621 Unit 2.pdf thermak methods of analysis

THERMAL METHODS OF
ANALYSIS
Dr Ashwini Wadegaonkar

Unit 2: Thermal methods of analysis
 Principle of thermal analysis
 Classification of thermal techniques
 Principle, instrumentation and applications of TGA
and DTA
 Factors affecting the thermal analysis
 Numerical problem

Thermal analysis
 A group of techniques in which a property of the
sample is monitored against time or temperature
while the temperature of the sample, in a specified
atmosphere, is programmed.

Introduction
 Thermal methods of analysis include a group of
techniques in which change in physical property of a
material is measured as a function of temperature while
the substance is subjected to a controlled temperature
programme.
 In modern era the new materials are studied with
respect to – composition, stability, chemical reactions
and dynamic properties.
 Thermal methods are useful for these kind of analysis.
 These methods also provide information about the
structure, composition, purity and the temperature
phase change of a material.

Principle of thermal analysis
 Thermal Analysis includes all the methods of measuring the
sample properties while the sample temperature is program-
controlled.

Thermal Analysis instrument includes the followings:
 Detection Unit: Furnace, sample and reference
holder, and sensor, heat and cool the sample in the
furnace, and detects the sample temperature and
property.
 Temperature Control Unit: Controls the furnace
temperature.
 Data Recording Unit: Records the signals of sensor
and sample temperature, and analyzes them.

 Temperature control, data recording and analysis
are all computer-controlled.
 The combination of the furnace and sensor enables
the various types of the measurement techniques.
 This computer can be connected to the several
instruments which has the other types of
measurement techniques, enables the simultaneous
measurement and analysis.

Classification of thermal techniques

TGA and DTA
Principle, instrumentation and applications
TGA – Thermogravimetry analysis
DTA – Differential Thermal Analysis

Principle
 The substance under study is either heated or cooled at a
controlled rate and the weight of the substance is
recorded as a function of time and temperature.
 If the temperature varies during the study, then the weight
is plotted as function of temperature.
 If the temperature is kept constant , the weight is plotted
as a function of time.
 The change in weight is used for quantitative analysis
and the temperature at which the change in weight
takes place can be used for qualitative analysis

Types of TGA –
1. Isothermal or static thermogravimetry
2. Quasistatic thermogravimetry
3. Dynamic thermogravimetry

Instrumentation
The apparatus that simultaneously heats the sample and
monitors its weight is called a thermobalance.

 Important features of thermobalance –
- It should be capable of continuously registering the weight change of the sample
studied
- The furnace should reach the maximum desired temperature – at least 15000C or
more
- The rate of heating should be linear and reproducible
- The sample holder should be in the hot zone of the furnace and this zone should be
of uniform temperature.
- There should be a provision to carry out heating in a variety of controlled
atmospheres and in vaccume
- The balance should be protected from the effect of corrosive gases
- The temperature and the change in weight must be measured accurately as possible
- The balance must be sensitive enough to study small changes in the weight of the
sample.
- It must have the facility for rapid heating and cooling of the furnace so that large
number of analyses could be carried out in short time.

Thermogravimetric curve

Factors affecting Thermogravimetry
 Instrumental factors
 Characteristics of the sample

Factors affecting Thermogravimetry
 Instrumental factors
1. Heating rate
2. Furnace atmosphere – static air, dynamic air, inert atmosphere
3. Sample holder
Characteristics of the sample
 Weight of the sample
 Particle size of the sample
 Heat of reaction
 Compactness of the sample
 Source of the sample

Applications –
 The determination of purity and thermal stability of
both primary and secondary standards used in
volumetric analysis
 Determination of correct drying temperatures and the
suitability of various weighing forms for gravimetric
analysis
 Determination of composition of complex mixtures
 Determination of suitable ignition temperature
 Direct application to analytical problems

Principle
 It is the analytical technique in which the temperature
difference between the sample and a non reactive
reference material is monitored when the two
substances are subjected to an identical heating
programme.
 The thermal curve is a plot of the difference between in
temperature between the sample and the reference as
a function of the temperature difference between the
sample and the reference material when the sample
does not undergo any chemical or physical change and
the reference material.

 DTA curve

 If the change is endothermic, the sample temperature is
lower than the reference material.
 Conventionally exotherms are plotted upwards whereas
endotherms are plotted downwards.
 Both the shape and size of the peaks give a large
amount of information about the nature of the sample.
 Sharp endothermic peaks signify the changes in
crystallanity or fusion processes whereas broad
endothermic peaks arise from dehydration reactions.
 Physical changes usually result in endothermic peaks
while oxidative chemical reactions show exothermic
peaks.

 Instrumentation

 Sample cell
 Furnace
 Temperature programmer and controller
 Amplifier and recorder

Applications
 It provides information about the temperature of transitions
 It can be used to study the process in which heat is absorbed or
evolved.
 The thermal curves can be used in qualitative analysis
 It is used to determine specific heat of substances
 It is used to detect energy changes occurring during melting and
polymorphic transitions. This technique is useful in constructing phase
diagrams of systems.
 It is used to determine directly the melting point of a substance and
hence check the purity of compounds.
 Thus can be used for the quality control of a large number of
substance like cement, soil, glass, catalysts, explosives and polymers.

 Comparison of TGA and DTA techniques
 book

Factors affecting the thermal analysis
 book

621 Unit 2.pdf thermak methods of analysis

More Related Content

Similar to 621 Unit 2.pdf thermak methods of analysis (20)

More from Miratunnoor1 (6)

Recently uploaded (20)

621 Unit 2.pdf thermak methods of analysis