For analytical students
Differential thermal analysis is a technique through which we can measure the change in temperature as a function of time or temperature
you can surely get concept of this technique along with the applications of this technique
4. Limitations Of TGA
ī The Chemical or physical changes which are
not accompanied by the change in mass on
heating are not indicated in
thermogravimetric analysis
ī During TG, Pure fusion reaction, crystalline
transition, glass transition, crystallization
and solid state reation with no volatile
product would not be indicated because they
provide no change in mass of the specimen.
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5. 1.Principle
Differential Thermal analysis is a technique in
which the temperature of the substance
under investigation is compared with the
temperature of a thermally inert material
such as a-alumina, and is recorded with
furnace temperature as the substance is
heated or cooled at a predetermined uniform
rate.
The principle of method consists in measuring
the change in temperature associated with
physical or chemical changes during the
gradual heating of the substance.
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6. Phenomena causing changes in
Temperature
Physical:
ī Adsorption (exothermic)
ī Desorption (endothermic)
ī A change in crystal structure
(endo â or exothermic)
ī Crystallization (exothermic)
ī Melting (endothermic)
ī Vaporization (endothermic)
ī Sublimation (endothermic)
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7. Chemical:
ī Oxidation (exothermic)
ī Reduction (endothermic)
ī Break down reactions
(endo â or exothermic)
ī Chemisorption (exothermic)
ī Solid state reactions
(endo â or exothermic)
ī Generally speaking, phase transitions,
dehydration, reduction and some decomposition
reactions produce endothermic effects whereas
crystallization, oxidation and some
decomposition reaction produce exothermic
effects.
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8. Historical Aspects:
īIn 1899 Robert Austen
improved this technique
by introducing two
thermocouples, one
placed in sample and
other in the reference
block.
īThis technique was later
on modified by
Burgess(1909),
Norton(1939),
Grim(1951), Kerr(1948),
Kauffman(1950), Fold
Vari(1958).
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10. Characteristics Of DTA Curves
īTemperature of the sample is greater
for an exothermic reaction, than that of
reference, for endothermic the sample
temperature lags behind that of
reference
īThe peak temperature corresponds to
the maximum rate of heat of evolution.
It does not represent the maximum rate
of reaction nor the completion of the
exothermic process.
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11. Characteristics Of DTA Curves
ī DTA curves are not only help in the
identification of materials but their peak
areas provide quantitative information
regarding mass of sample, heat of reactions
(Enthalpy change) and factors such as sample
geometry and thermal conductivity.
ī If latter two factors are expressed by a factor
âKâ called calibration factor, then peak area
can be expressed as:
Peak Area (A) = Âąâ H m K
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13. Instrumentation:
1. Source of Uniform Heating:
ī Nichrome (up to 1300 0C )
ī Platinum and its alloys (up to 1750 0C )
ī Molybdenum (up to 2000 0C )
2. Temperature Regulating System:
ī Electronic Temperature Regulators
3. Specimen Holder:
ī Pt, Ni, stainless steel, Ag and alloy such as Pt-
Rh
ī Ceramic material: sintered alumina, silica
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14. Instrumentation:
4. Measurement of Temperature:
ī Rare metal alloys Pt - Rh (Pt 10% - Rh 13%)
used as thermocouple
ī W - Mo also used as thermocouple
5. Temperature Recording System:
īGalvanometric Observation ( For few
samples)
īAutomatic pen
īInk electronic Recorder
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15. Factors Affecting DTA Curves:
DTA is a dynamic temperature technique.
Therefore, a large number of factors can affect.
These factors can be divided into the two
groups:
i) Instrumental factors
ii) Sample factors
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16. Factors Affecting DTA Curves:
Instrumental Factors:
īSize and shape of sample holder
īSample holder material
īHeating rate of sample
īSensitivity of recording system
īLocation of thermocouple in the sample
īAtmosphere around sample
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17. Factors Affecting DTA Curves:
Sample characteristics:
īAmount of sample
īParticle size
īPacking density
īHeat capacity
īThermal conductivity
īDegree of crystallinity
īDilutes of diluents
īSwelling and shrinking of sample
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20. Applications
The important applications of DTA are:
īRapid identification of the composition
of mixed clays
īStudying the thermal stabilities of
inorganic compounds
īCritically examining in a specific
reaction whether a new compound is
actually formed or the product is
nothing but an unreacted original
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21. Applications
īDTA offers a wide spectrum of useful
investigations related to reaction
kinetics, polymerization, solvent -
retention, phase-transformations, solid-
phase reactions and curing or drying
properties of a product
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