modern pharmaceutical analytical technique,pharmaceutics

1. THERMOGRAVIMETRIC
ANALYSIS-PRINCIPLE, FACTORS
AFFECTING THE RESULTS,
ADVANTAGES, DISADVANTAGES
M. SIMRANA FATHIMA
M.PHARMACY
(PHARMACEUTICS)
1ST YEAR
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2. INTRODUCTION
• Thermogravimetric analysis is an evaluation technique that measures
masses of different Substances at temperature change or at a
constant temperature over a given time
• Thermogravimetric analysis provides a quantitative measure of any
weight change associated with thermal induced transition
2 TYPES OF TGA
DYNAMIC TGA – sample subjected to increase temperature with time
ISOTHERMAL/STATIC TGA- sample subjected to constant temperature
for a period of time during which any change in weight is noted 2

3. PRINCIPLE
• Principle can be illustrated by weight loss
curve of hypothetical compound MCO32H2O.
• This curve is quantitative, in the calculation
that can be made to determine the
stoichiometry of compound at given
temperature.
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4. • The water is evolved at the beginning at A (minimum weight
loss temperature)
• A break is obtained at B. further heating results in the
formation of anhydrous MCO3 weight levels from C and D.
• The drying temperature of MCO3 should therefore be placed
somewhere between C and D, the value of C and D depends on
the heating rate of the furnace, a slow heating rate will shift
these temperatures to lower values.
• At point D the compound MCO3 starts to evolve Co2 giving MO
weight level from E to F.
• The thermal stabilities of original compound, the intermediate
compound and the final compound can be ascertained from the
examination of various region in the curve.
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5. METHODS
Thermogram of Calcium oxalate monohydrate
Curve is known as decomposition curve.
• The plateau in the curve indicates constant
weight, represent stable phase over
particular temperature interval
• Inflection in curve represents
A) formation of intermediate compounds
B) Absorption of volatile compound in the
new solid phase.
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6. OTHER IMPORTANT EXAMPLES
• PRECIPITATED SILVER CHROMATE : filtered in gooch crucible is
heated in a thermobalance, Curve is obtained
• From 92°C the weight remains constant up-to 312°C.
• Oxygen is liberated between 312°C – 540°C and
loss in weight. So decomposition curve appears as
2Ag2CrO4 2O2 + 2Ag + Ag2Cr2O4.
• Therefore the residue contains a mixture of silver
and silver chromite.
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7. • MERCUROUS CHROMATE
Decomposition occurs after 256°C.But stable between
52°C – 256°C
Hg2CrO4 Hg2O + CrO3
• the Mercurous oxide is lost by sublimation and
the chromium trioxide remains at constant weight
above 671°C.
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8. CALCIUM OXALATE AND MAGNESIUM OXALATE
• The decomposition of calcium oxalate takes place in 2 steps
CaC2O4 CaCo3 + Co
CaCo3 CaO + Co3
• The decomposition of magnesium oxalate takes place in 1 step only
MgC2O4 MgO + Co2 + CO
• The calcium oxalate and magnesium oxalate
remains stable from 228°C - 398°C
and 233°C - 397°C
• CaCo3 is stable from 420°C - 660°C
• MgO and CaO are stable above 480°C
• and 842°C
 AT 500°C - CaCo3 and MgO are stable
 AT 900°C - both calcium oxalate and
magnesium oxalate exist as simple oxide.
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9. FACTORS AFFECTING THE RESULTS
• There are two important factors upon which the TGA depends
1. INSTRUMENTAL FACTORS
 EFFECT OF HEATING RATE
EFFECT OF FURNACE ATMOSPHERE
GEOMETRY OF SAMPLE HOLDER
2. SAMPLE CHARACTERISTICS
AMOUNT OR WEIGHT OF SAMPLE
SAMPLE PACKING
SAMPLE PARTICLE SIZE
METHOD OF PREPARATION 9

10. 1.INSTRUMENTAL FACTORS
Various factors under instrumental factors are furnace heating
rate, recording chart speed, furnace Atmosphere, geometry of
the sample holder, geometry of furnace, sensitivity of recorder,
recording balance and composition of sample container.
 EFFECT OF HEATING RATE
• The decomposition of polystyrene in an atmosphere of
nitrogen: for 10% decomposition of polystyrene the
temperature are 375°C and 394°C for a heating rate of 1°C and
5°C per minute.
• For reversible reaction, the rate of reaction has little or no
effect but position of the intermediate compound may be
affected.
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11. • For example: At a heating rate of 2.5°C/min, the TG curve of
NiSo47H2O indicates one curve breaks and causes NiSO41H2O,
but if NiSO47H2O is heated at a heating rate of 0.6°C/min, the
TG curve shows various breaks such as NiSO46H2O, NiSO44H2O,
NiSO42H2O, NiSO41H2O.
FURNACE ATMOSPHERE
• Decomposition of calcium carbonate takes place at a much
higher temperature, if is carried out in an atmosphere of CO2
instead of nitrogen Some reversible and irreversible reactions
takes place
A) X(solid) Y(solid) + Z(gas)
B) X(solid) Y(solid) + Z(gas)
C) X(solid) + Y(gas) Z(solid) + Z(gas)
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12. • The function of inert gas in the furnace is to remove the
gaseous product liberated during thermogravimetric analysis in
reaction (A) and (B) and to stop reaction (C) from taking place
• The reaction (A) and not (B) is affected if the atmosphere in the
furnace has the same gas evolved during TGA.
• If the composition of gas (Y) in reaction (C) is changed then the
effect of reaction will be dependent upon the nature of the gas
in the atmosphere.
• It is desirable to maintain the nature of atmospheric gas as
constant as possible throughout the experiment
• In TGA 3 main furnace atmosphere are used they are static air,
dynamic air, inert atmosphere. 12

13. GEOMETRY OF THE SAMPLE HOLDER
• Flat plates, crucible of various capacities have been used as
sample holder and are made of glass, Alumina, ceramic, metals
and metallic alloys.
• A shallow dish is used because of rapid exchange of gaseous
material between sample and surrounding atmosphere in the
furnace
• The slope of TG curve is not affected by the geometry of
sample holder when the atmosphere in the furnace is same as
the gas evolved in the TGA
FOR EXAMPLE : In the decomposition of CaCO3 in CO2
atmosphere, the geometry of container has no effect on TG curve
but If the Decomposition is carried out in dry oxygen free
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14. 2.SAMPLE CHARACTERISATION
The factors that affect the TG curve may include Amount of
sample, solubility of evolved gas, sample particle size, heat of
decomposition of reaction, sample packing, nature of sample,
and thermal conductivity of sample.
AMOUNT/WEIGHT OF SAMPLE
• If a large amount of Sample Is used, a derivation from the
linearity with the rise in temperature takes place in exothermic
reaction
EXAMPLE : liberation CO during the decomposition of calcium
oxalate to calcium carbonate
CaC2O4 CaCO3 + CO
CaCO3 CaO + CO2
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15. • The presence of intermediate compound can be detected by
making use of small sample in preference to the larger ones.
EXAMPLE : 1800mg of CuSO4.5H2O, when studied
thermogravimetrically does not indicate the plateau corresponding
to CuS043H2O, but the plateau corresponding to intermediate
compound appear in TG curve when 0.426mg of CuSO4.5H2O
sample is employed.
SAMPLE PARTICLE SIZE
• The rate of reaction and shape of TG curve is altered by using
sample of different particle size.
• The decomposition takes place much earlier and at low
temperature with the sample having particle size of smaller
dimension.
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16. • Decomposition temperature has been found to decrease with
decrease in sample particle size which in turn decrease in
temperature at which complete decomposition takes place
SAMPLE PACKING
A compressed sample decomposes at a higher temperature. If
the decomposition is exothermic, sample temperature leads the
furnace temperature but if the decomposition is endothermic,
sample temperature lags behind the furnace temperature
METHOD OF PREPARATION
• Magnesium hydroxide prepared by precipitation method and
naturally occurring has Different temperature of
decomposition .
• The TGA studies indicate that source of method of formation
of sample should be ascertained before going for TGA. 16

17. ADVANTAGES OF TGA
• Any type of solid can be analyzed With minimal sample
preparation (at least 0.1mg) eg : powders, pellets, flakes
• TGA has high accuracy of balance used as well as precise
control of heating/cooling rate and atmospheric condition
• TGA may be convenient and time saving, performance of
technique does suffer due to construction requirements
• Easy sample changing and easy change of sample holder
• Fast heating rate with good resolution can be maintained
• In TGA one can hold the furnace at 1000°C without any balance
drift, which is not balanced in other thermobalance
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18. • Cooling time is very short therefore Thermogram can be
recorded
• TGA is a rapid process.
DISADVANTAGES OF TGA
• Solid sample only must be used in quantitative and qualitative
analysis
• Data interpretation is not always straight forward
• Very small Amount of samples are used but non-homogeneous
material cannot be tested
• Sensitive to heating rate and sample masses results in shift in
temperature
• Limited to sample which undergoes weight change. Melting,
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19. REFERENCE
• Instrumental method of analysis by Willard 7th edition
page(767-768).
• Instrumental method of analysis by Chatwal page(2.712-
2.719).
• Instrumental method of analysis by B. K. Sharma page(232-
239).
THANK YOU.
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