Analytical Methods and Techniques
Analytical Methods and Techniques CHE 221
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Date Created: 09/30/15
Chemistery 221 Chapter 14 ComplexFormation Titrations Complexformation reagents aere used widely for titrating cations Most metal ions react with electronpair donors to form coordination compounds or complexes Many of us have utilized the physical properties color of such complexes in the laboratory Ammonia complexes are widely used to test for cations During the Electrogravimetric Determination of Copper and Lead in Brass experiment ammonia was added to an analyte solution to test for the presence of copper II which in the presence of ammonia would produce the copperammonia complex CuNH342 a bright blue complex The donor species or ligand must have at least one pair of unshared electrons available for bond formations Ligands are de ned as ions or molecules that form covalent bonds with a cation or a nuetral metal atom by donating a pair of electrons which are then shared by the two Ammonia as mentioned above along with water and halide ions are common inorganic ligands The number of covalent bonds that a cation forms with a ligand is referred to as its coordination number For example copper II has coordination number of four The species formed from such coordination or complexing can be electrically positive nuetral or negative Copper when complexed with ammonia results in a cationic complex CuNH342 when complexed with glycine a nuetral complex CuNHzCHzCOO2 and when complexed with chloride an anionic complex CuCl4239 A chelate is produced when a metal ion coordinates with two or more donor groups of a single ligand to form a ve or six membered heterocyclic ring The copper complex of glycine is an example of a chelate 0 0 0 Cu mi N The copper bonds to both the oxygen of the carbonyl group and the nitrogen of the amine group forming a heterocyclic ring A ligand that has one donor group such as ammonia is called unidentate Dentate is a latin word that means having toothlike projections thereby unidentate singletoothed Glycine which has two groups available for covalent bonding the carbonyl oxygen and the aminal nitrogen is called bidentate As titrants multidentate ligands particularly tetradentate and hexadentate chelating agents those having four or six donor groups have tow advantages over their unidentate titrants First these multidentate titrants generally react more completely with cations thereby providing sharper more accurately end points Second they ordinarily react with metal ions in a singlestep process whereas wiht unidentate ligands usually involves two or more intermediate species Again this would provide a sharper end point in shorter period of time This advantage of a singlestep reaction is illustrated by the titration curves shown below taken from Skoog 239 scan and insert fig 141 The titration concerns a reaction that has an overal equilibrium constant of 1020 Curve A is derived for the reaction in which a metal ion M that has a coordinatioin number of four reacts with a tetradentate ligand D to form the complex MD a 11 complex Curve B depicts the reaction of M with a bidentate ligand B to form the complex MBz a 12 complex in two steps The formation constant for this reactions would be 1012 and 108 respectively for the first and second reactions Curve C depicts the reaction of M with a unidentate ligand A that forms the complex MA4 a 14 complex in four steps The formation constants for this reaction would be 108 105 104 102 for the first second third and fourth reactions This figure shows how a sharper end point is obtained for a reaction that takes place in a single step Schwarzenbach in 1945 first recognized the potential of tetriary amines that also contain carbonyl group as chelating agents V2