Download 5 Complexation

Complexation Reactions
and Titrations
Dr. Mohammad Khanfar
The formation of complexes
• Complex compounds are those formed as a result of a
coordinate bond formation between a donor group and
acceptor group, to produce a complex which has different
properties from those of the free metal ion
• Donor group (ligand): has at least one pair of unshared
electrons available for bond formation.
• ligands: water, ammonia, and halide ions, carboxyl
• Acceptor group or atom: has an has one or more of empty
orbitals. The acceptor is usually a metal atom (e.g. Mg2+, Ca2+,
Al3+,…)
The formation of complexes
• The number of coordinate bonds that a cation tends to form
with electron donors is its coordination number. Typical values
for coordination numbers are 2, 4, and 6.
• The species formed as a result of coordination can be
electrically positive, neutral, or negative. For example,
copper(II), which has a coordination number of 4, forms a
cationic ammine complex. Cu(NH3)42+; a neutral complex with
glycine, Cu(NH2CH2COO)2 and an anionic complex with
chloride ion, CuCI42-.
The formation of complexes
• Chelates, is produced when a metal ion coordinates with two or
more donor groups of a single ligand to form a five- or sixmember heterocyclic ring. Reaction called chelation reaction.
• Chelate complexes are more stable than non-chelate complex.
• The Cu complex of glycine is an example chelation
The formation of complexes
• A ligand that has a single donor group, such as ammonia, is
called unidentate NH3-Cu2+
• Diaminoethane, which has two groups available for
coordinate bonding, is called bidentate.
H 2N
NH 2
M
• Multidentate: tridentate, tetradentate, pentadentate, and
hexadentate chelating agents are also known. e.g.
Ethylenediaminetetraacetic acid (EDTA)
The formation of complexes
• Complexation reaction are useful in titrimetric analysis as they
can be fast and complete
• However, they are reversible reactions that can be
represented by the general equation:
Kf
• Where L is the ligand (donor) and M is the metal
• The ligand can be charged or neutral
• The higher the formation constant (Kf) the more complete and
spontaneous the reaction would be, the more it would be
suitable for titrimetric analysis
The formation of complexes
• Complexation reactions occur in a stepwise fashion,
Complexometric titrations
• Titration curve is usually a plot of pM = - log [M] as a function of the
volume of titrant added.
• Most often, in complexometric titrations the ligand is the titrant
and the metal ion the analyte, although occasionally the reverse is
true.
• As titrants, multidentate ligands, particularly those having four or
six donor groups, have two advantages over their unidentate
counterparts.
• First, they generally react more completely with cations and thus
provide sharper end points.
• Second, they ordinarily react with metal ions in a single step
process (1:1 ratio), whereas complex formation with unidentate
ligands usually involves two or more intermediate species
Complexometric titrations
Curve A: M having a coordination number of 4 reacts with a tetradentate
ligand D to form the complex of MD. Curve B: is for the reaction of M with
a bidentate ligand B to give MB2 in two steps. Curve C: involves a
unidentate ligand A that forms MA4 in four steps
Complexometric titrations
• Ethylenediaminetetraacetic acid (EDTA), is the most widely
used complexometric titrant.
• The EDTA molecule has six potential sites for bonding a metal
ion: the four carboxyl groups and the two amino groups, each
of the latter with an unshared pair of electrons. Thus, EDTA is
a hexadentate ligand.
• EDTA is fully protonated (H4Y) at pH < 3, half protonated (H2y2) between pH 3-10, and fully deprotonated (Y4-) at pH > 10
• EDTA is commercially available as H4Y and the dihydrate of the
sodium salt, Na2H2y.2H20
• EDTA is a valuable titrant because it form stable 1:1 complex
with almost all cations except alkali metals
Complexometric titrations
• This great stability results from the several complexing sites within
the molecule that give rise to a cage-like structure, in which the
cation is effectively surrounded by and isolated from solvent
molecules.
Complexometric titrations
• EDTA is found to be sparingly soluble in water (0.2% w/v)
whereas its corresponding disodium salt is almost 50 times
more soluble than the parent compound (solubility 10% w/v).
Therefore, it is the disodium salt of EDTA (H2Y2-) which is
normally used in complexometric titrations.
• Therefore, if a solution is made such that [Y] = [MY], pM = -pKf
(or pM = pK` where K` = dissociation constant)
Complexometric titrations
• Effect of pH on complex formation
• EDTA has four protonated states, and since the actual
complexing species is Y4-, complexes will form more efficiently
and stable in alkaline solution.
• However, some complexes of divalent and trivalent metals,
e.g. copper, lead, nickel, cobalt are stable down to pH 3.
• Therefore, solutions are usually buffered at a pH at which the
complex is most stable and at which the color change of the
indicator is most distinct.
Complexometric titrations
• Indicators for EDTA Titrations
• The indicator is a dye which is capable of acting as a chelating
agent to give a dye-metal complex. The latter is different in
color from the dye itself and also has a lower Kf constant than
the EDTA-metal complex.
• The color of the solution, therefore, remains that of the dye
complex until the end point, when an equivalent amount of
sodium edetate has been added. As soon as there is the
slightest excess of edetate, the indicator is displaced and the
metal-indicator complex decomposes to produce free
indicator; this is accomplished by a change in color.
Complexometric titrations
• Eriochrome Black T is a typical metal ion indicator that is used in
the titration of several common cations.
• Its behavior as a weak acid is described by the equations:
pKa 6.3
pKa 11.5
• It has blue color in the pH range (7-11) in the uncomplexed form
• The metal complexes of Eriochrome Black T are generally red,,
therefore, it is necessary to adjust the pH to 7 or above so that the
blue form of the species, HIn2-, predominates in the absence of a
metal ion.
Complexometric titrations
• Until the equivalence point in a titration, the indicator complexes
the excess metal ion so that the solution is red. With the first slight
excess of EDTA, the solution turns blue as the free uncomplexed
indicator accumulated
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Other indicators:
Alizarine flurine blue
Calcon
Catechol violet
Diphenylcarbazone
Methyl thymol blue
Tiron
Murexuide
General principles involved in disodium edetate
titrations
• Direct Titrations:
• A suitable buffer solution and indicator are added to the metal ion
solution and the solution titrated with standard disodium edetat
until the indicator just changes color. A blank titration may be
performed, omitting the analyte as a check on the presence of
traces of metallic impurities in the reagents.
• Example: 0.3 g sample of magnesium sulphate (MgSO4, 120.36
g/mol) was dissolved in water and buffered with ammonia at pH 10,
mixture of Eriochrome Black T and sodium chloride (1:99) was
added as indicator. The mixture was titrated with 45 ml of 0.05 M
disodium edetate until the solution becomes full blue. 0.3 ml was
consumed in blank titration. Calculate the %w/w of MgSO4.
• Answer: 89.7%w/w
General principles involved in disodium edetate
titrations
• Back titartion
• Back-titration procedures are used when no suitable indicator
is available. when the reaction between analyte and EDTA is
slow, or when the analyte forms precipitates at the pH
required for its titration.
• The sample is heated with measured excess of disodium
edetate to form the soluble complex with the analyte (metal),
and then the excess EDETA is back-titrated with Mg2+ or Zn2+
of known concentration using a suitable indicator.
General principles involved in disodium edetate
titrations
• Example: Determination of the % of Ca3(PO4)2, since
tricalcium phosphate is insoluble, the sample is dissolved with
the aid of heat in excess HCl. One mole of tricalcium
phosphate react with three moles of EDTA.
• 0.7 g sample of tricalcium phosphate (310.18 g/mol) was
dissolved and heated in 250 ml of diluted HCl solution. After
15 min, 50 ml aliquot was treated with 55mL of 0.05M
disodium edetate. Ammonia buffer was added to bring the pH
to 10 and the disodium edetate not required by the sample is
back titrated with 32 mL of 0.05M zinc chloride using mordant
black II as indicator to red color (complexed indicator).
Calculate %w/w of Ca3(PO4)2.
• Answer: 84.9%
General principles involved in disodium edetate
titrations
• Water Hardnes
• Water "hardness" was defined in terms of the capacity of cations
(Ca, Mg, and other heavy metals) in the water to replace the
sodium or potassium ions in soaps and to form precipitated
products that cause "scum" in the sink or bathtub.
• The determination of hardness is a useful analytical test that
provides a measure of the quality of water for household and
industrial uses. The test is important to industry because hard
water, on being heated, precipitates calcium carbonate, which clogs
boilers and pipes.
• To determine Ca2+ hardness only, murexide is used as indicator in
strongly alkaline solution (pH 12) since, under these conditions, it
chelates with Ca2+ only and Mg2+ precipitates as Mg(OH)2.
• Titration of Ca2+ and Mg2+ in a 50.00mL sample of hard water
required 23.65 mL of 0.01205 M EDTA. A second 50.00mL
aliquot was made strongly basic with NaOH to precipitate
Mg2+ as Mg(OH)2. The supernatant liquid was titrated with
14.53 mL of the EDTA solution. Calculate:
• (a) the total hardness of the water sample, expressed as ppm.
• (b) the concentration in ppm of CaC03 in the sample.
• (c) the concentration in ppm of MgCO3 in the sample.
How to convert molarity to normality
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The general formula to convert molarity to normality:
N=MxF
F depends on the reaction type.
For acid-base reaction, F is the number of accepted or donated
protons per molecule for basic and acidic substances respectively.
It’s more complex for complexation reaction.
For the general equation of complexation reaction :
YL + XMN+
LM
Where L is the ligand (donor group), M is the metal (acceptor group
or atom), and N is the charge of the cation.
For the ligand (L), F = X/Y N
For the metal (M), F = N
How to convert molarity to normality
• What is the normality of 0.05 M of disodium edetate and 0.1
M solution of Ca3(PO4)2.