Tutorial 7

Compleximetry

2

Objectives1.Define complex formation reaction.

2.Differentiate between types of ligands.

2.Define properties of EDTA titrations.

3.Describe different types of EDTA titrations.

4.Determination of water hardness.

5.Describe how to increase EDTA selectivity.

6. Apply EDTA titrations.

Is the reaction of a metal ion with electron donnating group or nucleophile to form a metal-ligand complex.

Metal Ligand complex

Ex.: Cu2+ + 4:NH3 [Cu(NH3)4]2+

The metal ion acts as Lewis acid (electron acceptor) and is used as a titrant for complex forming agent (Ligand) which acts as a Lewis base (neutral group or negatively charged group {stronger Lewis base}) that can donate electrons.

Complex formation reaction

The coordination number (n) of the metal ion represents the number of coordinate bonds around the central metal, which is the number of monodentate ligands coordinated to the metal ion.

Cu2+ + 4NH3 [Cu(NH3)4]2+ n= 4

Coordinate bond is the bond formed between electron donnating group (ligand) and an electron acceptor (metal ion).

Monodentate ligands: contain only one center of donnation e.g.: CN-, F-, :NH3, H2O:

Bidentate ligands: contain 2 centers of donnation. e.g. ethylene diamine H2N:-CH2-CH2-:NH2

Multidentate ligands: contain more than 2 centers of donnation. Ethylene diamine tetraacetic acid (EDTA).

Each element can’t donate more than one lone pair of electrons.

Types of ligands:

EDTA is a tetrabasic acid that has six potential sites for bonding with a metal ion, four carboxyl groups and the two amino groups , each of the latter with unshared pair of electrons. Thus EDTA is a hexadentate ligand

EDTA is not a selective reagent :as it can complex with all metal ions with a ratio 1:1 regardless of the charge on the cation.

e.g. Ag+ + Y4- ---- AgY3-

Al3+ + Y4- ---- AlY-

EDTA:a multidentate ligand

Tri and tetravalent valent metal ions such as Bi3+, Fe3+, Cr3+, Th4+, V4+ form highly stable EDTA complexes. These can be titrated with EDTA in acid medium pH 1-3 using 0.2N HNO3 to adjust pH.

Ca2+ can be titrated in highly alkaline medium pH>12 using murexide as indicator.

Divalent metal ions such as Pb2+, Cd2+, Zn2+ can be titrated in acidic medium using hexamine buffer and xylenol orange as indicator.

Most of divalent metal ions as Mg2+,Ca2+, Ba2+, Pb2+, Zn2+ can also be titrated in alkaline medium (pH 10) using ammoniacal buffer and Eriochrome black T as indicator

The main determining factor in the pH dependence is the stability of the formed complex in comparison to the free EDTA in a given pH; the complexes should have higher stability constants so that the titration is possible.

The stability of the formed complexes is highly dependent on the pH of the medium:

Many metal ions can be titrated using EDTA taking into account choosing a suitable indicator and pH medium.

This type has a very important application in the determination of: hardness of water (hard water can’t form foam with soap)

There are two types of water hardness: Temprorary Hardness, this can be removed by boiling and it is due to

carbonate salts.

Permenant hardness, this can’t be removed by boiling and this is mainly due to other types of salts that are not affected by heating as sulphates, silicates….etc

Types of EDTA titrationsA. Direct titration:

Water hardness is represented in terms of the presence of Ca2+ and Mg2+ salts.

Total Ca2+ and Mg2+ are determined by direct titration with EDTA at pH 10 using ammoniacal buffer and EBT as indicator.

Ca2+ alone can be determined at pH 12 using 0.4N NaOH and murexide as indicator.

Hardness of Water:

1.Back titrations are useful for the determination of cations that form stable EDTA complexes and for which no suitable indicator is available as in case of determining Thallium.

2. The method is also useful for cations as Cr(III) and Co(III) that react only slowly with EDTA. A measured excess is added and after the reaction , the excess EDTA is titrated against standard zinc or magnesium.

3. It can also be used when the analyte forms precipitates at the required pH of its titration.

B. Back titration:

In displacement titrations ,an unmeasured excess of a solution containing the magnesium or zinc complex of EDTA is introduced into the analyte solution.

The analyte which forms more stable complex than that of magnesium or zinc will undergo the following displacement reaction:

MgY2- + M2+ --- MY2- + Mg2+

Where M2+ can be Hg(II), Pd(II), Ti(II), Mn(II) and V(II).

The liberated Mg2+ or Zn 2+ can be then titrated with standard EDTA solution.

Displacement titrations are used on lacking sharp end point or when no suitable indicator is available.

C. Displacement titration:

How to increase selectivity of EDTAControl of pH Precipitation Masking with

CyanideAt highly acidic medium , pH 1-3, only tri, and tetravalent metals can be detected

If a metal is precipitated as hydroxide on adding buffer or NaOH, EDTA can’t react with metal so it will be masked.e.g. Mg2+, Sr2+, Ba2+, Pb2+

Metals that can react with CN- are: Ag(I), Cu(II), Hg(II), Cd(II), Zn(II), Co(II), Ni(II), Fe(II), Fe(III), Cr(III).

At highly basic Medium pH> 12, Calcium and Barium can be determined using murexide indicator and NaOH to Adjust pH.

Metals that can’t react with CN- are: Ca, Sr, Ba, Mg, In, Pb and Mn

( N.B. , in highly basic medium, most of the metals will precipitate as hydroxides which is more stable than forming a complex with EDTA, i.e. can’t be titrated with EDTA)

ONLY Zn and Cd cyano complexes that the metal ions can be released (demasked) from the complex using formaldehyde, acetone or chloral hydrate

Suggest a suitable method to determine each metal in the following mixture: 1. Fe2+,Zn2+,Ca2+

1- Back titration to find the equivalent volume for total metals.

2- On a new portion, add CN- , Fe2+ and Zn2+ will be masked, therefore by direct titration, the volume of EDTA will be equivalent to Ca2+.

3- On the same flask, add acetone, Zn2+ will be de-masked, therefore by direct titration, the volume of EDTA will be equivalent to Zn2+.

4- By subtraction, u can obtain the equivalent volume to Fe2+.

Questions

2-Cr3+,Cu2+,Cd2+,Mg2+

1- Back titration to find the equivalent volume for total metals.

2- On a new portion, adjust the medium at highly acidic pH, (by direct titration, the volume of EDTA will be equivalent to Cr3+.

3- On a new portion, add CN- , Cu2+ , Cd2+ and Cr3+ will be masked, therefore by direct titration, the volume of EDTA will be equivalent to Mg2+.

4- On the same flask, add acetone, Cd2+ will be de-masked,

therefore by direct titration, the volume of EDTA will be equivalent to Cd2+.

5- By subtraction, u can obtain the equivalent volume to Cu2+.

3.Cd2+, Fe2+,Fe3+,Ca2+

1- Back titration to find the equivalent volume for total metals.

2. On a new portion, adjust the medium at highly acidic pH. By direct titration, the volume of EDTA will be equivalent to Fe3+.

3. On a new portion, add CN- , Fe3+,Fe2+ and Cd2+ will be masked, therefore by direct titration, the volume of EDTA will be equivalent to Ca2+.

3- On the same flask, add acetone, Cd2+ will be de-masked, therefore by direct titration, the volume of EDTA will be equivalent to Cd2+.

4- By subtraction, u can obtain the equivalent volume to Fe2+.

4.Cd2+ , Ca2+, Ni2+,Mn2+

1- Back titration to find the equivalent volume for total metals.

2. . On a new portion, adjust the PH to be highly alkaline (PH>12) using NaOH and murexide. By direct titration, the volume of EDTA will be equivalent to Ca2+ .

3. On a new portion, add CN- , Cd2+ and Ni2+ will be masked, therefore by direct titration, the volume of EDTA will be equivalent to Ca2+ and Mn2+.

By subtraction: volume of 3-2, you can obtain the equivalent volume to Mn2+.

3- On the same flask, add acetone, Cd2+ will be de-masked, therefore by direct titration, the volume of EDTA will be equivalent to Cd2+.

4- By subtraction, u can obtain the equivalent volume to Ni2+.

For each of the following COMPLEX FORMING AGENTS, draw the mode of chelation with a metal ion:

1. Note that the final complex should have a coordination number of 6.

 

M

OHH

OHH

+

For each of the following COMPLEX FORMING AGENTS, draw the mode of chelation with a metal ion:

1. Note that the final complex should have a coordination number of 6.

Metal ion interacts with this ligand within the ratio of 1:2.What is the coordination number?

 

H2N

OO

NH2

OO

M

+

n = 4