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Prepared by:Priyanka jain

THERMOGRAVIMETRIC ANALYSIS CONTENTS:      

INTRODUCTION PRINCIPLE TYPES INSTRUMENTATION METHODOLOGY APPLICATIONS STRENGTH AND LIMITATION

INTRODUCTION DEFINITION:-

TGA is a technique in which mass of a substance is monitored as a function of temperature or time as the sample specimen is subjected to a controlled temperature program in a controlled atmosphere. It is a technique in which upon heating a material, its weight increases or decreases. TGA is commonly used to determine selected characteristics of materials that exhibit either mass loss or gain due to decomposition, oxidation, or loss of volatiles (such as moisture).

PRINCIPLE OF THERMOGRAVIMETRIC ANALYSIS

Large number of chemical substances invariably decomposes upon heating and this idea of heating

a sample to observe weight changes is the underlying principle of TGA.

TYPES OF TGA 1. Static (or Isothermal) TGA 2. Dynamic TGA 1. STATIC (ISOTHERMAL) TGA:- In this technique sample is maintained at a constant temp. for a period of time during which any changes in weight are observed. 2. DYNAMIC TGA:- In this sample is subjected to under controlled continuous increase in temp. conditions (linear with time).

INSTRUMENTATION

1. A high precision balance 2. Furnace 3. A sensitive recorder

Thermo gravimetric analyzer also known as thermobalance.

A Thermo gravimetric analyzer consists of a sample pan that is ed by a precision balance. The pan resides in a furnace and is heated or cooled during the experiment. The mass of the sample is monitored during the experiment. A sample purge gas controls the sample environment. The gas may inert or a reactive gas that flows over the sample and exist through an exhaust.

A TGA analysis is performed by gradually raising the temperature of a sample in a furnace as its weight is measured on an analytical balance that remains outside of the furnace. In TGA, mass loss is observed if a

thermal event involves loss of a volatile component. Chemical reactions, such as combustion, involve mass losses, whereas physical changes, such as melting, do not. The weight of the sample is plotted against temperature or time to illustrate thermal transitions in the material – such as loss of solvent and plasticizers in polymers, water of hydration in inorganic materials, and, finally, decomposition of the material.

RECORDING BALANCES

These are of two types:-

Deflection type

Beam type

Helical type

Null Type

Cantilevered beam

Torsion wire

DEFLECTION TYPE:This type of balance is based on conventional analytical balance consist of beam type, helical spring, cantilever beam and torsion wire involving the conversion of deviation directly into a record of weight change.

NULL TYPE:It consists of a sensing element which detects any slightest deviation of the balance beam and provides the application of restoring force, directly proportional to the change in weight, there by returning the beam to its original null point. The restoring force is subsequently recorded.

FURNACE Furnace is designed in such a way for maintaining a fixed temp. or a predetermined linear heating programme. (10° C – 600º C/h). The temperature control of the furnace is achieved via a thermocouple mounted very close to the furnace winding. S.No.

Specification

Max temp.ºC

1.

Nickel-chrome (Nichrome)

1100

2.

Platinum-Rhodium

1450

3.

Graphite- tube furnace

>1500

RECORDER

The recording device must be such so as to:-

• Record both temp. and weight continuously. • Make a definite periodic record of the time.

METHODOLOGY Example The thermogram for calcium oxalate(CaC2O4.H2O) Decomposition of calcium oxalate monohydrate exhibits three weight losses with temperature in an inert ƒ atmosphere (e.g. N2). CaC2O4• H2O → CaC2O4→ CaCO3→ CaO

Calcium oxalate monohydrate (20mg)

H2O, (12.3%, 2.46 mg)

CO (19.2%,3.84mg)

CO2 (6.02%,3.01 mg)

Weight decreases as lose H2O, CO and CO2

The successive plateaus correspond to the anhydrous oxalate (100250 °C), calcium carbonate (400- 500°C) and finally calcium oxide (700-850°C). In other words, these plateaus on the decomposition curve designate 2 vital aspects, namely: 1)Clear indication constant weight 2)Stable phase within a specified temperature interval. Step-1 The water of hydration from calcium oxalate monohydrate is lost which corresponds to 2.46mg (12.3%) equivalent to 1 mole of water in the temperature range 100-250°C. Step-2 One mole of carbon monoxide is evolved subsequently from calcium oxalate, corresponding to 3.84 mg (19.2%) in the temperature range 400-500°C. The 19.2% weight loose that occurred between 400-500°C should corresponds to 19.2% of the original formula weight of 146.Therefore the product being given out has a formula weight of 1.92*146=28.0 that corresponds to carbon monoxide.

Step-3 Finally, a mole of carbon dioxide is evolved from calcium carbonate that corresponds to 6.02 mg (3.01%) of the original formula weight of 146.Therefore ,the product being evolved has a formula weight of 0.301*146=43.946(=44), and it corresponds to carbon dioxide. In situation whereby an inert material is present along with a pure substance, from the generated thermogram the composition of the mixture may be derived from the percentage weight variation which takes place relative to the percentage weight variation observed with the pure compound (A), by employing the following expression: Compound A (wt %)={ %wt.change for mixture/%wt.change for pure compound A}*100(%)

MECHANISM OF WEIGHT CHANGE IN THERMOGRAVEMETRIC ANALYSIS •Weight Loss:

–Decomposition: The breaking apart of chemical bonds. –Evaporation: The loss of volatiles with elevated temperature. –Reduction: Interaction of sample to a reducing atmosphere (hydrogen, ammonia, etc). –Desorption. •Weight Gain: –Oxidation: Interaction of the sample with an oxidizing atmosphere -Absorption

FACTORS AFFECTING TG CURVE Heating rate Sample size (Increases the temp at which sample decomposition occurs) Particle size of sample Packing Crucible shape Gas flow rate (Affects the progress of the reaction)

EVOLVED GAS DETECTION AND ANALYSIS The analysis of the purge gas exit stream from thermogravimetric analyzers is useful in establishing mechanism and stoichiometric relationships of thermal decompositions. In evolved gas analysis(EGA),

the absolute identities of the gaseous components are determined, whereas in evolved gas detection(EGD), the presence of only a single, preselected component of the evolved gas is sensed. An approriate analyzer may be coupled to a thermogravimetric system for performing either EGA or EGD. The resulting hyphenated methods are powerful analytic tools. Two analyzers Mass spectrometers(MS) and flame ionization detectors

(FID) are successfully coupled to TG systems. The TG-MS combination is used for evolved gas analysis whereas the TG-FID combination provides evolved gas detection.

APPLICATIONS • Evaluation of Gravimetric precipitates • Evaluation of suitable standards •Testing of purity of samples • Analysis of polymeric material (also polymer stability) • Phase transition measurement • Compositional analysis • Reaction Kinetics (computer software allows the computation ofdw/dt,

which is imp. In kinetic interpretation of reactions and processes. • Determining moisture, volatile and ash contents

STRENGTH OF THERMOGRAVIMETRIC ANALYSIS

 Any type of solid can be analyzed, with minimal

sample preparation  Qualitative or quantitative analysis LIMITATIONS OF THERMOGRAVIMETRIC ANALYSIS

 Data interpretation not always straightforward. Analysis in combination with other techniques is often helpful.

Facilities of Thermogravimetric analysis available in India

• IIT Madras, Chennai (Department of Chemistry) • IIT Delhi • IIT Patna

• Tata advanced materials Ltd. Bangalore • Indian institute of science Bangalore • AIMS New Delhi

INSTRUMENTATION OF TGA

The basic instrumental requirements for TGA are a precision balance with a pan loaded with the sample, and a programmable furnace. The

furnace can be programmed either for a constant heating rate, or for heating to acquire a constant mass loss with time. Regardless of the furnace programming, the sample is placed in a small, electrically heated furnace equipped with a thermocouple to monitor accurate measurements of the temperature by comparing its voltage output with that of the voltage-versus-temperature table stored in the computer’s memory. The atmosphere in the sample

chamber may be purged with an inert gas to prevent oxidation or other undesired reactions.

The TGA instrument continuously weighs a sample as it is heated to temperatures of up to 2000°C for coupling with FTIR and Mass spectroscopy gas analysis. As the temperature increases, various components of the sample are decomposed and the weight percentage of each resulting mass change can be measured. Results are plotted with temperature on the X-axis and mass loss

on the Y-axis.