Thermogravimetric analysis (TGA) involves measuring the weight of a substance as it is heated, and can be used to analyze mixtures, determine drying temperatures, and study reaction kinetics. TGA works by heating a sample in a furnace while precisely measuring its weight on a high-precision balance. Derivative thermogravimetric analysis (DTGA) measures the rate of weight change during heating. TGA can identify different components in a mixture based on their unique thermal decomposition profiles, and determine optimum drying ranges to isolate compounds like calcium oxalate. Kinetic parameters of reactions can also be extracted from TGA or DTGA curves using dynamic or isothermal methods.

1. BY
JILANI S M
I M PHARM
GCP, BANGALORE
THERMOGRAVIMETRIC
ANALYSIS

2. CONTENTS
1. INTRODUCTION
2. INSTRUMENTATION
3. DERIVATIVE THERMOGRAVIMETRIC ANALYSIS
4. FACTORS AFFEVTING TGA
5. APPLICATION OF TGA

3. INTRODUCTION
 In thermometric methods of analysis, some property of
the system is measured as a function of the temperature.
Thermoanalytical methods:
Designation Property measured Apparatus
Thermogravimetric analysis Change in weight Thermobalance
Derivative
thermogravimetrivc
analysis(DTGA)
Rate of change of weight Thermobalance
Deferential thermal
analysis(DTA)
Heat evolved /absorbed DTA apparatus
Calorimetric DTA Heat evolved/absorbed Differential calorimeter
Thermometric titrations Change of temperature Titration calorimeter

4.  Fractional thermogravimetric analysis - it involves the
measurement of weight loss due to the escape of all volatile
matters from a sample during heating(FTGA).
 In thermogravimetric analysis – it involves the weighing of the
substance under investigation while it is being heated at a
predetermined rate.
 In DTA – measures the change in heat content as a function of
the difference in temperature between the sample and inert
reference compound.
 In thermometric titration – measures the changes in solution
temperature and plotted against the volume of titrant.
 Thermograms - in thermal methods, the thermal behavior of
substance is recorded as a function of temperature/ time and
the recorded curves may be considered as thermal spectra.

5. Types of thermogravimetric analysis
1. Static/isothermal gravimetry – in which the sample
weight is recorded as a function of time at a constant
temperature
2. Quasistatic TGA - the sample is heated to constant
weight at each step in a series of increasing
temperatures.
3. Dynamic thermogravimetry – the sample is heated at a
uniform rate, but unfortunately the results are subjected
to errors due to the effect of changing air buoyancy and
convection, the measurement of temperature, heating
rate, and heat of reaction which are often looked.

6. Instrumentation
 In thermogravimetric analysis, the determination of
temperature at which a material of reproducible
stoichiometry is produced is facilitated by the use of the
instrument called Thermobalance.
 Basically it consists of
o Precision balance,
o Heating device
o Temperature and atmosphere control devices.

7. A schematic modern thermogravimetric analyser

8. 1. Weight measurement:
 Weighing mechanism in TGA may be a modification of a
single or double pan balance, an electronically self
balancing device, a torsion balance or simple spring
balance is used depending on the requirement.
 For the preliminary work in the laboratory, a precision
torsion balance may be easily adapted for use as a
thermobalance.
Characteristic of balance:
 It should be simple to operate
 It should have an adjustable range of weight change
 It should be able to respond rapidly to changes in weight
 It should be rugged, accurate, very sensitive and
mechanically stable

9.  The thermobalance that have been used are either the null
point or the deflection type of instruments.
 As a weight change occurs, and the balance beam start to
deviate from its normal position, a sensor detects the
deviation and triggers the restoring force to bring the
balance beam back to the null position. The restoring
position is directly proportional to the weight change.

10. Deflection type of balance

11.  In some modern commercial instruments, a beam of light
abstracted by a wave attached to the end of a quartz spring
has been used in quartz spring balance. Photoelectric
detection was used in that case.
 Torque motor galvanometer- here sample is attached to the
needle to some null point. Null is detected by pair of split
photocells behind a wave in a light beam.
2. Heating and temperature measurements:
 The furnace design and control have taken various forms
depending on the degree of sophistication of the
instrument.
 The main requirement is that the heating rate be smooth
so that it can maintain either a linear heating program or
fixed temperature.

12.  The simplest temperature programme is a variable
transformer, the other equipments use conventional
thermocouples or resistance thermometers situated as close
to the furnace winding as possible.
 Platinum and tungsten windings are commonly used, the
nichrome windings permit a maximum temperature of
11000 C to 14500C
 The size of the furnace is very important. Small furnace cool
very quickly and it is difficult to control linear heating rate.
 Although large furnace maintain isothermal temperature
and are suitable to maintain linear heating rate, but attain
the required temperature very late.

13.  The difficult problems in TGA is the measurement of
temperature. The self heating during oxidation, the gaseous
products produced during heating and the reaction of the
sample with the atmosphere are the problems which
introduce error in temperature measurement.
 The usual rate of heating is 4-50C per minute
3. The sample cups:
 Several shape of sample cups have been used for TGA work
depending on the nature of the system being investigated.
 Generally there are 4 designs of sample cups:
1. Shallow pan
2. Deep crucible
3. Covered cup
4. Retort cup

14.  Shallow pans are used when volatile products are produced
during heating and it is necessary to allow the gases to
diffuse to the surface to escape.
 Deep crucible are used in industrial scale calcinations or
surface area studies.
 Covered cups are used where the studies have to be carried
out in the self generated atmosphere.
 Retort cups are useful in boiling point studies.
4. Atmosphere control:
 Atmosphere control is extremely important in TGA because
the weight change in the sample is due to the gaseous
products formed during sample heating or the reaction of
the sample with the balance atmosphere.
 The control of atmosphere can be achieved either in the
sample or in the balance

15.  The another way of maintaining the atmosphere is by
flushing the whole balance with an inert gas, such as
nitrogen, argon, CO2 , hydrogen or helium.
 Because of low thermal conductivity and very low density,
helium is usually preferred.

16. Derivative thermogravimetric analysis
(DTGA):
 several commercial thermobalances are provided with
electronic circuits to record the derivative automatically.
 As in thermogravimetry, the DTGA involves the measurement
of the rate of change of weight of the specimen as it is being
heated at a uniform slow rate.

17. Relation between TGA and DTG curves for the pyrolysis of
CaCO3 and MgCO3

18.  The plateau in thee thermogram at 700˚C is quite clear, but
the shoulder at about 870˚C is not very clear.
 However in the thermogram obtained in DTGA, the shoulder
has been clearly resolved into a peak.

19. Factors affecting TGA:
1. Effect of changing air buoyancy and convection:
 The most favored arrangement for thermo balance is to
support the inert sample from below in the centre of a
cylindrical, capped tube furnace
 Under this condition the inert sample may show an
apparent weight gain up to 10mg. Therefore to be applied
to the recorded weight change.
 The causes for the weight gain are
 Decreased air buoyancy
 Increased convection
 Effect of heat of balance mechanism

20. 2. Measurement of temperature:
 The usual practice in TGA is to measure the temperature in
the furnace near the sample. The temperature so determined
are usually higher than those determined by the more
common process of measuring the temperature directly.
 The cause of this difference is partly due to the thermal log
and partly due to the finite time required to cause a
detectable change in weight.
3. Effect of atmosphere:
 When sample is dried or decomposed in thermobalance in
ambient air, the atmosphere near the sample is continuously
modified due to the addition of gaseous decomposition
products or the loss of the part of original gas by the reaction
with the sample.

21.  Even a small changes in composition of this atmosphere
can affect the thermogram. It is therefore it is necessary to
flush the thermobalance continuously with inert gas in
order to maintain as constant an atmosphere as possible.
4. Effect of heat of reaction:
 The heat of reaction will affect the difference between
sample temperature and furnace temperature, causing
the sample temperature to lag or lead the furnace
temperature depending on whether the reaction is
endothermic or exothermic.
 The reaction is endothermic, the effect of temperature
lag is to increase the furnace temperature and the
differential temperature will be additive.
 But when the reaction is thermic effect will tend to
compensate each other.

22. 5. Effect of heating rate:
 Heating rate has been found to affect the thermogram
appreciably.
 The effect of heating rate is important if the thermogram is to
be used for kinetic analysis.
 TGA of calcium oxalate monohydrate, the plateau from the
room temperature to 100o c corresponds to monohydrate
 The loss of CO commence at 413o c and that of CO2 at 685o c .

23. Effect of heating rate on the
thermogram of CaC2O4 H2O

24. 6. Sample characteristics:
 Weight of the sample – large sample affects the TG curve
and the curve deviates from linearity as the temperature
rises. Hence, small samples are preferred. If 20mg of copper
sulphate.5H20 is used, no platuex corresponding to
CuSO4.3H2O is obtained, but if only 0.5mg of the sample is
used this plateau can be observed.
 Particle size – particle size also affects the Tgcurve. Smaller
particles decomposes at lower temperature while larger
sized particles of the sample take longer time and
decompose at higher temperature.
 Compactness of the sample – compact sample decompose
at higher temperature than loose samples.

25.  Previous history of the sample – the source and method
of preparation of the sample also have been found to
affect TG curve.
Ex: precipitated Mg(OH)2 decomposes at a different
temperature than naturally occurring Mg(OH)2

26. Application of TGA :
1. Determination of optimum drying temperature range
2. Analysis of mixtures
3. Kinetic studies
4. Surface area measurements

27. 1. Determination of optimum drying temperature:
 Thermogravimetric analysis being widely used in
qualitative, quantitative analysis and thermal stability
studies etc.
 Much of the work in thermogravimetry has been done to
establish optimum temperature for the conditioning of
precipitates for conventional gravimetric analysis.

29.  The curve A: shows the thermogram of calcium oxalate. It
is normally precipitated as the monohydrate from hot
solution
 The curve exhibit several platuex corresponding
respectively to monohydrate from room temperature to
100o c, anhydrous calcium oxalate from 226o to 398o c,
calcium carbonate from 460 to 635o c and calcium oxide
from above 840o c
 The curve B: represents the thermogram of magnesium
oxalate which reveals that dihydrate is stable up to 160o c
 The anhydrous compound exists between 220o and 400o c
and the oxide is stable from 480o to 1oooo c.
 The curve C : represents the T.G curve of silver chromate
 The initial drop in weight indicates the excess water,

30. And just about 92o c the weight becomes constant and remains
so up to 812o c after which oxygen is lost.
 The decomposition of silver chromate can, therefore, be
represented as
2Ag2 Cro4 Ag2CR2O4 +2Ag +2o2
 Silver chromate is thus stable over a temperature range of
about 90o to 800o c. hence in gravimetric analysis of
chromium, the precipitate of silver chromate can dried
anywhere in this region.
 The curve D: represents the excess water. The curve
mercurous chromate is stable over the range of 52-256o c it
commences to decompose in to Hg2O and the weight
becomes constant at about 671o c

31. 2. Analysis of mixtures:
 The curves A and B reveals a significant difference in
behavior between the oxalates of calcium and magnesium
as represented by their decomposition reactions.
 It will be noted that magnesium oxalate does not pass
through the carbonate stage. This difference in their
thermal behavior permits their simultaneous
determination by TGA .
 They can be determined in a mixture by igniting at two
temperatures
At 500o c at which CaCo3 and Mgo are stable
At 900o c both the metals exists as simple oxides.
 The weights of these two precipitates will permit
calculation of the calcium and magnesium contents of the
original sample.

32.  The TGA of Cu – Ag alloy is based on the relative stabilities
of their nitrates. The content of Cu and Ag is determined by
TGA .
 TGA curves indicate that AgNo3 is stable up to 473o c after
which it losesNo2 and o2 and the weight becomes constant
above 608o c when it is converted in to metallic Ag. Cupric
nitrate, on the other hand, decomposes in to Cuo in two
steps:

33.  CuO being stable above 700oc. Thus weight of the mixed
precipitate at 400o c will permit the analysis of the Cu – Ag
alloy.
3. Kinetic studies:
 TGA, DTA and other thermoanlaytical methods can be used
to study kinetics of a chemical reaction and to determine basic
kinetic constants such as the rate constant (k), activation
energy(E), order of reaction(x), and frequency factor(A), in
these methods, a change in some physical property (weight,
enthalpy, volume/length) as a function of temperature is
measured continuously and automatically
 There are two approaches for kinetic studies:
1. Isothermal /static method: which involves the
determination of degree of transformation at constant
temperature as a function of time .

34. 2.Dynamic method : which involves the determination of degree of
transformation as a function of time during a linear increase of
temperature.
4. Surface area measurement:
 It is well known that solid surfaces adsorb gases. The gas
adsorbed on the solid surface will contribute to its weight.
 TGA can be applied to determine the nitrogen and the other
gases
 In a conventional method, a cleaned sample surface is thermo
stated at the desired temperature in a thermobalance and a gas
at the known pressure is allowed to be adsorbed slowly and the
weight again with time is recorded.
 After the equilibrium has been established the pressure can be
increased to next desired level.

35. Reference:
 Instrumental approach chemical analysis by A.K. Srivastav
and P.C. jain
 Thermal methods by James W. Dodd and Kenneth H. Tonge

36. THANK YOU