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 oftechniques in which change in physical property of amaterial is measured as a function of temperature whilethe substance is subjected to a controlled temperatureprogramme.
In modern era the new materials are studied withrespect to – composition, stability, chemical reactionsand dynamic properties.
Thermal methods are useful for these kind of analysis.
These methods also provide information about thestructure, composition, purity and the temperaturephase 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
TGA – Thermogravimetry analysis
Principle
The substance under study is either heated or cooled at acontrolled rate and the weight of the substance isrecorded as a function of time and temperature.
If the temperature varies during the study, then the weightis plotted as function of temperature.
If the temperature is kept constant , the weight is plottedas a function of time.
The change in weight is used for quantitative analysisand the temperature at which the change in weighttakes place can be used for qualitative analysis
TGA – Thermogravimetry analysis
Types of TGA –
1. Isothermal or static thermogravimetry
2. Quasistatic thermogravimetry
3. Dynamic thermogravimetry
TGA – Thermogravimetry analysis
Instrumentation
The apparatus that simultaneously heats the sample and
monitors its weight is called a thermobalance.
TGA – Thermogravimetry analysis
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 ormore
- 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 beof uniform temperature.
- There should be a provision to carry out heating in a variety of controlledatmospheres 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 thesample.
- It must have the facility for rapid heating and cooling of the furnace so that largenumber of analyses could be carried out in short time.
TGA – Thermogravimetry analysis
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
TGA – Thermogravimetry analysis
Applications –
The determination of purity and thermal stability ofboth primary and secondary standards used involumetric analysis
Determination of correct drying temperatures and thesuitability of various weighing forms for gravimetricanalysis
Determination of composition of complex mixtures
Determination of suitable ignition temperature
Direct application to analytical problems
DTA – Differential Thermal Analysis
Principle
It is the analytical technique in which the temperaturedifference between the sample and a non reactivereference material is monitored when the twosubstances are subjected to an identical heatingprogramme.
The thermal curve is a plot of the difference between intemperature between the sample and the reference asa function of the temperature difference between thesample and the reference material when the sampledoes not undergo any chemical or physical change andthe reference material.
DTA – Differential Thermal Analysis
DTA curve
If the change is endothermic, the sample temperature islower than the reference material.
Conventionally exotherms are plotted upwards whereasendotherms are plotted downwards.
Both the shape and size of the peaks give a largeamount of information about the nature of the sample.
Sharp endothermic peaks signify the changes incrystallanity or fusion processes whereas broadendothermic peaks arise from dehydration reactions.
Physical changes usually result in endothermic peakswhile oxidative chemical reactions show exothermicpeaks.
DTA – Differential Thermal Analysis
Instrumentation
DTA – Differential Thermal Analysis
Sample cell
Furnace
Temperature programmer and controller
Amplifier and recorder
DTA – Differential Thermal Analysis
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
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Factors affecting the thermal analysis
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