CHAPTER 15

Complexation Titrations: Taking Advantage of Complexing Agents

Terms Defined

• Complex formation – the process whereby a species with one or more unshared electron pairs forms coordinate bonds with metal ions.

• Ligand – an ion or molecule that forms a covalent bond with a cation or a neutral metal atom by donating a pair of electrons that are then shared by the two.

• Chelating agent – substance with multiple sites available for coordination bonding with metal ions. Such bonding typically results in the formation of five or sic membered rings

• Dentate – (Latin) having toothlike projections

Forming Complexes

• Metals ions are Lewis acids, because they accept electrons from Lewis bases. When metal cations combine with Lewis bases, the resulting species is called a complex ion, and the base is called a ligand.1

• The coordination number is the number of covalent bonds that the metal cation tends to form with the electron donor.

• For example, copper (II) has coordination number of four. The species formed from such coordination or complexing, can be electrically positive, neutral or negative. Copper when complexed with ammonia results in a cationic complex, Cu(NH3)4

2+, when complexed with

glycine, a neutral complex, Cu(NH2CH2COO)2,

and when complexed with chloride, an anionic complex, CuCl4

2-.

• When a metal cation is complexed to ligands forming a neutral compound, the complex is called coordinated compound.1

• A chelate is produced when a metal ion coordinates with two or more donor groups of a single ligand to form a five or six membered heterocyclic ring. The copper complex of glycine, is an example of a chelate:

 

2+ H

N H 2

C

H

C

O

O H

OO

N H 2

C u2 + O

O

N H 2

C u

• Some common inorganic ligands are ammonia, water, and halides.

• A ligand that has one donor group such as ammonia, is called unidentate. 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 two 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 single-step process, whereas with unidentate ligands usually involves two or more intermediate species.

• An example of a hexadendate ligand is EDTA (Ethylenediaminetetraacetic Acid). It has six potential sites for complex formationthe electron pairs on the two nitrogen atoms and the four electron-rich carboxyl groups.

• The ligands of EDTA wrap around the metal ion and effectively form a cage around the ion forming a more stable metal complex. This ability is called the chelate effect, and EDTA is called a chelating agent. The properties of EDTA were demonstrated in Lab 27E-3 and 4 Determination of Manganese and Hardness of Water.

• EDTA, H6Y2+, is a hexaprotic system; it has six

acidic hydrogens that are lost upon metal-complex formation. The first four pK values apply to carboxyl protons, and the last two are for the ammonium protons (pK1 = 0.0, pK2 = 1.5, pK3 = 2.0, pK4 = 2.66, pK5 = 6.16, and pK6 = 10.24). The pH of an EDTA solution affects the equilibrium constant of complex formation. Solutions of high pH used in analytical procedures do not significantly effect the stability of a complex.

• In order to determine the concentration of all possible EDTA containing species in titration, we use:

CT = [H6Y2+]+[H5Y+]+[H4Y]+[H3Y-]+[H2Y-2]+ [HY-

3]+[Y4-]• If we want to determine the relative concentrations

of individual species of EDTA during titration, we use the alpha values.

• The alpha value for a species is given by

aspecies = [species]/CT

For a given hydronium ion concentration such as [Y4-]/ CT,

,a4=K1+K2+K3+K4/[H3O+]4+K1[H3O+]3+K1K2[H30+]2+K1K2K3[H3O+]+K1K2K3K4

• Conditional Formation Constants can also be computed using alpha constants, but only at a single pH.The overall formation constants are called beta values.

K/ = a4KMY = [MY(n-4)+]/[Mn+] a4CT

Summary Uni vs Multidentate ligands

• In order to provide an overall comparison of a unidentate ligand, such as ammonia, vs a multidentate ligand such as EDTA, we can examine the following titration curves.

• 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 1:1 complex. Curve B, depicts the reaction of M with a bidentate ligand B to form the complex MB2, a 1:2 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 1:4 complex in four steps.

The formation constants for this reaction would be 108, 106, 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.

Examples

Page 383 15-14 aa) 26.37 mL X 0.0741 mmol Mg(NO3)2/mLX 1mmol

EDTA/mmol Mg(NO2)2 X1mL EDTA/0.0500 mmole EDTA = 39.1 mL EDTA

Page 384 15-22(13.31 X 0.03560) mmol EDTA/9.76 g sample X 1mmolTI2SO4/2mmolEDTA X

0.5048g TI2SO4 / mmol TI2SO4 X 100% = 1.228% TI2SO4

Examples

Page 384 15-24# mmol Fe3+ = (13.73 X 0.01200) mmol EDTA X 1mmol Fe2+/mmolEDTA =

0.16476 mmol

#mmol Fe2+ = (29.62 –13.73) mL EDTA X 0.01200 mmol EDTA/mL EDTA X 1mmol Fe2+/mmol EDTA = 0.19068 mmol

(0.16476 mmol Fe3+X 55.847 mg Fe3+/mmol)/50.00 mL X 10-3 L/mL = 184.0 ppm Fe3+

(0.19068 X 55.847)/0.05000 = 213.0 ppm Fe2+