Thermogravimetric analysis (TGA) By Thermogravimetric analysis(TGA) by Vikram University, Ujjain

A very good note

1. THERMOGRAVIMETRIC
ANALYSIS(TGA)
Dr. Anshumala Vani
S.S. in Chemistry & Biochemistry
VIKRAM UNIVERSITY,
UJJAIN
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2. Contents: •Introduction
•Types
•Principal
•TGACurve
•
Instrumentation
•
Instrumentation
•Factors affectingresults
•Advantages
•Limitations
•Pharmaceuticalapplications
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3. Thermogravimetricanalysis(TGA) •Thermogravimetric analysis or thermal gravimetric analysis
(TGA)isamethodofthermalanalysisinwhichthe massofa
sampleismeasuredovertimeasthetemperature changes.
•This measurement
phenomena,
provides
such
information about physical
as phase
transitions, absorption and desorption as well as chemical
phenomena including chemisorptions, thermal decomposition,
andsolid-gasreactions(e.g.,oxidationorreduction)
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4. Three types ofthermogravimetry: 1. Isothermal or static thermogravimetry: In this technique
the sample weight is recorded as function of time atconstant
temperature.
2.
Quasistatic thermogravimetry:
In this technique the
sample
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2.
Quasistatic thermogravimetry:
In this technique the
sample
is heated to constant weight at each of series of increasing
temperatures.
3. Dynamic thermogravimetry: In this technique the sampleis
heated in an environment whose temperature is changing in a
predetermined manner generally at linear rate. This type is
generallyused.

5. Principle ofTGA: •Inthermo-gravimetricanalysis,thesampleisheatedinagiven
environment(air,N2,CO2,He,Ar,etc.)atcontrolledrate.The
changeintheweightofthesubstanceisrecordedasafunction
oftemperatureortime.
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•The temperature is increased at a constant rate for a known
initial weight of the substance and the changes in weights are
recorded as a function of temperature at different timeinterval.
•This plot of weight change against temperature is called
thermo-gravimetric curve or thermo-gram, this is the basic
principle ofTGA.

6. TGAcurve: •The instrument used for
themo-gravimetry is a
programmed precision
balance for rise intemperature
known as
Thermo
-
balance.
known as
Thermo
-
balance.
•Results are displayed by aplot
of mass change versus
temperature or time and are
known as Thermogravimetric
curves or TGcurves.
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7. TGAcurve: •TG curves are normally plotted with the mass change (D
m) in
percentage on the y-axis and temperature (T) or time (t) onthe
x-axis.
•
There are
two
temperatures in the reaction,
Ti(procedural
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•
There are
two
temperatures in the reaction,
Ti(procedural
decomposition temp.) and Tf(final temp.) representing the
lowest temperature at which the onset of a mass change is seen
and the lowest temperature at which the process has been
completedrespectively.
•The reaction temperature and interval (Tf-Ti) depend on the
experimental condition; therefore, they do not have anyfixed
value.

8. TGA curve of AgNO
3:
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9. Instrumentation ofTGA:
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10. Instrumentation ofTGA: •Recordingbalance
•Sample holder
•Furnace
•Temperature programmer /controller(thermocouple)
•Recorder
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11. Recording balance:
A microbalance is used to record a change in mass ofsample/
substance.
An ideal microbalance must possess followingfeatures:
•It should accurately and reproducibly record the change in
mass of sample in ideal ranges of atmospheric conditions and
temperatures.
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temperatures.
•It should provide electronic signals to record the changein
mass using arecorder.
•The electronic signals should provide rapid response tochange
inmass.

12. Recording balance: •It should be stable at high ranges, mechanicallyand
electrically.
•Its operation should be userfriendly.
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After the sample has been placed on microbalance, it is leftfor
10-15min to stabilize. Recorder balances are of totypes:
I. Deflection-type instrumentsand
II. Null-typeinstruments

13. Recording balance: I. Deflection balances :they are followingtypes-
i. Beamtype
ii. Helicaltype
iii. Cantileveredbeam
iv.
Torsion
wire
iv.
Torsion
wire
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14. Recording balance: •II.Null point balances: It consist of a sensor which detects the
deviation from the null point and restores the balance to its
null points by means of restoringforce.
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15. Sample holder: •The sample to be studied is placed in sample holder or
crucible. It is attached to the weighing arm ofmicrobalance.
•There are different varieties of crucibles used. Some differin
shape and size while
some
differ
in materials
used.
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shape and size while
some
differ
in materials
used.
•They are made up from platinum, aluminum, quartz or alumina
and some other materials like graphite, stainless steel, glasset c

16. Sampleholder: Crucibles:
Crucibles should have temperature at least 100K greaterthan
temperature range of experiment and must transfer heat
uniformly to sample. Therefore the shape, thermal
conductivity and thermal mass of crucibles are important
which depends on the
weight
and nature of
sample
and
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which depends on the
weight
and nature of
sample
and
temperaturerange.
There are different types of crucibles. Theyare:
1. Shallow pans(used for volatilesubstances)
2. Deep crucibles (Industrial scalecalcination)
3. Loosely covered crucibles (self generated atm.Studies)
4. Retort cups (Boiling pointstudies)

17. Types ofcrucibles:
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18. Furnace •The furnace should be designed in such way that it produces a
linear heatingrange.
•It should have a hot zone which can hold sample and crucible
and its temperature corresponds to the temperature of
furnace.
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and its temperature corresponds to the temperature of
furnace.
•There are different combinations of microbalance andfurnace
available. The furnace heating coil should be wound in sucha
way that there is no magnetic interaction between coil and
sample or there can cause apparent masschange

19. Temperatureprogrammer/controller: •Temperature measurement is done in no. of ways
thermocouple is the most commontechnique.
•The position of the temperature measuring device relativeto
the
sample
is very
important.
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the
sample
is very
important.
•The major typesare:
a . The thermocouple is placed near the sample containerand
it has no contact with the sample container. This isn’t a good
arrangement where low-pressure areemployed.

20. Temperatureprogrammer/controller: b. The sample is kept inside the sample holder but not incontact
with it. This arrangement is better than that of (a) because it
responds to small temperaturechanges.
c .
The
thermocouple
is placed either in contact with
sample
or
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c .
The
thermocouple
is placed either in contact with
sample
or
with the sample container. This is the best arrangement ofsa mple
temperaturedetection.

21. Temperatureprogrammer/controller: Thermocouple in athermo-balance:
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22. Recorder: The recording systems are mainly of2types
1. Time-base potentiometric strip chartrecorder.
2. X-Yrecorder.
In some instruments, light beam galvanometer, photographic
paper recorders or one recorder with two or
more
pens are
also
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paper recorders or one recorder with two or
more
pens are
also
used.
In the X-Y recorder, we get curves having plot ofweights
directly againsttemperatures.
However, the percentage mass change against temperatureor
time would be more useful.

23. Factors affectingTGA: Factors affecting the TG curve The factors which may affectthe
TG curves are classified into two maingroups.:
(1)Instrumentalfactors:
(a)Furnace heatingrate
(b)
Furnace
atmosphere
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(b)
Furnace
atmosphere
(2)Sample characteristics includes :
(a)Weight of thesample
(b)Sample particlesize

24. Factors affectingTGA: 1.Instrumentalfactors:
Furnace Heating rate: The temperature at which the
compound (or sample) decompose depends upon the heating
rate. When the heating rate is high, the decomposition
temperatureisalsohigh.Aheatingrateof3.5°Cperminuteis
usually
recommended
for
reliable
and
reproducible
TGA
.
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usually
recommended
for
reliable
and
reproducible
TGA
.
Furnace atmosphere: The atmosphere inside the furnace
surrounding the sample has a profound effect on the
decompositiontemperatureofthesample.ApureN
2gasfrom
acylinderpassedthroughthefurnacewhichprovidesaninert
atmosphere.

25. Factors affectingTGA: 2.Samplecharacteristics:
(a)Weight of the sample: A small weight of the sample is
recommended using a small weight eliminates the existenceof
temperature gradient throughout the
sample.
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temperature gradient throughout the
sample.
(b)Particle size of the sample: The particle size of the sample
should be small and uniform. The use of large particle or
crystal may result in apparent, very rapid weight lossduring
heating.

26. Other factors affecting TGAcurve: •Sample holder
•Heat ofreaction
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•Compactness of sample
•Previous history of thesample

27. Advantages ofTGA: •A relatively small set of data is to betreated.
•Continuous recording of weight loss as a function of
temperature ensures Equal weightage to examination overthe
whole range of
study.
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whole range of
study.
•As a single sample is analyzed over the whole rangeof
temperature, the variation in the value of the kinetic
parameters, if any, will beindicated.

28. Limitations ofTGA: •The Chemical or physical changes which are notaccompanied
by the change in mass on heating are not indicated in thermo-
gravimetricanalysis.
•
During
TGA,
Pure
fusion
reaction, crystalline transition,
glass
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•
During
TGA,
Pure
fusion
reaction, crystalline transition,
glass
transition, crystallization and solid state reaction with no
volatile product would not be indicated because they provide
no change in mass of thespecimen.

29. Applications ofTGA: •From TGA, we can determine the purity and thermalstability
of both primary and secondarystandard.
•Determination of the composition of complex mixtureand
decomposition
of
complex
OR composition of
complex
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decomposition
of
complex
OR composition of
complex
systems.
•For studying the sublimation behavior of varioussubstances.
•TGA is used to study the kinetics of the reaction rateconstant.

30. Applications ofTGA: •Used in the study of catalyst: The change in the chemical
states of the catalyst may be studied by TGA techniques.(Zn-
ZnCrO4) Zinc-Zinc chromate is used as the catalyst in the
synthesis ofmethanol.
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•Analysis of the dosageform.
•Oxidative stability ofmaterials.
•Estimated lifetime of aproduct.

31. Applications ofTGA: •TGAisoftenusedtomeasureresidualsolventsandmoisture,
butcanalsobeusedtodeterminesolubilityofpharmaceutical
materialsinsolvents.
•
The
effect
of reactive or corrosive atmosphere on
materials.
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•
The
effect
of reactive or corrosive atmosphere on
materials.
•Moisture and volatiles contents onmaterials.

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