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A seminar on
Dr. (Mrs.) Anju Goyal
Professor & Head
Dept. of Pharmaceutical Chemistry
Different thermal analytical methods
Types of Thermogravimetry
Recording of result
Information obtained from a TG curve
Factors affecting a TG curve
The term “thermal analysis” incorporates those techniques in
which some physical parameters of the system is determined
and/or recorded as a function of temperature.
When matter is heated it undergoes certain physical and
chemical changes. These changes that take place when an
unknown sample is heated provide us with information that
enables us to identify the material.
Based on the above definition, the various techniques of
thermal analysis are summarised in the table:
S.No NAME OF THE
1. THERMOGARVIMETRY TG Thermobalance Mass Mass vs.
DTG Thermobalance dm/dt dm/dt vs.
DTA DTA Apparatus ΔT ΔT vs.
DSC Calorimeter dH/dt dH/dt vs.
- Calorimeter Temperature Temperature
DIFFERENT THERMAL ANALYTICAL TECHNIQUE
S.No NAME OF THE
Reflectance % Reflectance
7. EVOLVED GA S
TMA Dilatometer Volume or
EC Electrometer or
I or R vs.
ETA ETA Apparatus Radioactivity
According to Wendlandt,
“ Any analytical instrument technique is regarded as a
thermal analysis method if the physical parameter is
measured as a function of temperature (or time).
According to this definition,
PROTON NUCLEAR MAGNETIC RESONANCE,
ELECTRON SPIN RESONANCE,
X- RAY DIFFRACTION,
UV, Visible and IR SPECTROPHOTOMETRY
are thermal methods.
It is a method of thermal analysis in which a physical
property of substance is measured as a function of
temperature whilst the substance is subjected to a
controlled temperature programmer.
An Internationally accepted definition of
thermogravimetry is as follows:
"It is a technique where by weight of substance in an
environment heated or cooled at a controlled rate, is
recorded as a function of time or temperature.”
TYPES OF THERMOGRAVIMETRY :
There are three types of Thermogravimetry:
Isothermal / Static Thermogravimetry
1. Isothermal/ Static Thermogravimetry: In this technique the sample
weight is recorded as a function of time at constant temperature.
2. Quasistatic Thermogravimetry: In this technique the sample is heated
to constant weight at each of the series of increasing temperature.
3. Dynamic Thermogravimetry: In this technique a sample is heated in an
environment whose temperature is changing in predetermine manner
generally at linear rate.
Most of the studies are generally carried out with dynamic
thermogravimetry. Therefore it is generally referred to as
The principle of thermogravimetry is based on the simple fact
that the sample is weighed continuously as it is being heated
to elevated temperatures and changes in the mass of a sample
Changes in temperature affect the sample. Not all thermal
changes/events bring a change in mass of sample i.e. melting,
crystallization but some thermal events i.e. desorption,
absorption, sublimation, vaporization, oxidation, reduction
and decomposition bring a drastic change in mass of sample.
It is used in analysis of volatile products, gaseous products
lost during the reaction in thermoplastics, thermosets,
elastomers, composites, films, fibers, coatings, paints, etc.
It is a technique which is studied under thermal analysis and is
employed for detection of such type of materials which undergo
mass change (gain or loss) when subjected to thermal events
viz. decomposition, oxidation, reduction, etc.
For this reason, it is very significant to optimize those
conditions/factors on which the change of mass of sample
depend throughout the operation/experiment.
RECORDING OF RESULT:
The instrument used for themogravimetry is a programmed precision
balance for rise in temperature known as Thermobalance.
Results are displayed by a plot of mass change versus temperature or
time and are known as Thermogravimetric curves or TG curves.
TG curves are normally plotted with the mass change (Dm) in
percentage on the y-axis and temperature (T) or time (t) on the x-axis.
A typical TG curve has been shown (Figure 1).
Fig.1.Characteristics of a
There are two temperatures in the reaction,
Ti (procedural decomposition temp.) representing the lowest
temperature at which the onset of a mass change is seen
Tf (final temp.) representing the lowest temperature at which
the process has been completed respectively.
The reaction temperature and interval (Tf-Ti) depend on the
experimental condition; therefore, they do not have any fixed
INFORMATION OBTAINED FROM A TG
Plateau: A plateau (AB, Fig.2.) is that part
of the TG curve where the mass is essentially
constant or there is no change in mass.
Procedural Decomposition Temperature:
The initial temperature, Ti, (B, Fig.2.) is
that temperature (in Celsius or Kelvin) at which the cumulative-mass
change reaches a magnitude that the Thermobalance can detect.
Final Temperature: The final temperature, Tf, (C, Fig.3.), is that
temperature (in Celsius or Kelvin) at which the cumulative mass change
reaches a maximum.
Reaction Interval: The reaction interval is the temperature difference
between Tf and Ti.
It can be concluded that Thermogravimetry is
concerned with the change in weight of a material
as its temperature changes.
First, this determines the temperature at which the
material loses weight. This loss indicated
decomposition or evaporation of the sample.
Second, the temperature at which no weight loss
takes place is revealed, which indicates stability of
These temperature ranges are physical properties
of chemical compounds and can be used for their
FACTORS AFFECTING THERMOGRAVIMETRIC
►EFFECT OF FURNANCE
► WEIGHT OF THE SAMPLE
►SAMPLE PARTICLE SIZE
►HEAT OF REACTION
►COMPACTNESS OF THE
►PREVIOUS HISTORY OF
EFFECT OF FURNANCE ATMOSPHERE: the test samples are
generally heated in vacuo or in the presence of an inert gas, in
order to remove the gases formed during sample heating and also
to prevent the occurrence of any undesirable reactions. The
common atmospheres involved in thermogravimetry are as
1. Static air: In this type air from atmosphere is allowed to flow
through the furnace.
2. Dynamic air: In this type compressed air from a cylinder is
allowed to pass through the furnace at a measured flow rate.
3. Inert atmosphere: Nitrogen gas (oxygen free) is used as inert
HEATING RATE: If a substance is being heated at a fast
heating rate, the temperature of decomposition will be higher
than that obtained at a slower rate of heating.
Eg.:- for a 10% decomposition of polystyrene, the
temperatures are : 375˚C for a heating rate of 1˚C / min and
394 ˚C for a heating rate of 5˚C / min.
SAMPLE HOLDER: The geometry of the sample holder can
change the slope of TG curve. Sample holders range from
flat plates to deep crucibles of various capacities. Materials
used in their construction may vary from glass, alumina, and
ceramic compositions to various metals and metallic alloys.
When the atmosphere is solely the gas, the shape of crucible
has no effect on the slope of the curve. Generally shallow
dish is preferred as there is a rapid exchange of gases
between sample and the surrounding atmosphere.
Weight of the sample: If a large sample is used, there occurs a
deviation from linearity as the temperature rises, especially for an
exothermic reaction. Eg.: evolution of CO during decomposition
of calcium oxalate to CaCO3.
Sample particle size: With the particle size of smaller dimension
the decomposition takes place earlier, while with greater particle
size the decomposition proceeds only at higher temperatures.
Previous history of the sample: Eg.: TG studies showed that
Mg(OH)2 prepared by precipitation method has a different
temperature of decomposition from that of the naturally
occurring material. This factor shows that one should be sure
Heat of reaction: This effect was studied by
Newkirk. The heat of reaction alters the difference
between the sample temperature and the furnace
temperature. If the heat effect is exothermic or
endothermic, this will cause the sample temperature
to lead or lag behind the furnace temperature.
Compactness of the sample: A compressed sample
will decompose at higher temperatures than a loose
Components of Instrumentation: :
A. Recording balance
B. Sample Holder
D. Furnace temperature programmer/Controller
TG curves are recorded using a Thermobalance. It consists
of an electronic microbalance, a furnace, a temperature
programmer and a recorder (instrument connected to
Thermobalance to record the output/curves).
It is the most important component of Thermobalance.
A microbalance is used to record a change in mass of sample/substance.
An ideal microbalance must possess following features:
a. It should accurately and reproducibly record the change in mass of
sample in wide ranges of atmospheric conditions and temperatures.
b. It should provide electronic signals to record the change in mass using a
c. The electronic signals should provide rapid response to change in mass.
d. It should be stable at high ranges, mechanically and electrically.
e. Modern microbalances have the ability to be not affected by vibrations.
f. Its operation should be user friendly.
After the sample has been placed on microbalance, it is left for 10-15 min
to stabilize. Recorder balances are of two types:
1. Deflection-type instruments and
2. Null-type instruments.
Deflection Balances: They are of following types:
I. Beam Type
ii. Helical Type
iii. Cantilevered Beam
iv. Torsion Wire
● Null-Point Balances : It consists of a sensor
which detects the deviation from the null point
and restores the balance to its null point by
means of a restoring force.
Different Types of Deflection Balances
Null Type balance
Sample Holder or Crucible:
The sample to be studied is placed in sample holder or
crucible. It is attached to the weighing arm of microbalance.
There are different varieties of crucibles used. Some differ
in shape and size while some differ in materials used.
They are made from platinum, aluminum, quartz or
alumina and some other materials like graphite, stainless
steel, glass, etc.
Crucibles should have temperature at least 100 K greater
than 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
There are different types of crucibles. They are:
1. Shallow Pans: These are used for such samples in which diffusion is
the rate controlling step. Volatile substances produced during
reaction must escape out which is determined as weight loss.
2. Deep Crucibles: These are used in such cases where side reactions
are required such as in study of industrial scale calcinations, surface
area measurements, etc.
3. Loosely covered Crucibles: These are used in self-generated
atmospheric studies. Rate of temperature or weight loss is not
important because the studies are done isothermally.
4. Retort Cups: These are used in boiling point studies. It provides single
plate of reflux for a boiling point determination.
Different types of crucibles are used for different materials i.e. Flat
crucibles with small lip are used for powdered sample whereas walled
crucibles are used for liquid samples. Therefore, the form of crucibles
used will determine the temperature gradients in sample.
The furnace should be designed in such a way that it produces a linear
It should have a hot zone which can hold sample and crucible and its
temperature corresponds to the temperature of furnace.
There are different combinations of microbalance and furnace available. The
furnace heating coil should be wound in such a way that there is no magnetic
interaction between coil and sample or there can cause apparent mass change.
Coils used are made of different materials with variant temperature changes
Nichrome wire or ribbon for T<1300 K,
Platinum for T>1300 K,
Platinum-10% rhodium Alloy for T<1800 K.
The size of furnace is important. A high mass furnace may have a high range
of temperature and obtain uniform hot zone but requires more time to achieve
the desired temperature. Comparatively, a low mass furnace may heat quickly
but it’s very difficult to control rise in temperature and maintain hot zone.
Position of furnace with respect to
It is done with the help of thermocouple.
Different materials are used for measuring different ranges of
temperatures i.e. chromal or alumel (alloys of Platinum) thermocouples
are used for T=11000 C, tungsten or rhenium thermocouples are used
for higher temperature.
The position of thermocouple is important. It can be adjusted in
i. Thermocouple is placed near the
sample container and has no contact
with sample container. This arrangement
in not preferred in low-pressures.
ii. The sample is kept inside the sample
holder but not in contact with it. It responds to small temperature changes
iii. Thermocouple is placed either in contact with sample or with sample
container. This method is best and commonly employed.
Position of thermocouple in a Thermobalance
The recording systems are mainly of 2 types:
1. Time-base potentiometric strip chart recorder.
2. X-Y recorder.
In some instruments, light beam galvanometer,
photographic paper recorders or one recorder with two
or more pens are also used.
In the X-Y recorder, we get curves having plot of
weights directly against temperatures.
However, the percentage mass change against
temperature or time would be more useful.
Used to analyze filler content in polymers; carbon black in
oils; ash and carbon in coals.
Automatic Thermogravimetric Analysis
Evaluation of gravimetric precipitates
Evaluation of suitable standards
Testing of purity of samples
Curie point determination
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Analysis, 5th edition New york 2001.
H.H.Willard, L.L Merrit Jr.J.A Dean, F.A Settle Jr.Intrumetal Method Of
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Gurdeep R.Chatwal, Sham K. Anad, Instrumental Method Of Chemical Analysis,5th
edition, Himalaya Publishing house,Page No-2.701.
T. Hatakeyama and F.X. Quinn, Jhon Wiley and Sons Publications, 1999. Thermal
Analysis Fundamentals and Applications to Polymer Science: Thermogravimetry, pg.
45-71, 2nd Ed.
Coats, A. W.; Redfern, J. P., 1963. "Thermogravimetric Analysis: A Review", pg. 88:
Fleming Polymer Testing And Consultancy, http://www.flemingptc.co.uk/our-
Sharma B.K. Goel Publishing House “Instrumental Methods of Analysis” pg. 234-
Thermal Analysis Dr. S. Anandhan, Asst. Professor, Dept. of Met. And Mat. Engg.,