Mr.Deepak Pradhan Asst. Prof. Dept. of Pharmaceutical Chemistry GGSCOP, YNR Concept of complexation and chelation Werner’s theory Coordination number and electronic structure of complex ions Stability constants Titration curves Masking and Demasking agents Types of complexometric titration Metal ion indicators Factors influencing the stability of complexes EDTA-metal ion complexes Determination of hardness of water. Complexometric tirition is the titration of metal ion with a complexing agent and the end point is detected by electrically or using suitable a metal ion indicator. Complexes are compounds formed from combination of metal ions with ligands (complexing agents). A metal is an electron deficient species while a ligand is an electron rich, and thus, electron donating species. A metal will thus accept electrons from a ligand where coordination bonds are formed. Electrons forming coordination bonds come solely from ligands. Hence metal ion acts as Lewis acid (electron acceptor) and the ligand is used as a titrant or a complex forming agent which acts as a Lewis base (electron pair donor or a negatively charged group). Is a combination of metal ion with electron donating group or nucleophile to form a complex.----------------[Ag(CN)2] Cu2+ + 4NH3--------------.[Fe(SCN)6]3- . M2+ + nL -----------------[MLn]2+ e. Ag+ + 2CN.[Cu(NH3)4]2+ Fe3+ + 6SCN.g.--------------. . in this case. like EDTA. is called a chelate. When the ligand is a large organic molecule having two or more of the complexing groups. the ligand is called a chelating agent and the formed complex. :NH2CH2CH2H2N:) donates two pairs of electrons.A ligand is called a monodentate if it donates a single pair of electrons (like :NH3) While a bidentate ligand (like ethylenediamine. The ligand can be as simple as ammonia which forms a complex with Cu2+. for example. Ethylenediaminetetraacetic acid (EDTA) is a hexadentate ligand. giving the complex Cu(NH3)42+. Unidentate Ligands: Ligands that are bound to metal ion only at one place are called unidentate ligands (one toothed). Ex: Halide ions. cyanide ions and NH3 . for example. NH3. is unidentate ligand capable of complexing with cupric ions Cu(NH3)42+. Classification of Ligands 1. Bidentate and Multidentate Ligands: Many ligands are known that contain more than one group. tridentate ligands (3 donar atoms). H2N-CH2-CH2-NH2 . They include bidentate ligands (2 donar atoms). quadridentate ligands. Such ligands are known as multidentateligands or chelating agents.2. etc. capable of binding with metal ions. Thus. ethylene diamine is an example of bidentate ligand. Ligands having more than one electron donating groups are called chelating agents.e.. Thus. those forming only one bond are described as coordination compound. . i.Complexes involving simple ligands. The most effective complexing agent in ligands are amino and carboxylate ions. The stability of a chelate is usually much greater than that of corresponding unidentate metal complex. ring influence the stability of compound. a chelate can be described as a heterocyclic ring structure in which a metal atom is a member of ring. There is no fundamental difference between co- ordination compound and a chelate compound except that in a chelate compound. Werner’s theory • Werner (1891) first noticed that for each atom there were an observed maximum number of small groups which can be accommodated around it. . This number. and this is probably the driving force for complex formation. • Within the limits imposed by Werner’s co-ordination number. • Co-ordination number depends purely upon steric factors and is in no way related to the valency of the ion. which is called Werner’s co-ordination number. there is a tendency for the metal to attain or approach inert gas structure. Stability of Complex The use of term stability is general because the coordination compounds are stable in one reagent but dissociate or decompose in presence of another reagent. The stability of complex is expressed qualitatively in terms of i) Thermodynamic stability ii) Kinetic stability The general equation for the formation of a 1:1 chelate complex. MX. . It is also possible that term stability can be referred as an action of heat or light or compound. is M+X [MX] Where M is the metal ion and X the chelating agent. . Thermodynamic stability It is the measure of extent of formation or transformation of complex under given set of condition at equilibrium. Thermodynamic stability depends on the strength of bond between metal ligand. The most of complexes are highly stable but they are dissociated in aqueous solution to some extent. Term “Inert” for thermodynamically stable complexes and “labile” for reactive complexes. Kinetic stability It is referred to rate or speed at which a complex formation or transformation reaction proceeds at equilibrium. . It deals with the rate and mechanism of a chemical reaction. Time factors play an important role in deciding the kinetic stability of the complex. Number of Rings c. Chelate Effect a. Ligand Concentration 3. Effect of Substitution d. Ionic Size b. Ionic Charge 2. Ring Size b. Nature of the Ligand a.Factor affecting Stability of the complex 1. Macrocyclic effect . Size and Charge b. Basic Character c. Nature of the Central Metal Ion a. the small and highly charged cation can form more stable coordinate compounds because of most stable coordinate bonds.74Ao and Cd(II) 0. Nature of the Central Metal Ion a. Ionic Charge If the charge of the central metal ion is more and the size is small. then the stability of the complex is more. Ex: . In general. Ex: Zn(II) forms more stablecomplexes than Cd(II).The size of Zn(II) ion is 0. Ionic Size The stability of coordination compound (complex) decreases with increase in size of metal ion.97Ao b. More Size Less Stable Less Charge Small Size More Stable More Charge . Ligands with higher charge have small size and form more stable compounds. Size and Charge Ligands with less charge and more size are less stable and form less stable coordination compounds. Nature of the Ligand a. higher will be the stability of coordination compounds. . CN. Ex: Ligands like NH3. Basic Character Calvin and Wilson [13] suggested that the higher basic character or strength of the ligand. It is defined that a strong base or higher basic strength of the ligand means it forms more stable compounds or its donating tendency of electron to central metal ion is higher.b.etc have more basic character that means they form more stable compounds. c. SO3H. When both acidic and basic groups are present. whereas. NH2and OH. the complex will be soluble over a wide range of pH. . When hydrophilic groups are absent. but they will be soluble in organic solvents. Ligand Concentration The solubility of metal chelates in water depends upon the presence of hydrophilic groups such as COOH. forming insoluble complexes. dimethylglyoxime and salicylaldoxime are chelating agents. or poly-valent metal ions. the solubility of both the chelating agent and the metal chelate will be low. Ethylenediaminetetra-acetic acid is a typical sequestering agent. The term sequestering agent is generally applied to chelating agents that form water-soluble complexes with bi. Chelate Effect a. For chelate (saturated chelate) rings the following is the decreasing order of stability with increasing ring size . Ring Size The stability of chelate is depending on the size of chelate ring. It is found that 4-membered rings are unstable and rare than 5-membered rings which are common and stable. The stability of coordination complex increases with number of chelatering. (having identical donor sites). similarly an n-dentate macrocyclic ligand gives even more stable complexes than the similar n-dentate open chain ligand. This is called as macrocyclic effect . Effect of Substitution (Steric effect) The clashing of groups on two coordinated ligands will result in distortion of bond angles and a decrease in stability. As steric effect is decreasing. Number of Rings The stability of the complex goes up with an increase in the number of groups available for coordination. Increase in the number of rings increase the stability of compounds c. Macrocyclic effect Just as any n-dentate chelating ligand gives more stable complexes than n-unidentate ligand of comparable type.b. the stability of a complex is increasing d. . the free metal ions disappear as they are changed into complex ions. Principle of Complexometric Titration In complexometric titration. A standard plot was plotted by considering pM (negative log of metal ion concentration) v/s volume of titrant. End point can be detected usually with an Indicator or instrumentally by potentiometric or conductometric (electrometric) method. . This sudden pM raise results from removal of traces of metal ions from solution by EDTA. Titration curves . Types of complexometric titration • Direct Titration • Back Titration • Replacement Titration • Indirect Titration . Direct Titration Direct Titration: It is the simplest and the most convenient method used in chelometry.-calcium gluconate injection. E.g. This method is analogous to simple acid- base titrations. In this method. calcium lactate tablets and compound sodium lactate injection for the assay of calcium chloride Limitations: -slow complexation reaction -Interference due to presence of other ions . the standard chelon solution is added to the metal ion solution until the end point is detected. and the excess is back titrated with a standard solution of a second metal ion. excess of a standard EDTA solution is added to the metal solution. . This metal cannot be directly titrated with EDTA because of precipitation of Mn(OH)2.Back Titration In this method.g.Determination of Mn. An excess of known volume of EDTA is added to an acidic solution of Mn salt and then ammonia buffer is used to adjust the pH to 10 and the excess EDTA remaining after chelation. . E. is back titrated with a standard Zn solution kept in burette using Eriochrome blackT as indicator. which is to be analyzed. excess quantity of Mg EDTA chelate is added to Mn solution. The freed Mg metal is then directly titrated with a standard EDTA solution. Mn displaces Mg from Mn EDTA solution. the metal may be determined by the is placement of an equivalent amount of Mg or Zn from a less stable EDTA complex. By this method Ca. This displacement takes place because Mn forms a more stable complex with EDTA. Hg may be determined using Eriochrome blackT indicator. Pb. In this method.Replacement Titration In this method the metal. Mn quantitatively displaces Mg from Mg EDTA chelate. When direct or back titrations do not give sharp end points. which is to be analyzed. displaces quantitatively the metal from the complex. . It is used for the determination of ions such as anions. e. which do not react with EDTA chelate.Barbiturates do not react with EDTA but are quantitatively precipitated from alkaline solution by mercuric ions as 1:1 complex.g. . .Indirect Titration This is also known as Alkalimetric titration. Protons from disodium EDTA are displaced by a heavy metal and titrated with sodium alkali. when an equivalent amount of sodium EDTA has been added. therefore. As soon as there is the slightest excess of EDTA. remains that of the dye complex until the end point. the metal-dye complex decomposes to produce free dye. The colour of the solution. this is accomplished by a change in colour. The latter is different in colour from the dye itself and also has a low stability constant than the chelate-metal complex. Indicators The pM indicator is a dye which is capable of acting as a chelating agent to give a dye-metal complex. . It should form 1:1 complex which must be weaker than the metal chelate complex. The indicator should not compete with the EDTA. . Colour of the indicator and the metal complexed indicator must be sufficiently different. Metal indicators must comply with the following requirements- Compound must be chemically stable throughout the titration. Colour reaction should be selective for the metal being titrated. At the onset of the titration. Finally. the reaction medium contains the metal- indicator complex (MI) and excess of metal ion. indicator by I and chelate by EDTA. Mechanism of action of indicator Metal be denoted by M. Since the metal- indicator complex (MI) is weaker than the metal-EDTA chelate. The overall reaction is given by: . When EDTA titrant is added to the system. EDTA removes the last traces of the metal from the indicator and the indicator changes from its complexed colour to its metal free colour. at the end point. the EDTA which is being added during the course of the titration is chelating the free metal ions in solution at the expense of the MI complex. a competitive reaction takes place between the free metal ions and EDTA. . . The special structure of its anion which has 6 ligand atoms. It forms strainless five-membered rings. Disodium EDTA is used as M/20 solution . 2.EDTA Titration Disodium salt of EDTA is a water soluble chelating agent and is always preferred. 3. It has low price. It is non- hygroscopic and a very stable sequestering agent (Ligands which form water soluble chelates are called sequestering agents). EDTA has the widest general application in analyses because of the following important properties: 1. EDTA-metal ion complexes . .6 gm of disodium EDTA in water and make the volume up to 1000 ml and standardize the prepared solution.Preparation of M/20 Disodium EDTA Dissolve 18. HCl to dissolve CaCO3. Add 300 mg of hydroxyl naphthol blue indicator and titrate with the prepared M/20 disodium EDTA solution. until the solution is deep blue in colour. Add 50 ml of water and minimum quantity of dil. . Adjust the pH of the solution to 12 by adding NaOH.Standardization of Disodium EDTA Weigh accurately about 200 mg of CaCO3 in a titration flask. 1% is permissible. Selectivity of Titration EDTA is a very unselective reagent because it complexes with numerous doubly. When a solution containing two cations which complex with EDTA is titrated without the addition of a complex-forming indicator. then the ratio of the stability constants of the EDTA complexes of the two metals M and N must be such that KM/KN ≥ 106 if N is not to interfere with the titration of M. . and if a titration error of 0. triply and quadruply charged cations. Classical separation .Use of selective metal indicators.pH control .Use of masking and demasking agents .Removal of anions Kinetic masking .Solvent extraction .Steps to increase the selectivity The following procedures will help to : . . ferrocyanide for Zn and Cu. These are filtered. fluoride for Ca. Mg and Pb. can be separated either in the form of insoluble sulphides using Sodium sulphide. Masking by Complex formation: Masking agents form more stable complexes with the interfering metal ions.Co.Masking agents Masking agents act either by precipitation or by formation of complexes more stable than the interfering ion-EDTA complex. Masking by Precipitation: Many heavy metals e.g. The most important aspect is that the masking agent must not form complexes with the metal ion under analysis.. common precipitating agents are sulphate for Pb and Ba. or as insoluble complexes using thioacetamide. decomposed and titrated with disodium EDTA. . Cu and Pb. oxalate for Ca and Pb. using a NH4Cl /NH4OH buffer and a colorimetric detection of the equivalent point. This application note also uses EDTA titration with a potentiometric determination of the equivalent point by means of a Calcium ion-selective electrode. we use a mixture of TRIS and acetylacetone. This buffer solution allows separation between Calcium and Magnesium when a Calcium selective electrode is used as measuring electrode . The above- mentioned standard lays down a titration with EDTA at pH 10. Instead of the NH4Cl /NH4OH buffer. Introduction: Hardness of water measures the sum of calcium and magnesium ions present in the water.Determination of hardness of water.00.