EXP. NO. 6 Coplexometric Titration Ligands are classified regarding the number of donor groups available: If more than bidentate called (polydentate ligand), also sometime called (chelating agent). Note all types of complex formation used for titration. Only the complexes which are: 1. Stable 2. Fast complex formation 3. There is a method for measuring E.P. Tetra dentate Ligand: The Ligand attached to metal at 4 sites Tridentate Ligand: The Ligand attached to metal at 3 sites Diethylene triamine Triethylene tetramine

The most famous compound used is: EDTA (ethylene Di amine Tetra Acetic acid) Chelate : It is a complex formed between the ligand containing two or more donor groups and metal to form ring structure. (heterocyclic rings or chelate rings). Chelating agents: organic molecules containing two or more donor groups which combine with metal to form complex having ring structure. Chelates are usually insoluble in water but soluble in organic solvent. Sequestering agent : Ligands which form water soluble chelates e.g. EDTA. Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution. EDTA, has four carboxyl groups and two amine groups that can act as electron pair donors, or Lewis bases. The ability of EDTA to potentially donate its six lone pairs of electrons for the formation of coordinate covalent bonds to metal cations makes EDTA a hexadentate ligand.

Disodium EDTA is commonly used to standardize aqueous solutions of transition metal cations. Disodium EDTA (often written as Na2H2Y) only forms four coordinate covalent bonds to metal cations at pH values ≤ 12. In this pH range, the amine groups remain protonated and thus unable to donate electrons to the formation of coordinate covalent bonds There is 6 points of coordination bond as shown (four hydrogen's & two nitrogen's) and can make 6 of five membered ring when complexed with metal ion [M+]. So it can be combined with metal ions [M+] in 6 points (more stable complex). So EDTA called [hexadentate ligand]. EDTA is a weak acid because 4H+ can be replaced. Any (H+) can be replaced by [Na+, Li+, K+ and NH4+] forming the salts of EDTA. But if the metal ion was more than mono valent, it will form complexes. The famous salt is [Na2H2Y](disodium salt), or tetra sodium salt [Na4Y]. EDTA has four acid dissociation steps pKa1= 1.99, pKa2= 2.67, pKa3= 6,16, pKa4= 10.26 So first and second (H) can be donated easily. Na2H2Y.2H2O (EDTA) (disodium salt) is used because it is more soluble than the acid (EDTA) (H4Y) itself, although EDTA can be used as primary Std. substance. Na4Y is very alkaline (it is like NaOH solution) so it is not used and also it is (very hygroscopic), so the salt (Na2H2Y) is easily used (dissolved easily). 5 forms of EDTA, (H4Y, H3Y-, H2Y2-, HY3-, Y4-) EDTA combines with all metal ions in 1:1 ratio

Stoichiometry: The stoichiometry of (M + EDTA) reaction is usually 1:1 and the change is of the metal ion not affected, so we use it in the calculations (M molarity). Properties of these reactions are: 1- 1:1 reaction. 2- Always produce same amount of hydrogen ions (2H+). 3- pH affected on titration results. The titration must carried out at alkaline medium to consume H+ which produced from above reactions, so that the equilibrium goes to right and the stability of the complex formed MY will more stabile so most of the EDTA titrations carried in alkaline medium but in more stabile MY complexes it can be carried out in acid medium also (for example Fe3+ , Cd2+) which the (Ks) is too high also. We need buffer solution to neutralize H+ continuously which produced as a result of the titration, so that the stability of MY increased.

Indicators: These indicators have properties of acid-base indicators. Metal ion indicator or metalochromic indicator These indicators have properties of acid-base indicators.

Procedure: Standardization with std. 0.02M ZnSO4 solution: A/ Preparation and standardization of EDTA solution. Preparation of approximate 0.02M EDTA soln. 1- weight out roughly 1.86gm of EDTA di sod. salt on a watch glass, dissolve it in a beaker by D.W. 2- transfer it quantitatively to a 250ml vol.flask complete the volume to the mark. Standardization with std. 0.02M ZnSO4 solution: 1- weight out accurately on a sensitive balance 0.5751gm of pure solid ZnSO4.7H2O, dissolve it n D.W. transfer it quantitatively to a 100ml vol. flask complete the volume to the mark. 2- Pipette 10ml of the prepared solution to a conical flask add 30ml of D.W. add 1ml of buffer solution pH=10, add 0.1gm of solid E.B.T. indicator. 3- Titration the solution with EDTA solution from burette until color changes from red to blue. 4- Calculate the accurate M of the EDTA solution.

B/Determination of Mg2+ in unknown solution: 1- Pipette 10ml of Mg2+ solution to a conical flask, add 30ml D.W. add 1ml of buffer pH=10, add 0.1gm of E.B.T indicator. 2- Warm the solution of 400C, titrate it with EDTA solution from burette until the color change red to blue. C/ Hardness of water. Total hardness 1- Pipette 25ml of tap water to a conical flask, add 0.5ml of buffer pH=10, add 0.1gm of E.B.T indicator. 2- Titrate with EDTA solution from burette until the color change from red to blue. Record vol. of EDTA as V1. Calculate the total hardness of water as ppm CaCO3. Temporary hardness 1- Place 250ml of tap water in a beaker, boil for 20-30min. filter to a 250ml vol. flask. Complete the volume by D.W. 2- Pipette 25ml of the water to a conical flask, add 0.5ml of buffer pH=10, add 0.1gm of E.B.T. indicator. 3- Titrate with EDTA solution until the color change from red to blue. Record vol. of EDTA needed as V2. 4- Calculate the permanent hardness as ppm CaCO3.Then calculate the temporary hardness as ppm CaCO3.