Chapter 11 EDTA Titrations

Polar groups have positive and negative regions that attract neighboring molecules by electrostatic forces.

Nonpolar groups have little charge separation and are soluble inside the nonpolar cell membrane.

Metal cations dissolve in water and are said to be hydrophilic (“water loving”).

Cell membranes exclude water and are described as hydrophobic (“water hating”).

EDTA is a merciful abbreviation for ethylenediaminetetraacetic acid, a compound that forms strong 1:1 complexes with most metal ions (Figure 12-1) and finds wide use in quantitative analysis.

11-1 Metal-Chelate Complexs

Metal ions are Lewis acids, accepting electron pairs from electron-donating ligands that are Lewis bases.

Cyanide (CN-) is called a monodentate ligand because it binds to a metal ion through only one atom (the carbon atom).

A ligand that attaches to a metal ion through more than one ligand atom is said to be multidentate (“many toothed”) , or a chelating ligand ( pronounced KEE-late-ing).

We say that ethylenediamine is bidentate because it binds to the metal through two ligand atoms.

The chelate effect is the ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands.4

An important tetradentate ligand is adenosine triphosphate (ATP), which binds to divalent metal ions (such as Mg2+, Mn2+, Co2+, and Ni2+) through four of their six coordination positions (Figure 12-2).

The octadentate ligand in Figure 12-3 is being evaluated as an anticancer agent.5

The chelate is covalently attached to a monoclonal antibody, which is a protein produced by one specific type of cell in response to one specific foreign substance called an antigen.

A titration based on complex formation is called a complexometric titration. The stoichiometry is 1:1 regardless of the charge on the ion.

BOX 11-1 Chelation Therapy and Thalassemia

11-2 EDTA

Acid-Base Properties

pK applies at 25oC and µ = 0.1, except pK1 applies at µ = 1 M

Fraction of EDTA in the form Y4-:

where [EDTA] is the total concentration of all free EDTA species in the solution.

EDTA Complexes

The equilibrium constant for the reaction of a metal with a ligand is called the formation constant, Kf, or the stability constant:

Formation constant:

Conditional Formation Constant

[Y4-] = αY4-[EDTA]

Conditional formation constant:

The number K’f = αY4-Kf is called the conditional formation constant, or the effective formation constant.

Mn+ + EDTA = MYn-4 K’f = αY4-Kf

11-3 EDTA Titration Curves

Mn+ + EDTA = MYn-4 K’f = αY4-Kf (12-7)

Region 1: Before the Equivalence PointRegion 2: At the Equivalence Point

MYn-4 = Mn+ + EDTA

Region 3: After the Equivalence Point

Titration Calculations

Ca2+ + EDTA CaY2-

K’f = αY4-Kf = (0.30)(1010.65) = 1.34 X 1010

Region 1: Before the Equivalence Point

Fraction remaining

(=4/5)

Original concentration

of Ca2+

Dilution factor

Total volume of solution

Initial volume of Ca2+

=0.029 1 M pCa2+ = -log[Ca2+] = 1.54

Region 2: At the Equivalence Point

Original concentration

of Ca2+

Dilution factor

Total volume of solution

Initial volume of Ca2+

Initial concentration (M)Final concentration (M)

Region 3: After the Equivalence Point

Original concentration

of EDTA

Dilution factor

Total volume of solution

Volume of excess EDTA

Original concentration

of Ca2+

Dilution factor

Total volume of solution

Initial volume of Ca2+

The Titration Curve

11-4 Do It with a Spreadsheet

Mass balance for M:

Mass balance for L:

Spreadsheet equation for titration of M with L:

Spreadsheet equation for titration of M with L:

11-5 Auxiliary Complexing Agents

To permit many metals to be titrated in alkaline solutions with EDTA, we use an auxiliary complexing agent.

Metal-Ligand Equilibria17

M + L = ML β1 = [ML]/([M][L]) (12-13)

M + 2L = ML2 β2 = [ML2]/([M][L]2) (12-14)

The equilibrium constants, βi, are called overall or cumulative formation constants.

αM = [M]/CM (12-15)

CM = [M] + [ML] + [ML2]

CM = [M] + β1[M][L] + β2[M][L]2

= [M]{1 + β1[L] + β2[L]2}

Fraction of free metal ion:

EDTA Titration with an Auxiliary complexing Agents

K’’f = αZn2+αY4-Kf (12-18)

Box 11-2 Metal Ion Hydrolysis Decreases the Effective Formation Constant for EDTA Complexes

K’’’f = {(αFe3+αY4-)/αFeY-}Kf

Take-home message: In this book, we restrict ourselves to cases in which there is no hydrolysis and αMm+ is controlled by a deliberately added auxiliary ligand. In reality, hydrolysis of Mm+ and MY influences most EDTA titrations and makes the theoretical analysis more complicated than we pretend in this chapter.

11-6 Metal Ion Indicators

Metal ion indicators (Table 12-3) are compounds whose color changes when they bind to a metal ion. Useful indicators must bind metal less strongly than EDTA does.

MgIn + EDTA MgEDTA + In (12-19) Red Colorless Colorless Blue

Demonstration 11-1 Metal Ion Indicator Color Changes

If a metal does not freely dissociate from an indicator, the metal is said to block the indicator.

11-7 EDTA Titration Techniques

Direct Titration

In a direct titration, analyte is titrated with standard EDTA.

Auxiliary complexing agents such as NH3, tartrate, citrate, or triethanolamine

may be employed to prevent metal ion from precipitating in the absence of EDTA.

Back Titration

In a back titration, a known excess of EDTA is added to the analyte.

Displacement Titration

Hg2+ does not have a satisfactory indicator, but a displacement titration is feasible.

Mn+ + MgY2- MYn-4 + Mg2+ (12-20)

2Ag+ + Ni(CN)42- 2Ag(CN)2

- + Ni2+

Indirect Titration

Anions that precipitate with certain metal ions can be analyzed with EDTA by indirect titration.

Box 11-3 Water Hardness

Hardness is the total concentration of alkaline earth (Group 2) ions, which are mainly Ca2+ and Mg2+, in water.

Ca2+ + 2RCO2- Ca(RCO2)2(s) (A)

Soap PrecipitateR is a long-chain hydrocarbon such as C17H35 ㅡ

Hard water leaves solid deposits called scale on pipes when it evaporates.

Hardness is beneficial in irrigation water because alkaline earth ions tend to flocculate (cause to aggregate) colloidal particles in soil and thereby increase the permeability of the soil to water.

CaCO3(s) + CO2 + H2O Ca(HCO3)2(aq) (B)

The fraction of hardness due to Ca(HCO3)2(aq) is called temporary hardness because this calcium is lost (by precipitation of CaCO3) upon heating. Hardness arising from other salts (mainly dissolved CaSO4) is called permanent hardness because it is not removed by heating.

Masking

A masking agent is a reagent that protects some component of the analyte from reaction with EDTA.

When cyanide is added to a solution containing Cd2+ and Pb2+, only Pb2+ reacts with EDTA. (Caution: Cyanide forms toxic gaseous HCN below pH 11. Cyanide solutions should be strongly basic and only handled in a hood.)

Fluoride masks Al3+, Fe3+, Ti4+, and Be2+. (Caution: HF formed by F- in acidic solution is extremely hazardous and should not contact skin and eyes. It may not be immediately painful, but the affected area should be flooded with water and then treated with calcium gluconate gel that you have on hand before the accidents. First aid providers must wear rubber gloves to protect themselves.)

Demasking releases metal ion from a masking agent.