COMPLEXATION TITRATIONScx

Complexation Methods

These are class of reactions that deal with complex formation.

Complexation Methods

Current method of choice for the determination of metal ions except

Group 1A metals

Because monovalent metal ions yield relatively weak or unstable complex

monovalent

Because _ metal ions yield relatively weak or unstable complex

Al3+, Bi3+, Ca2+, Cu2+, Hg2+, Mg2+, and Zn2+

the complex formation is rapid and quantitative with polyvalent metal ions such as:

polyvalent metal ions

the complex formation is rapid and quantitative with _ such

metal ions

Complexation Methods - Current method of choice for the determination of _ except Group 1A metals

equivalence point

During titration, metal ion reacts with a suitable ligand to form a complex, and the _ is determined by an indicator or appropriate instrumental method

metal ion

During titration, _ reacts with a suitable ligand to form a complex, and the equivalence point is determined by an indicator or appropriate instrumental method

indicator or appropriate instrumental method

During titration, metal ion reacts with a suitable ligand to form a complex, and the equivalence point is determined by an _

when a metal ion combines with a molecule then donates electron

how does a complex form?

Ligand

species that has a molecule with at least one pair of unshared electron

Ligand

an electron donor, while a metal is an electron acceptor

Ligand

based on lewis theory

Complex

If a ligand contains 2 or more molecules or groups that can donate electron. The resulting complex is called chelate.

molecules/groups that can also donate electrons

chelate

If a ligand contains 2 or more molecules or groups that can donate electron. The resulting complex is called _.

metal +

ligand nucleophilic (excess e-) ?

monovalent metal ions

yield relatively weak or unstable complex

electron donor

a ligand is an _; while metal is an electron acceptor

complex

a _ is formed because of sharing of electrons: metal being positive, will accept electrons, while ligand (having unshared electrons), will donate electrons. this follows the principle of lewis acid-base theory

metal

a complex is formed because of sharing of electrons: _ being positive, will accept electrons, while ligand (having unshared electrons), will donate electrons. this follows the principle of lewis acid-base theory

ligand

a complex is formed because of sharing of electrons: metal being positive, will accept electrons, while _ (having unshared electrons), will donate electrons. this follows the principle of lewis acid-base theory

lewis acid-base

a complex is formed because of sharing of electrons: metal being positive, will accept electrons, while ligand (having unshared electrons), will donate electrons. this follows the principle of _ theory

electron acceptor

a ligand is an electron donor; while metal is an _

Al3+, Bi3+, Ca2+, Cu2+, Hg2+, Mg2+, Zn2+

examples of metals

Al3+

astringent

Bi3+

hyperacidity

Ca2+

bone, homeostasis, vit d, 2nd most abundant extracellular

Cu2+

antiseptic

disodium ethylenediaminetetraacetate (Na2EDTA)

the most commonly used analytical reagent for complexation reaction is the

unshared electrons

groups that contain _ and therefore can be accepted by the metal analyte

multidentate (“many toothed”) or a chelating agent

essentially, a ligand that attaches to a metal ion through more than one ligand atom is said to be _ 3

chelate effect

is the ability of a ligands to form more stable metal complexed

stable

If there are more molecules/atoms in a ligand that can form a bond (making additional ring in the complex)

with the metal ion, it is considered more _. Therefore:

Stability of ligands:

MULTIDENTATE LIGANDS >> MONODENTATE LIGANDS

>>

If there are more molecules/atoms in a ligand that can form a bond (making additional ring in the complex)

with the metal ion, it is considered more stable. Therefore:

Stability of ligands:

MULTIDENTATE LIGANDS_ MONODENTATE LIGANDS

pKn value or (Kn)

the dissociation constant of a chemical reaction

opposite

if we do the _ (decrease H+/increase the pH of the soln), the rxn proceeds forward, producing the product species

product

if we do the opposite (decrease H+/increase the pH of the soln), the rxn proceeds forward, producing the _ species

pK

higher _, low K

1

note that 1 mole of edetate reacts 1 mole of metal producing a complex, therefore:

the factor used in all complexometric titrations using EDTA is _

formation constant, Kf, stability constant

the equilibrium constant for the reaction of a metal with a ligand is called the _ 3

↑

_Kf of a complex, ↓pH at which the complexation titration can be run.

Edetate disodium

_ may contain trace of moisture, so it must

be dried at 80oC to obtain the required hydrates

(C10H14N2Na2O8.2H2O)

13

Sodium Hydroxide (NaOH) alkalinizes the solution to a pH of about _,

so that the Ca-EDTA complex would be stable and any magnesium

which might be present as a contaminant would not react. Magnesium

can form complex with EDTA at pH 10.

we use an indicator

how do we determine the endpoint?

Organic dyes

_which form colored chelates with many metal ions. The resultant color of the

metal-indicator complex is distinctly different from that of the dye itself.

sharpened

The endpoint may be _ by the addition of a nonchelating screening dye that is

complementary to the color observed either just before or just after the endpoint is reached.

Metallochromic indicators

_ are also affected by pH, since they are also acid-base indicators.

10-100x

Metal-dye complex should be _ less stable than the corresponding metal-EDTA complex

metal ions

If the metal-indicator complex color is similar to the free indicator color, the color change occurs when one or more hydrogen ions are replaced by _. Therefore, indicator is sensitive to change in pH and also concentration of metal ions.

complexometric

Chelometric means _. So chelometric calcium

carbonate is used as primary standard for complexometric

titration. (In this case, as the analyte for the standardization

of Na2EDTA VS)

↓

↑Kf of a complex, _pH at which the complexation titration can be run.

8

For successful titrations with EDTA, the Kf (as log) of the

complex formed must be greater than _.

complexometric titrations

_ require pH control at which metals are being analyzed

Hg2+

antiseptic, preservatives

Mg2+

2nd most abundant cation; antacid; laxative

Zn2+

astringent; antiseptic

Titrant (Chelating Agent/Complexing Agent/complexone)

most commonly used analytical reagent for complexation; disodium, ethylediamine

Titrant

(Chelating Agent/Complexing Agent/complexone)

Na2EDTA

acts as Lewis base

Metal (analyte)

acts as lewis base

groups containing unshared electrons

monodentate

bidentate

tridentate

tetradentate

hexadentate

heptadentate

octadentate

nonadentate

etc

If a ligand binds to a metal ion through only 1 atom, the ligand is said to be:

multidentate

A ligand attached to a metal ion through 1 or more ligand atom is said to be_ (many tooth)

hexadentate

Na2EDTA

Acts as Lewis Base

monodentate

How many dentate/s does the following ligand contain?

monodentate

How many dentate/s does the following ligand contain?

electron pairs, just the atom (depends parin)

In identifying the # of dentate/s dont look at the ___

monodentate

How many dentate/s does the following ligand contain?

bidentate

(carbonyl grp not kasama bcs stable bond w/carbon atom)

How many dentate/s does the following ligand contain?

bidentate

How many dentate/s does the following ligand contain?

tetradentate (oxygen not included)

How many dentate/s does the following ligand contain?

hexadentate

How many dentate/s does the following ligand contain?

hexadentate (mahirap magshare to other e-; delocalize)

How many dentate/s does the following ligand contain?

octadentate

How many dentate/s does the following ligand contain?

octadentate

most stable kasi 8 not most used in titration cuz its expensive

The CHELATE EFFECT

ability of a ligand to form more stable metal complexes

The CHELATE EFFECT

if more molecules/atoms in a ligand titration form a bond w/metal ions, more stable

>

Multidentate ligands ( _ ) monodentate ligands

chelating

multidentate ligands =

complexing

monodentate ligands =

chelating

stronger _ effect, more stable

bond breakage

high temp, high _

Octadentates

(such as DTPA) give more stable complexes than hexadentates (EDTA),

EDTA

_ has the

widest general application in the analysis because of its powerful complexing action and commercial

availability.

octadentates

DTPA

H4Y - 4 Hydrogens in EDTA

What are the factors that affect the stability of a complex?

Kn formula

pKn formula

K2 formula

pH

The dissociation/reaction of

EDTA is dependent on _.

pH

Therefore, _ affects the

stability of complex formed

by EDTA during

complexation reaction.

unprotonated ligand

In EDTA titrations, the _, Y4-

, forms complexes

with metal ions.

greater than 8

For successful titrations with EDTA, the Kf (as log) of the

complex formed must be _

most stable

AlY-

Al3+ Complex [MY-4]

BiY-

Bi3+ Complex [MY-4]

CaY-2

Ca2+ Complex [MY-4]

CuY-2

Cu2+ Complex [MY-4]