it is a method of miscellaneous instrumental analytical technique. it is one of the thermal analytical techniques used. it also has wide applications in the field of pharmacy.

1. Thermogravimetry
SHIVV

2. Definition:
It is a technique whereby the weight of a
substance, in an environment heated or
cooled at a controlled rate, is recorded
as a function of time or temperature.

3. Types of Thermogravimetry:
 Dynamic TGA: In this type of analysis, the sample is subjected to
condition of continuous increase in temperature usually linear with time.
 2. Isothermal or Static TGA: In this type of analysis, sample is
maintained at a constant temperature for a period of time during which
change in weight is recorded.
 3. Quasistatic TGA: In this technique sample is heated to a constant
weight at each of a series of increasing temperature.

4. Principle:
 In thermogravimetric analysis, the sample is heated in a given
environment (air, N2, CO2, He, Ar, etc.) at controlled rate.
 The change in the weight of the substance is recorded as a function of
temperature or time.
 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 time interval.
 This plot of weight change against temperature is called
thermogravimetric curve or thermogram, this is the basic principle of
TGA

5. Principle:
 First, This determines the temperature at which the material loses
weight.
 This loss indicates decomposition or evaporation of sample.
 Second the temperature at which no weight loss takes place is revealed.
 T his indicates stability of the material.
 These temperature ranges are physical properties of chemical
compounds and can be used for their identification.

6. Thermogravimetric curve:
 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.

7. A typical TG Curve:

8. Information from TG curve:
 Types of TGA curve:
 TG curves are classified according to their shapes into seven types.
 Type A- this curves shows no mass change over the entire range of
temperature. It can be concluded that the decomposition temperature for
sample is greater than the temperature range of instrument.
 Type B- this curves shows that there is large mass loss followed by mass
plateau and is formed when evaporation of volatile product(s) during drying,
desorption or polymerization takes place. If a non-interacting atmosphere is
present in the chamber, type B curve will change into type A curve.
 Type C- this curve shows the single-stage decomposition temperatures (Ti
and Tf).

9. Types :

10.  Type D- this curve shows the multi-stage decomposition processes
where reaction is resolved.
 Type E- this curve shows the multi-stage decomposition reaction where
reaction is not resolved.
 Type F- this curve shows the increase in mass in the presence of an
interacting atmosphere e.g. surface oxidation reactions.
 Type G- this curve shows multiple reactions one after the other e.g.
surface oxidation reaction followed by decomposition of reaction
product(s).

12. Plateau:
 A plateau (AB, in fig) 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,in Fig.) 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, in Fig.), 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.

13. Instrumentation:

14.  The balance is the most important component of thermobalance.
 A good balance must fulfill:
 Accuracy, sensitivity, reproducibility and capacity should be similar to those
of analytical balance.
 Should have an adequate range of automatic weight adjustment.
 Should have high degree of mechanical and electronic stability.
 Should have rapid response to weight changes.
 Should be unaffected by vibration.
 Simple to operate and versatile.
The balance:

15.  Types of recording balances:
 Deflection type: these are of following types
 Beam type - in these balances, the conversion of deflected beams takes place into
the weight change.
 Helical type - in these balances, elongation or contraction of spring occurs with
change in weight which is recorded by the help of transducers.
 The cantilevered beam - in these balances, one end of beam is fixed and on other
end sample is placed. It undergoes deflection which can be recorded by the help
of photographic recorded trace
 Torsion wire - in these balances, the beam is attached to hard torsion wire which
acts as fulcrum. The wire is attached to one or both ends of balance to make the
deflection of beam proportional to weight changes.

16. Deflection balances:

17. Null point balances:
 It has sensor to detect the deviation of the balance from its null position.
 Then a restoring force is applied ( electrical or mechanical) to the beam
to restore its null position.
 This force is proportional to wt change.

18. Sample holder:
 The geometry, size and material with which it is made have an important effect on the TGA
curve.
 Materials used for construction are glass, quartz, alumina, stainless steel, graphite, etc.
 Types:
 Shallow pans – used for substances where it becomes necessary to eliminate diffusion as rate
controlling step. the sample is placed after forming a thin layer of it so that as soon as volatile
substance is formed, it will escape.
 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.
 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.
 Retort cups - These are used in boiling point studies. It provides single plat of reflux for a
boiling point determination.

19. Furnace:
 The furnace and control system( furnace controller) should be designed
to produce a linear heating rate over the whole working temperature
range of furnace.
 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 viz. Nichrome wire or ribbon for T<1300 K, Platinum for T>1300
K, Platinum-10% rhodium Alloy for T<1800 K and Silicon Carbide for
T<1800 K.

20.  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.
 The position of furnace is also important.
 Quartz spring balance has the weighing system below the furnace but
the beam balance has weighing system at several positions .

21. Temperature measurement:
 It is done with the help of thermocouple.
 Different materials are used for measuring different ranges of
temperatures i.e.
 Chromel or alumel (alloys of Platinum) for T=11000C.
 tungsten or rhenium thermocouples are used for higher temperature.

22.  The position of thermocouple is important. It can be adjusted in following
ways :
 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 only.
 iii. Thermocouple is placed either in contact with sample or with sample
container. This method is best and commonly employed.

24. Recorder:
 Two types:
 Time – based potentiometric strip chart recorder, and
 X – Y recorders. We get curves having plot of weights directly against
temperatures
 In some light – beam – galvanometer, photographic paper recorders or
one recorder with two or more pens are used.

25. Thermobalance:
Points to be kept in mind while purchasing a
thermobalance:
 Capable of recording continuously the wt changes of the sample as
function of time and temperature.
 Should cover wide range of temperature.
 Temperature and wt loss should be recorded to an accuracy range of
better than +/- 1.
 Linear heating should be there.
 Radiation and convection currents, and magnetic effects due to furnace
heaters must not affect the weighing system.

26.  Sensitivity of the balance should be commensurate with the size of the
samples being used.
 There should not ne any chemical attacks of volatile products on the
apparatus.
 Crucible should be located within the hot zone.
 Balance has to be protected from furnace.
 Capable of adjusting various speeds of the chart that is being used to
record the mass lose or temperature rise.
 Should facilitate rapid heating or cooling of the furnace to record several
TG curves in short span of time.

27. Atmosphere controller:
 To stop the reaction of gases present in the furnace with the sample
atmospheric controller is required.
 Inert gases will be circulated through that atmosphere to stop the
reactions.

28. Factors affecting thermogravimetric
curve:
Two main types
Instrumental effects or factors.
The characteristics of the sample.

29. Instrumental factors:
Heating rate:
 The temperature at which the compound (or sample) decompose
depends upon the heating rate.
 When the heating rate is high, the decomposition temperature is also
high.
 A heating rate of 3.5°C per minute is usually recommended for reliable
and reproducible TGA.
Furnace atmosphere:
 The atmosphere inside the furnace surrounding the sample has a
profound effect on the decomposition temperature of the sample.

30.  The common atmospheres involved are:
 Static air: air from atmosphere is allowed to flow through the furnace.
 Dynamic air: compressed air from cylinder is allowed to pass through the
furnace at a measured flow rate.
 Inert atmosphere. A pure N2 gas from a cylinder passed through the furnace
which provides an inert atmosphere.

31. Sample characteristics:
Weight of the sample
 A small weight of the sample is recommended using a small weight
eliminates the existence of temperature gradient through the sample.
Particle size:
 Various particle sizes of the sample alter the reaction rate and hence the
curve shape.
 Smaller dimensions – decomposition earlier
 Larger size – decomposition proceeds at higher temperatures.
 The particle size of the sample should be small and uniform generally.

32. Heat of reaction:
 It alter the difference between the sample temperature and 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 sample.

33. Processes that leads to weight
gain or loss in TGA :

34. Applications:
 Thermal Stability: related materials can be compared at elevated
temperatures under the required atmosphere. The TG curve can help to
elucidate decomposition mechanisms.
 Material characterization: TG curves can be used to "fingerprint"
materials for identification or quality control.
 Compositional analysis: by careful choice of temperature programming
and gaseous environment, many complex materials or mixtures may be
analyzed by decomposing or removing their components. It is used to
analyze e.g. filler content in polymers; carbon black in oils; ash and
carbon in coals, and the moisture content of many substances.

35.  Simulation of industrial processes: the thermobalance furnace is
thought as mini-reactor and has ability to perform operations like some
types of industrial reactors.
 Kinetic Studies: by understanding the controlling chemistry or predictive
studies, a variety of methods can be used to analyze the kinetic features
of weight loss or gain.
 Corrosion studies: TG provides a means of studying oxidation or some
reactions with other reactive gases or vapors.
 To study purity
 To determine decomposition temperature. Forced degradation study.