ADVANCED ANALYTICAL CHEMISTRY – 1S 2019
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
ADVICES
1) Ponctuation
2 Tests (30/05/19 e 18/07/19)
1 Seminar (11/07/19)
2 Atividades: 27/06/19 task 1 (sample preparation)
11/07/19 sampling (experiment at home)
2) Sampling experiment
“M&M Sampling Study”
Extra Activity (Submit Report)
ADVICES
3) Seminaries
“methods about acid-base equilibrium”
Sugestions already discussed
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
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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
Colour varies with central metal and from complex to complex
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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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Types 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*:
1,100,000 results ("chelators")
420,000 results ("complexing agents")
* Consultation completed 05-08-19
Other complex exemples :
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Complexes with TWO or more CENTRAL IONS
NH2
NH3)3Co Co(NH3)3
OH
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 ligands:
they connect 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 ligands 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 non complexed
EDTA 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
(ligand 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+ concentration 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 diethyldithio 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 ten
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 titration 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
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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