate bonds in which both electrons are contributed by theligand. Thus, the hexacyanoferrate ion may be considered to consist of three ordinarycovalent bonds and three co-ordinate bonds, although in the complex the bonds are identicalhybrid bonds which have been shown to be directed towards the apices of a regularoctahedron.   The negative charge on the complex ion is equal to the total number of the negative groups minus the valency of the metal ion. When neutral groups only are involved, the charge onthe complex is positive and is equal to the metal ion, e.g. [Cu (NH 3 ) 4 ] 2+ .  3 CLASSIFICATION OF LIGANDS 1.   Unidentate Ligands: Ligands that are bound to metal ion only at one place are called unidentate ligands (one toothed) . NH 3 , for example, is a unidentate ligand capable of complexing with cupric ions. Halide ions, cyanide ions and NH 3 are common examplesof unidentate ligands. The formation of complex Cu (NH 3 ) 42+ proceeds in the followingsteps: 2. Bidentate and Multidentate Ligands: Many ligands are known that contain more thanone group, capable of binding with metal ions. Such ligands are known as multidentate ligands or chelating agents. They include bidentate ligands (2 donaratoms), tridentate ligands (3 donar atoms), quadridentate ligands, etc.Thus, ethylenediamine is an example of bidentate ligand. H 2 N-CH 2 -CH 2 -NH 2  Ethylene diamine tetra acetic acid (EDTA) is an example of multidentate ligand. COORDINATION COMPOUND OR CHELATE COMPOUNDS : Complexes involving simple ligands, i.e., those forming only one bond are described as co-ordination compound . A complex of a metal ion with 2 or more groups on a multidentateligand is called a chelate or a chelate compound . There is no fundamental differencebetween co-ordination compound and a chelate compound except that in a chelatecompound, ring influence the stability of compound. Thus, a chelate can be described as aheterocyclic ring structure in which a metal atom is a member of ring. The stability of achelate is usually much greater than that of corresponding unidentate metal complex. CHELATING AGENTS: Ligands having more than one electron donating groups arecalled chelating agents . The most effective complexing agent in ligands are amino andcarboxylate ions. All the multidentate ligands important in analytical chemistry contain thestructure component as follows:    4 The solubility of metal chelates in water depends upon the presence of hydrophilic groupssuch as COOH, SO 3 H, NH 2 and OH. When both acidic and basic groups are present, thecomplex will be soluble over a wide range of pH. When hydrophilic groups are absent, thesolubilities of both the chelating agent and the metal chelate will be low, but they will besoluble in organic solvents. The term SEQUESTERING AGENT is generally applied tochelating agents that form water-soluble complexes with bi- or poly-valent metal ions.Thus, although the metals remain in solution, they fail to give normal ionic reactions.Ethylenediaminetetra-acetic acid is a typical sequestering agent, whereas,dimethylglyoxime and salicylaldoxime are chelating agents, forming insoluble complexes.As a sequestering agent, ethylenediaminetetra-acetic acid reacts with most polyvalent metalions to form water-soluble complexes which cannot be extracted from aqueous solutionswith organic solvents. Dimethylglyoxime and salicylaldoxime form complexes which areinsoluble in water, but soluble in organic solvents; for example, nickel dimethylglyoximehas a sufficiently low solubility in water to be used as a basis for gravimetric assay.EDTA forms chelates with nearly all metal ions and this reaction is the basis for generalanalytical method for these ions by titration with a standard EDTA solution. Such titrationsare called complexometric or chilometric or EDTA titrations PRINCIPLE OF COMPLEXOMETRIC TITRATION Many principles of acid-base titrations are used in complexometric titration. Incomplexometric titration, the free metal ions disappear as they are changed into complexions. In acid-base titrations, the end point is marked by sudden change in pH. Similarly, inEDTA titration, if we plot pM (negative log of metal ion concentration) v/s volume of titrant, we will find that at the end point, the pM rapidly increases (Fig. 1). This sudden pMraise results from removal of traces of metal ions from solution by EDTA.Any method, which can determine this disappearance of free metal ions, can be used todetect end point in complexometric titrations. End point can be detected usually with anindicator or instrumentally by potentiometric or conductometric (electrometric) method.
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