ADVANCED ANALYTICAL CHEMISTRY – 1S 2018

2nd modulus

Equilibrium and Complexation Titrations

Samples Preparation

Statistics Applied to Analytical Chemistry

Class notes : www.ufjf.br/baccan

Prof. Rafael Arromba de Sousa

Departamento de Química - ICE

rafael.arromba@ufjf.edu.br

ADVICES

1) Ponctuation

2 Tests

1 Seminar (07/04/18)

2 Atividades: 05/30/18 task 1

06/20/18 sampling experiment

2) Sampling experiment

“M&M Sampling Study”

Room Activity (Submit Report)

Each student must bring at least 1 M & M package

(brown package with at least 104 g)

Also bring 1 pair of gloves for handling

ADVICES

3) Seminaries

About “complexation”

Sugestions:

- Titration with EDTA to determine the water hardness

- Spectrophotometric determination of total phosphorus

- Spectrophotometric determination of FeII with acetylsalicylic acid

REFERENCES

QUÍMICA ANALÍTICA QUANTITATIVA ELEMENTAR N. Baccan, J. C. Andrade, O. E. S. Godinho, J. S. Barone 3ª Ed, Editora Edgard Blücher Ltda: São Paulo, 2001

ANÁLISE QUÍMICA QUANTITATIVA *

D. C. Harris 8ª Ed, LTC, 2012 FUNDAMENTOS DE QUÍMICA ANALÍTICA * D. A. Skoog e col. 9th Ed, Cengage Learning, 2014 INTERNET: http://www.iq.usp.br/gutz (*) Look for English versions if necessary 4

Class 1 COMPLEXATION EQUILIBRIUN AND TITRATION

CLASSROOM PLAN

(Review) Chemical equilibrium in solutions Definition of complex ion and complexing agent (EDTA) Physical-chemical aspects of complexation reactions EDTA chemistry and complexometric titrations (principles)

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Chemical equilibria in aqueous solution

Chemical reactions in equilibrium

a A + b B c C + d D

IMPORTANT PHYSICAL-CHEMICAL ASPECTS

[C]c [D]d

1) Equilibrium constant, K: K=

[A]a [B]b

Proposed in 1864 as "Law of mass action ". Characteristics:

[species]= mol L-1

[solvent]= 1

K is dimensionless

a) In the REVERSE DIRECTION (from right to left) the constant is K´

K´= 1/K c C + d D a A + b B

direct

reverse

In the state of equilibrium the velocities of the direct and inverse reactions are equal

6 K´

b) When TWO REACTIONS ARE ADDED, the value of K is

equal to the product of the individual values :

HA H+ + A- , K1 [H+] [A-] [CH+] [A-] [CH+]

H+ + C CH+ , K2 K3= K1 K2= =

[HA] [H+] [C] [HA] [C]

HA + C CH+ + A- , K3

2) The Le Chatelier Principle

Allows to predict the changes that occur when reagents or products are

added to an equilibrium reaction

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The reaction moves in order to compensate the disturbance imposed

to the equilibrium state :

2) The Le Chatelier Principle

Predicts the changes that occur when reagents or products are

added to an equilibrium reaction

a A + b B c C + d D

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WHICH ARE THE ANALYTICAL IMPLICATIONS?

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The complexation equilibrium is one of the important examples

Species formed: metal complexes

3) There are several types of CHEMICAL EQUILIBRIUM ...

Cu (NH3)4 2+

COMPLEX or COMPLEX ION

Metal and ligands interact by means of a covalent bond

Ligand electrons occupy free metal orbitals

M Ln GENERAL FORMULAE

Cu 2+ + 4 NH3 Cu (NH3)4

2+

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COMPLEXES CHARACTERISTICS

MAXIMUM No OF LIGANDS, N metallic ion coordenation no

- Depends on ion electronic configuration

- The size of the ligands, among other factors

- The most common numbers are 2, 4 and 6

CENTRAL ION (usually) transition metals (24 Cr – 30 Zn)

LIGANDS neutral molecules or negative ions

EXAMPLE: TETRAAMINE-COPPER :

M = central Ion (Cu 2+); L = Ligand (NH3); N= 4

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COMPLEXES CARACTERISTICS

Metallic COMPLEXES CAN BE NEUTRAL OR CHARGED:

Antitumural compound

(chemotherapy)

Coordination compound

Fe(CN)6] 3-

Cisplatin: PtCl2(NH3)2

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M – L BOND

It involves acid - base type interactions

Lewis CONCEPT

Metal: ACID capable of receiving electron pairs

Ligand: BASE capable of donating electron pairs

SO... - To act as ligand: species must have at least 1 pair of "free“ electrons Complex: product of an acid + base (Lewis) can be mononuclear or polinuclear

greater analytical interest

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M – L Bond

Acid – base interaction

Number is related to “geometry”:

(Understanding the concept) Exerc 1:

Write the formation reaction of the hexaaquacobalt II complex and identify which

specie acts as acid and which one acts as a base, according to Lewis's theory.

B

F

F

F

Unused p orbital

M Ligand

Orbital not occupied

(accommodates electrons

from ligand) Ligand

Ligand

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Tipes of ligands

Monodentate ligands :

- “Simple” ligands (such as water, ammonia and halides)

-They bind to the metal ion by only a single “point”

Example: íon CN - Ag + + 2 CN - [AgCN2] –

Polydentate ligands :

- Organic ligands

- They bind to the metal ion by means of two or more “atoms"

Ex: Ethylenediamine H2N - CH2 - CH2 - NH2

bidentate ligand

Important ex. of polydentate ligand :

Ethylene diaminetetracetic acid (EDTA)

15 Chemical and biomedical uses...

Ligand groups:

carboxyl and amino radicals

(electrons associated with O and N)

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Biomedical uses

-"Chelation" therapies for metal ions

- Nutrients in excess (Fe)

- Contaminants (Pb and Pu) - occupational health

Chemical uses

- Analytical applications ...

- Technological Applications

- Detergents

- “Cleaning Products” prevents oxidation by metal ions

- Cosmetics

Important ex. of polydentate ligand :

Ethylene diaminetetracetic acid (EDTA)

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THE POLYDENTATED LIGANDS:

they are called CHELATORS and they originate the CHELATES

Chemical species capable of coordinating with positive ions forming

ionic compounds (generally soluble in water)

LIGAND “SITES”:

Nitrogen atoms

(preferably coordinate with Cd, Co, Cu, Hg, Ni, Zn)

Oxygen atoms

(preferably coordinate with Al, Bi, Pb)

EDTA: N and O atoms!

coordinates with a wide variety of metals

Solubilization in water of insoluble species

Going deeper...

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THE LIGANDS "IN THE MEDIA"

On Google*:

888,000 results ("chelators")

284,000 results ("complexing agents")

* Consultation completed 04-26-18

Other complex exemples :

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Complexes with TWO or more CENTRAL IONS

NH2

(NH3)3Co OH Co(NH3)3

NH2

BIOCHEMISTRY AREA :

IMPORTANT EXEMPLES OF QUELATES AND COMPLEXES

ATPs (adenosine triphosphate)

tetradentate ligands that coordinate to Mg 2+, Mn 2+, Co 2+ and Ni 2+

Hemoglobine red (blood) pigment: central ion: Fe 2+

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4) "Formation Constant" of complex Ions (K f):

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m M + n L MmLn

[MmLn]

K f=

[M]m [L]n

If the reaction occurs in "n" steps (forming intermediate complexes):

Kf = K1.K2.Kn

Situation that occurs with monodentate binders:

are connecting to the central ion 1 to 1 ...

The formation constant K f is also called the "stability constant“

The inverse of K f is called the "instability constant": k f = 1 / K inst.

PHYSICAL-CHEMICAL ASPECTS OF EQUILIBRIUM REACTIONS...

4) “Formation constant” of complex ions ( K f ):

Reactions with multidentate binders occur in a single step and are more

favorable (larger entropy)

K f monodentate ligand < K f bidentate ligand < K f polydentate ligand

Quelate effect:

Ex: [Cd(C2N2H8)2]2+ é MORE STABLE than [Cd(C2NH5)4]

2+

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Literature:

PHYSICAL-CHEMICAL ASPECTS OF EQUILIBRIUM REACTIONS...

EDTA PHYSICAL-CHEMISTRY :

Weak acid (4 ionizable H):

pK1 = 2.00

pK2 = 2.66

pK3 = 6.16

pK4 = 10.26

Represented by H4Y:

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IONIZATION REACTIONS:

H4Y H+ + H3Y-

[H+] [H3Y -]

K1 =

[H4Y]

Depends on pH ...

HOW ARE EDTA COMPLEXES?

n-4

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- EDTA forms 1:1 complexes with the most of metallic ions, independent of the

metal charge (except with Na+, Li+ and K+)

- The higher the cation charges, the higher the values of K f

M n+ + Y 4- MY n-4

pH > 10

n-4

In pH > 10 the α fraction of the specie Y4- is

more significant:

D. Harvey, Modern Analytical Chemistry,

McGraw-Hill: Boston, 2000 25

[Y 4-]

α4 =

Ca Ca = conc. of EDTA

non complexed species

HOW ARE EDTA COMPLEXES?

"CONDITIONAL CONSTANT", K f '(pH dependent):

[MY n-4]

K f =

[M n+] [Y 4-]

[MY n-4]

K f α4 = = K f´ [M] n+ Ca

[MY n-4]

K f =

[M] n+ α4 Ca

D. Harvey, Modern Analytical Chemistry, McGraw-Hill: Boston, 2000 26

M n+ + Y 4- MY n-4 pH [MY n-4]

K f´ =

[M] n+ Ca

[Y 4-]

α4 =

Ca

?

IN PRACTICE: Influence of pH and selectivity !

For different analytes there is a pH from

which the complex formation is favored (larger

Kf)

EDTA is used over a wide pH range

(binder widely applied)

The choice of pH promotes selectivity for

some species (Ex Ca e Mg)

Skoog DA e col., Fundamentals of Analytical Chemistry, 8th Ed, Thomsom Broks Cole: Belmont, 2004 27

Understanding the conditional forming constant

Exerc 2 (Harries, 7th ed, pg 260):

Calculate the free Ca2+ conc. in a CaY2- 0.100 mol L-1 solution in pH 6.00

and in pH 10.0. Data Kf CaY 2 - = 1010.65, α Y 4-= 1.8.10 -5 (pH 6.00) and α Y 4-= 0.30 (pH

10.0).

Answer: in pH 6 [Ca2+] ≈ 3.5 10-4 mol L-1

in pH 10 [Ca2+] ≈ 2.7 10-6 mol L-1

SHOWS the pH importance...

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CLASSICAL AND INSTRUMENTAL APPLICATIONS 1) Determination of different metal species and anions by gravimetry or

titration - Determination of Ni with dimethylglyoxime: precipitation in ammoniacal

medium followed by weighing the precipitate (after drying)

2) Separation of inorganic species (metals) - Separation of AgCl and Hg2Cl2 using NH4NO3: formation of Ag(NH3) 2+

- Masking of Mn II (with triethanolamine) in the determination of Ca and Mg in

calcium by EDTA titration 3) Formation of colored complexes for colorimetric detection of metallic cations - Complexation of Cu with sodium diethyldithiocarbamate and extraction with

chloroform - Spectrophotometric determination of Fe II/ Fe III with the 1-10-

phenanthroline...

Analytical applications of complexation equilibrium

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Case “3”: Formation of colored complexes

ALLOWS to make a liquid-liquid extraction of metallic ions in water Pay attention to the pH of the medium

To use appropriate solvent

Exemples: - Extraction of various metals (Al, Be, Ce, Co (III), Ga, In, Fe ...) Acetyl ketone (chelant) + CCl4 (solvent)

- Ni and Pd extraction Dimethylglioxime (chelant) + HCCl3 (solvent)

Several other chelators: Cupferron, diphenylthiocarbazone, sodium diethyldithiocarbamate ...

Applications - other examples

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EXAMPLES OF LIGANTS USED IN ANALYTICAL PROCEDURES

Sodium diethyl diethyl carbamate (DDTC)

Ethylenediamine

Triethylenetetraamine

Di(3-aminopropyl) amine

O-phenanthrolin

Di-n-hexyloctanoamide

Tri-n-butyl-phosphate

Cetyltrimethylammonium bromide (CTAB)

8-hydroxyquinoline

1-phenyl-3-methyl-4-benzoyl-5-pyrazone

2-Hydroxypyridine-3-carboxylic acid

Are employed in research

carried out in the last nine

years in works with analytical

and / or environmental focus

Case “1”) Titrations

envolve EDTA as titrant or titled

The use as titrant is more comum:

Sample solution + satandard solution Stoichiometric product

(titled) (titrant)

The calculation of the analyte concentration

is based on the volumes used

(titled and titrant)

IMPORTANT:

This type of titrattion will be the main focus of this class

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Applications (deepening...)

COMPLEXOMETRIC TITRATION PRINCIPLE :

Titration of the analyte with a chelating agent complex-Ion

Titrant (EDTA is the more common)

Titled (SAMPLE containing the dissolved metallic ion)

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Complexometric Titrations

ATTENTION:

CHECK THE NEED TO USE A SUITABLE SAMPLE PREPARATION

The experimental aspects of the complexometric titrations are

similar to those of the other volumetries ...

Equivalence point x End point

abrupt change in some titled property

The titrant concentration should be known and reliable

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Titrant requirements (standard solution)

Stable solution of known and reliable concentration

Prepared from primary or secondary standards

The reaction with the test substance should be rapid,

occur at room temperature and have defined stoichiometry

Example of standard solution: EDTA

acid solution of ethylenediaminetetraacetic acid (0.01-0.10 mol L-1)

C10H16N2O8

292,2 g mol-1

White solid, soluble in water, must be oven dried before use and it is available

Commercially as di-sodium, tetra-sodium and calcium-di-sodium salts 35

REFERENCES USED

QUÍMICA ANALÍTICA QUANTITATIVA ELEMENTAR N. Baccan, J. C. Andrade, O. E. S. Godinho, J. S. Barone 3ª Ed, Editora Edgard Blücher Ltda: São Paulo, 2001

ANÁLISE QUÍMICA QUANTITATIVA D. C. Harris 7ª Ed, LTC: Rio de Janeiro, 2008 FUNDAMENTALS OF ANALYTICAL CHEMISTRY D. A. Skoog e col. 8th Ed, Thomsom Broks Cole: Belmont, 2004 INTERNET: www2.iq.usp.br/docente/gutz/curtipot.html 36

Continue in the next class!

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