oRGANO METALLIC COMPLEXES EQUILIBRIUM

1. 44
The metal chelate is formed when a proper chelating agent is added to a
solution of metal ion. The chelate formed may be water insoluble or water
soluble. The water insoluble chelate when formed quantitatively, it may be filtered
and the composition and structure of the chelate can be determined by various
analytical methods and by making use of various physico-chemical methods. If
the chelate is soluble and if its isolation is difficult, special methods have to be
used to find out its composition and other characteristics.
When a chelate is formed, there is change in the properties of the metal ion
solution and many characteristics of the solution are altered. The few important
properties which are changed on complexation are -
(i) change in conductivity of the solution,
(ii) change in colour of the solution,
(iii) change in qualitative properties of their solution,
(iv) change in colligative properties.
If such a change in property is followed by suitable physico-chemical
measurements then it may be of great importance in elucidating the composition
and structure of the complex [166].
Different workers have used different methods for the study of complexes
in solution as well as to study the properties of water insoluble complexes. Few of
these methods used by different workers are listed below:
(i) Potentiometric measurements [167],
(ii) Ion-exchange method [168-169],
(iii) Polarographic method [170,171],
(iv) Solubility method [172-173],
(v) Reaction kinetics [174],
(vi) Electrical conductance,
(vii) Thermogravimetric analysis [175,207],
(viii) Spectrophotometric method,
(ix) Infra-red spectra [176].

2. 45
In the present work, last three methods have been used to study one or
more complexes.
SPECTROPHOTOMETRIC METHOD:
H. Ley [177] recognised that the development of a characteristic colour is
one of the most important feature of chelate compound. He made use of
absorption spectra to distinguish between free metal ions and their chelates.
Besides to indicate the chelate formation, spectrophotometry can also be used to
study the composition of chelates or complexes in solution. As the quantity of the
chelate formed is directly proportional to the concentration of metal ion,
spectrophotometry can also be used to find out the concentration of metal ion if
the standard data are available. The advancement made in the design of
sophisticated spectrophotometer has helped to exploit this technique to the
maximum possible extent.
Two basic laws of spectrophotometry are of much importance. The first
known as the Lambert law and the second known as the Beer-Lambert law.
According to Lambert's law, when a monochromatic light is passed through a
transparent medium, the intensity of the transmitted light decreases exponentially
as the thickness of the absorbing medium increases.
Mathematically, this can be written as,
- (dI/db) = K.I … (1)
where, I is the intensity of the incident light, dI is the small decrease in intensity
on passing through the small thickness, db and 'K' is a constant. Integration of
above equation (1) gives,
ln (Io/It) = K.b
or
It = Io . e-K.b
… (2)
Where, Io and It are the intensities of incident radiation and that of transmitted
radiation, respectively.
Beer studied the effect of concentration of the coloured constituent in
solution on the absorption of radiation. He found that "the intensity of a beam of

3. 46
monochromatic radiation decreases exponentially as the concentration of the
absorbing substance increases."
Combining Lambert's law and Beer's law, we have the law known as
Lambert-Beer law or simply known as Beer's law. It is stated mathematically as
follows:
It = Io . e
- Cb
… (3)
where, C is the concentration of the solution and b is the thickness, or path length
of measuring cell.
Converting equation (3) to natural logarithm, we can write down,
log (Io/It) = bC
where, Io is the intensity of the incident radiation and It is the intensity of
transmitted radiation, is a constant which is a characteristic of absorbing
species and wavelength. It is termed as molar absorptivity. The term log(Io/It) is
known as absorbance denoted by A.
Thus,
A = bC or = A/bC
Concentration 'C' is expressed in mole/lit and path length 'b' in cm and so molar
absorptivity will have the unit, lit.mol-1
.cm-1
.
Beer's law is obeyed well in dilute solution. In concentrated solution, the
index of refraction for the absorbed radiation is changed and hence the system
shows deviation from Beer's law. In the present study, the maximum
concentration limit upto which the Beer law is obeyed is studied.
DETERMINATION OF SANDELL'S SENSITIVITY [178]:
The knowledge of sensitivity is of utmost importance in colorimetric
determination of traces of metals. It is defined as the smallest weight of
substance that can be determined in a column of solution having unit cross-
section. The weight is expressed as micrograms and the area in cm2
.
This is valid only if system obeys the Beer law indefinitely at low
concentration. This is true for all the reactions. Two factors are involved in
determining the sensitivity: (1) the intensity of coloured product, and (2) ability of
the observer directly or indirectly to detect small difference in absorption of the

4. 47
solution. In spectrophotometry, the maximum amount of coloured substance that
can be determined usually depends upon the reproducibility of the measurement
of transmittance of faintly coloured solution. If molar absorptivity of coloured
compound is known, we can calculate Sandell's sensitivity as,
No. of atom of metal present in complex
Sensitivity (S) = n[M/ ] = x Mol. wt.
Molar absorptivity of species
Organic reagents with high molecular weights furnish maximum sensitivity,
when used as chromogenic reagent.
ABSORPTION SPECTRA OF METAL COMPLEXES:
For a metal chelate, two types of light absorption occur in the visible and
ultraviolet region: (1) the absorption due to an electronic transition in a
conjugated system in which a metal may or may not have taken part, and (2) the
absorption resulting from electronic transition in the metal ion itself. The former
which is due to the ligand is very strong and is known as "K" type absorption. It
has been investigated in several cases [179-180]. The absorption bands
characteristics of a metal ion alone are relatively weak and they result from the
so-called forbidden transition in the electron shells of the metal itself. This
transition which corresponds to an electron shift in the unfilled 'd' orbitals of the
transition metals and which are frequently intensified due to co-ordination may be
observed in a free metal ion as well as metal chelate. M. Calvin and co-workers
[180,181] observed absorption in the visible region due to the forbidden transition
and also 'K' type absorption bands, characteristics of the ligands in the UV region
in case of Cu(II) ethyl acetoacetate and other substituted -diketo chelates. They
concluded from the experimental data that the influence of a metal ion on the
characteristic absorption is much weaker than the ligand which has a large
conjugated system.
The transition metal cations have characteristic absorption bands in the
visible and near UV region. These are considerably changed according to stereo-
chemical forms and the ligand strength with the same solvent. In case of non-
polar solvent being used the absorption bands may be due to electron transition

5. 48
from 'd' orbital of lower energy to 'd' orbital of higher energy or due to charge
transfer. According to Franck-Condon principle, during electronic transition the
atom in a molecule does not change the relative position. In charge transfer
process, the absorption of light occurs when an electron is transferred from an
orbital lying principally on the ligand to an orbital lying principally on the metal or
vice-versa. The absorption bands due to charge transfer are intense. Generally,
such bands are at higher frequencies compared to d-d transition bands. Polar
solvents shift the charge transfer bands to lower wavelength [182]. In case of
d-d transition, the selection rules may be obeyed.
(1) Transition in which the number of unpaired electrons changes in going
from the ground to excited state are referred to as "spin or multiplicity
forbidden."
(2) Transitions within a given set of p or d sub-shell are "Laporte forbidden" if
the molecule has a centre of symmetry.
In transition metal complexes, there is always a centre of symmetry
which does not change during transition and hence d-d transitions are forbidden.
However, the distortion in the orbital symmetry due to ligand field or solvent effect
causes the transition and as a result weak absorption is observed in many cases.
ML6 complexes of Ni(II) exhibits a simple spectrum involving 3 spin allowed
transitions in the range of 7000-13000 cm-1
, 11000-20000 cm-1
and 19000-27000
cm-1
. In addition, two spin forbidden bands also observed. ML4 square-planner
complexes of Ni(II) gives a strong absorption band between 15000-35000 cm-1
and 23000-30000 cm-1
. On the other hand, ML4 tetrahedral complex of Ni(II)
gives one less intense band nearly at 16000 cm-1
[183-184].
K. K. Desai and H. B. Naik [185] studied the absorption spectra of Cu(II),
Ni(II) and Pd(II) complexes of 2-hydroxy-4-ethoxypropiophenone oxime [HEPO].
Cu(II) complex with HEPO shows two bands. A weak band at 650 nm has been
assigned to the transition 1
A1g
2
B1g; while the other strong band at 365 nm is
due to charge transfer. Ni(II) complex of the same ligand shows two bands. A
weak band at 620 nm is assigned to 1
B1g
1
A1g transition and a strong band at

6. 49
380 nm has been assigned to charge transfer, Pd(II) complex of the same ligand
shows only one band at 360 nm which they assigned to the combination of all the
three spin allowed transitions: (1
A1g
1
A2g; 1
B1g
1
B1g).
METHOD FOR THE DETERMINATION OF THE COMPOSITION OF THE
METAL CHELATE:
Reaction between metal ion and ligand can be written in general form as
follows:
M + nL [MLn]
The number of ligand molecules 'n' required for each mole of metal ion, i.e.
metal:ligand ratio can be determined from absorption measurements. Several
methods have been used since long for this purpose:
(1) the method of continuous variation [186],
(2) the mole-ratio method [187],
(3) the slope ratio method [188],
(4) the logarithmic method [189],
(5) the method of isobestic point [190], and straight line method [191].
In the present work, the first two methods are employed for determining
the composition of the chelates and hence a brief account of these two methods
is given here.
METHOD OF CONTINUOUS VARIATION:
I. Ostromisslensky [192] in 1910 and R. B. Denison [193] in 1912 first
worked out the basis of the method of continuous variation. Job [186] published
the details of this method along with the discussion.
In a reaction of the type,
M + nL [MLn]
in which the complex MLn is formed from the metal ions M and ligand L. Here,
the solutions of metal ion and ligand of the same concentration are mixed in
varying proportion.
M + nL [MLn]
…(1)

7. 50
K = n
L
M
MLn
]
][
[
]
[
…(2)
where, [ ] represents activities molar concentrations. If we impose the
restriction,
Mt + Lt = constant
where, "Mt" and "Lt" are the total molarities of M and L respectively, it can be
shown that when concentration MLn is maximum,
t
dM
MLn
d ]
[
= 0 …(3)
or
L / M = n …(4)
In other words, for a constant total concentration of the metal and the
chelating agent, the concentration of the chelate is the greatest when the metal
and the chelating agent are brought together in the same ratio in which they exist
in the chelate. This can be evaluated in terms of the absorbance. If a solution of a
ligand "L" is mixed with a solution of a metal ion "M", so that the total molar
concentration of the ligand and metal ion is maintained constant then,
M = Mt - [MLn] …(5)
L = Lt - n[MLn] …(6)
Mt + Lt = constant …(7)
The absorbance "A" of the solution at a given wavelength represents the
total absorption by all the species in the solution and that is expressed by,
…(8)
where, l = length of the light path through the solution and 1, 2 and 3 are the
respective molar absorptivity of "M", "L" and "MLn" respectively.
Function "Y" which represents the difference in the absorbance of equation
(8) and the corresponding absorbance in absence of the reaction between the
solution of 'M' and 'L' can be given by,
where, the chelating agent is optically transparent and cell path is 1 cm. Equation
(9) may be written by putting 2 = 0 and l = 1 follows:
D = [ (M) + (L) + (MLn)] ... (8)
1 2 3
)
9
(
...
]
[
)]
(
)
(
)
(
[ 2
1
3
2
1 t
t L
M
MLn
L
M
Y

8. 51
Differentiation of the equation (10) with respect to "Lt" and combination with
differentiated form of equation (9) gives,
dY/dlt = ( 3- 1) . d(MLn)/dLt … (11)
Equation (11) represents the basis for the Job's method of continuous variation.
If the chelate is the only coloured substance present, the absorbance of the
solution is proportional to the concentration of the chelate and hence, the graph
of absorbance against the ratio of metal ion concentration to total concentration
of metal and ligand would give a curve showing maximum at the ratio
corresponding to the composition of the chelate.
This method has been used to determine the composition of Cu(II), Fe(II),
Fe(III) and UO2(II), V(V) chelate with several salicylic acid derivatives. R.T. Foley
and R.C. Anderson [194], S.E. Turner and R.C. Anderson [195] and J.H. Yoe
and R.E. Harvey [196] employed this method for various complexes. R.K.
Pandya [197] used this method to determine the composition of Cu(II), Ni(II) and
Co(II) chelates with o-hydroxy ketoximes.
A continuous variation plot generally will not produce a valid result if more
than one complex is formed. If a single complex is formed, the maximum of
continuous variation plot should be independent of wavelength. Consequently, it
is common practice to measure the absorbance of the prepared solutions at
several wavelengths. max that varies with solution suggest the presence of more
than one complex.
MOLE-RATIO METHOD:
The stoichiometric ratio of complex was also determined by the mole-ratio
method.
J.H. Yoe and A.L. Jones [187] described the mole-ratio method in which a
series of solutions are prepared containing a constant amount of the metal ion
but with increasing ratios of ligand to metal. For a stable complex, the curve rises
)
10
(
...
)
(
)
(
)
( 1
3
1 t
M
MLn
M
Y

9. 52
from the origin as a straight line and breaks sharply at a constant absorbance at
the molar ratio of the components in the complex, if both the interactants are
colourless. However, a complex that undergoes appreciable dissociation in
solution, gives a continuous curve which becomes approximately parallel to the
molar ratio axis only when an excess of the variable component is added.
In many cases, the results obtained by extrapolation of this curve are
uncertain. It is often seen that such a curve may be made to break sharply at the
correct molar ratio, if the ionic strength of the solutions adjusted to a suitable
value by the addition of an indifferent electrolyte. Thus, in such cases also, it is
possible to get information about the composition of the complex by this method.
Also the mathematical treatment of the mole-ratio method for deducing the
stoichiometry of complex in solution for situations in which several complexes
exist under a given set of conditions has been fully worked out by A.S. Meyer and
G.H. Ayres [198]. The mole-ratio method is generally superior to the method of
continuous variation for complexes having large ligand to metal ratios. For
example, the relative difference between the position of maximum for ML5 and
ML6 is 20% in mole-ratio method and 3% in continuous variation method.
DETERMINATION OF STABILITY CONSTANT:
One of the most important factors relating to the coordination compound is
its stability constants or formation constant. A reaction between a metal ion and a
ligand may be represented as,
… (1)
and stability constant or formation constant Ks of the chelate is given by,
If "a" represents the initial concentration of the metal, "b" that of the ligand
and "c" the concentration of chelate formed at equilibrium, then the stability
constant Ks is given by,
mM + nL MmLn
K =
[M L ]
[M] [L]
... (2)
S
m n
m n
K =
X
[a- mX] [b- nX]
... (3)
S m n

10. 53
Hence, the determination of "X" permits a calculation of the value of Ks, "a"
and "b" being known. The stability constant is a measure of the stability of the
complex in solution with reference to the dissociation into metal ions and free
ligands. From a precise knowledge of the stability constant, thermodynamic
constant may be evaluated. However, the method is accompanied by many
difficulties and it is doubtful whether true thermodynamic quantities of chelation
can be determined in a very simple cases.
The stability constants of chelates are studied mainly by two procedures.
G.N. Lewis and M. Randall [199] first introduced the concept of ionic strength
which later received theoretical justification from the Debye-Huckel theory. The
classical approach for the evaluation of thermodynamic equilibrium constant
involves the determination of equilibrium constant in media of low ionic strength
followed by an extrapolation to zero ionic strength (infinite dilution). Some
workers have used the value of a single determination and attempted to correct
this value to a thermodynamic equilibrium constant by the application of Debye-
Huckel theory. The second method was introduced by the G. Biederman and L.
G. Sillen [200] and the fundamental idea of this method is to control the activity
by keeping the ionic strength constant, because in dilute solution the activity
coefficient of a given strong electrolyte is the same in all solutions of identical
ionic strength. F.C. Rossotti and H.R. Rossotti [201] concluded while discussing
methods for deter-mining stability constant, "it would, therefore, seems better to
obtain reliable values of the stoichiometric constants (which describe the stability
of a species relative to the corresponding complexes with solvent molecules and
medium ions, then less certain values of the thermodynamic constants which do
not give absolute stability either, but only stability relative to the solvated
species)." The value of the stoichiometric constants are reliable under a given set
of experimental conditions and are useful for practical purpose. In the present
study, the constants determined are those obtained at room temperature and pH
as mentioned. At the particular pH, the effect of the hydrolysis of metal salt have
not been taken into account. This constant has been termed as stability constant
in the present work. Attempts to maintain the ionic strength with different

11. 54
electrolyte could not succeed in the present study as the metal chelates either
precipitated by the addition of an electrolyte or there was a gradual fading of
colour.
There are various methods for determining the stability constant. The mole
ratio method and Job's method which have been used here is described below in
brief.
DETERMINATION OF STABILITY CONSTANT BY MOLE-RATIO METHOD:
The stability constant may be calculated from the mole-ratio method. A
series of solutions is prepared which contain equal formal concentrations of the
metal ion, but different formal concentrations of the ligand. The ratio of these
concentrations should usually vary from about 0.1 to 10 or 20. The absorbances
of each solution is measured at a wavelength where the complex absorbs but the
aquometal ion does not. These absorbances are proportional to the equilibrium
concentration of the complex in the solutions and a plot of the absorbance
against the ratio of the number of moles of ligand to the number of moles of
metal-ion will resemble inverted obtuse angle. The extent of the curvature in the
vicinity of the end point depends, of course, on the degree of dissociation of the
complex. However, the stoichiometric formula of the complex can be found by
extrapolating the straight line portions of the graph, which is to say that the point
at which these lines intersect corresponds directly to the ratio of the ligand to
metal ion in the complex. This procedure works well for weakly dissociated
complex. But, if the dissociation constant of the complex is too high, the mole
ratio plot will become a smooth continuous curve and it will be impossible to
locate the stoichiometric point. In such cases, better results can often be secured
by the slope-ratio or continuous variation method.
Within a certain rather restricted range, however, the curvature around the
"end point" of a mole-ratio plot can be turned to good advantage and used for the
calculation of the stability constant of the complex. Let the dissociation of the
complex, be represented as,

12. 55
C 0 0 … Initial concentration
C(1- ) C n( C) … Equilibrium concentration
where, C is the total concentration of the complex in moles per litre assuming no
dissociation and is the degree of dissociation, the stability constant (reciprocal
of dissociation constant) may be written as,
The value of 'n' for the complex is obtained by Mole-ratio method and Job's
method. The value of may be obtained from the Mole-ratio curve and Job's
method plot by the following relationship:
where, Em is the maximum absorbance obtained from the curve, indicating that all
the reagent is present in the form of the complex. Es is the absorbance at the
correct stoichiometric molar ratio of the metal to reagent in the complex. As the
complex always little the value of Es smaller than Em.
INFRA-RED SPECTRA:
In contrast to the relatively few absorption bands observed in the UV region
for most organic compounds, the infra-red spectrum provides a rich array of
absorption bands. Many of the absorption bands can not be assigned accurately,
those that can, however, provide a wealth of structural information regarding the
molecule.
Comparison of the IR spectra of the ligands with that of complexes can
provide a very useful information regarding the nature of bonding in complexes.
In the metal complexes formed with the oximes, the metal ion is joined between
oximino group and o-hydroxy group. The exact linking is ascertained by above
comparison. Many workers have used this technique.
MLn M + nL
=
E - E
E
m s
m
n
S
C
n
C
C
K
)
(
)
1
(

13. 56
K. K. Desai and H. B. Naik [185] studied the complexes of Cu(II), Ni(II) and
Pd(II) formed with 2-hydroxy-4-ethoxypropiophenone oxime [HEPO]. They have
done the comparison of the IR spectra of complexes with that of ligand. In the IR
spectrum of HEPO, two band observed in the -OH stretch region are assigned to
two different type of hydroxyl group in the ligand. The first band observed around
3400 cm-1
disappears in the spectra of complexes is due to the intramolecularly
bonded -OH group i.e. 2-hydroxy group (phenolic -OH). The second band due to
oximino hydroxyl group of the ligand is observed at 2900-3000 cm-1
. The position
of this band is not affected on complexation. This clearly indicates that the
oximino hydroxyl group does not take part in coordination. The coordination of
metal ion through azomethine nitrogen is indicated by lowering of C=N band
from 1630 cm-1
in the ligand to 1615-1620 cm-1
in the complexes. This is also
supported by a slight downward shift of N=O (at 980 cm-1
in the ligand to
930-935 cm-1
in the complexes).
J. D. Talati and K. S. Parikh [202] used 2-hydroxy-4-n-butoxybutyro-
phenone oxime for the determination of Ni(II). They have done the comparison of
the IR spectra of complex with that of ligand. In the IR spectrum of ligand, two
bands are observed in the -OH stretch region, one at 3285 cm-1
due to the
2-hydroxy group and the other at 2840 cm-1
due to the oximino group. In the IR
spectrum of complex, the first band at 3285 cm-1
disappeared while the second
band shifted to 2880 cm-1
. This suggests that there is acid dissociation of the
phenolic 2-hydroxy group followed by the formation of Ni(II) complex through O of
the phenolic group and N of the oximino group, nitrogen forming a coordinate
bond.
J. D. Singh and S. P. Gupta [203] compared the IR spectra of
2,4-dihydroxy valerophenone oxime and Pd(II) complex. They observed new
bands in the spectra of complexes at 580 cm-1
and 515 cm-1
, which they assigned
to metal nitrogen and metal oxygen stretching modes respectively.
K. K. Desai, N. D. Naik and H. B. Naik [204] used 2-hydroxy-4-ethoxy-
acetophenone oxime as an analytical reagent for Ni(II). They have studied IR
spectra of ligand and chelate. The IR spectrum of chelate shows the band at

14. 57
2900-3000 cm-1
(=N-OH group). The band appearing at 3400 cm-1
(phenolic -OH
group) in oxime disappeared in the chelate. The band due to N=O appeared at
lower frequency in chelate than those in ligand. All these proved that N of oximino
group forms coordinate bond with metal while oxygen of 2-OH forms ionic bond,
with metal ion.
THERMOGRAVIMETRIC ANALYSIS :
Thermal methods of analysis may be defined as those techniques in which
changes in physical and/or chemical properties of a substance are measured as
a function of temperature. The various techniques under this heading are as
follows:
(i) Thermogravimetry [TG] :
A technique in which a change in weight of a substance is recorded as a
function of temperature.
(ii) Differential Thermal Analysis [DTA] :
A method for recording the difference in temperature between a substance
and an inert reference material as a function of temperature.
(iii) Differential Scanning Calorimetry [DSC] :
A method whereby the energy necessary to establish a zero temperature
difference between a substance and a reference material is recorded as a
function of temperature.
In the present work, thermogravimetry is used to study some
characteristics of the complexes.
The basic instrument required for thermogravimetry is a precision balance
with a furnace programmed for a linear rise in temperature, with time. The
heating rate may be controlled as desired. The results may be presented as-
(i) actual weight of substance as a function of temperature
(ii) the weight loss (in gms or percent) as a function of temperature
(iii) first derivative curve dw/dT as a function of temperature.

15. 58
Results of thermogravimetry are affected by various factors such as - (i)
heating rate, (ii) furnace atmosphere (iii) crucible geometry, and (iv) nature and
characteristics of a sample.
The early most wide-spread applications of thermogravimetry in analytical
chemistry had been in the study of the recommended drying temperature of
gravimetric precipitates. Duval [205] studied over a thousand gravimetric
precipitates by this method and gave the suitable drying temperature. For
instance, in case of Ag2CrO4, it was found from TGA that it may be dried at any
temperature between 100 C and 800 C, where there is plateau in TG curve.
Previously exact 135 C was specified.
In the present study, TGA of Cu(II), Ni(II) and Pd(II) complexes formed with
HMCO have been subjected to thermogravimetric analysis and from their
thermograms, temperature for safe drying of the precipitate without
decomposition have been found.
Further, since the TG curve is quantitative, calculations on stoichiometry of
compound can be made. In the present work, the weight of residue obtained after
the complete pyrolysis of the metal chelate is tried to correlate with the expected
weight of the residue from the chelates and this information has been used to
have idea about the stoichiometry of the complex.
EVALUATION OF KINETIC PARAMETERS :
Thermogravimetric analysis (TGA) has proven useful for evaluating kinetic
parameters of various reactions and materials which then provide viable
techniques for evaluation of potentially unstable nature of materials. TGA
methods for calculating kinetic parameters are proposed and are based on the
relationship between weight loss and temperature and often utilizing the
derivative rather than the integral curve. This methods have often been used to
evaluate degradation kinetics. The deviation of kinetic data from TGA curves
obtained under non-isothermal conditions has received considerable attention
and several comprehensive reviews are available.

16. 59
For the purpose of evaluating activation energy for thermal decomposition
of complexes of metal ion formed with HMCO, TGA thermograms obtained under
non-isothermal (dynamic) conditions have been used. Two types of methods of
TGA data treatment are available to evaluate activation energy (E) for thermal
decomposition;
(i) Single heating rate method, and
(ii) Multiple heating rate method
Here, evaluation of E based on a single heating rate method has been
described and the same is used in present work.
EVALUATION OF 'E' BASED ON SINGLE HEATING RATE METHOD :
Dynamic TGA thermograms obtained at a heating rate 10 C/min have
been analysed in terms of the graphical method proposed by Broido. [206].
METHOD OF BROIDO :
The Broido method is a simple and sensitive graphical method of treating
TGA data. According to this method, the weight at any time t, wt is related to the
fraction of initial molecules not yet decomposed Y, by the equation.
(1)
where, W0 is the initial weight of the material and W is the weight of the residue
at the end of degradation, and Wt is the weight of the residue at time t.
For isothermal pyrolysis,
dY/dt = -K. Yn
(2)
If, K = A. e- E/RT
(3)
and if T is a linear function of time t, i.e.
T = T0 + .t (4)
The equation (2), (3) and (4) may be combined,
dY/Yn
= -[A/ ]. e- E/RT
dT (5)
where, = dT/dt
On integration we get,

17. 60
= A/ . dT (6)
For first order kinetics (n=1) of complex degradation, it comes to
= -InY = In[1/Y] (7)
Putting this in equation (6) and on integration and taking logs of both sides of
equation (6), following equation is obtained.
In.In[1/Y] = (E/RTm + 1) InT + constant (8)
Thus, for first order reaction a plot of In.In(1/Y) Vs. InT yields a straight line
whose slope is related to E, assumption of e-E/RT
(Tm/T)2
.e-E/RT
leads to
InIn(1/Y) = -E/R(1/T) + constant (9)
Equation (9) is most accurate.
Patel, Ray and Patel [207] studied complexes of Ni(II), Co(II), Zn(II),
Mn(II), Cd(II) and U(VI) complexes with -oximinoacetoacet-o/p-anisidide
thiosemicarbazone [OAOATS and OAPATS]. The TG results revealed that the
metal chelate follow a single stage decomposition. From TG traces, it is observed
that the curves for metal chelates are steeper while the curve for ligand is
broader. On the basis of this, it is assumed that the rate of reaction for metal
chelate decomposition is faster than that of ligand. The Broido method was
applied to the TG data to determine the energy of activation and the order of the
reaction. The two water molecules in Mn(II) and UO2(II) complexes were lost at
180 C indicating that these molecules are probably coordinated to the respective
metal ions. The trend in thermal stability of the metal chelates on the basis of the
Ea values in the decreasing order is OAOATS > Zn > Ni > Mn > Hg > UO2 > Cd >
Co and OAPATS > Zn > Ni > Mn > Cd > Hg > UO2 > Co. The decomposition of
all chelates followed first order reaction.
N. B. Patel & H. H. Parekh [112, 113] have carried out TGA of complexes
and using the kinetic methods described above determined activation energy for
the decomposition of complexes.
In present study from TGA of Cu(II), Ni(II) and Pd(II) complexes formed
with HMCO, the kinetic parameters have been determined.