TGA

Thermogravimetric Analysis

1. Guided by: Prof. D.P. Ambhore Sir ( M.Pharm ) Presented by: Mr. Sanket Chordiya ( B.Pharm VII Sem.) Seminar on Thermo-Gravimetric Analysis Paramhansa Ramakrishna Maunibaba Shikshan Sanstha’s ANURADHA COLLEGE OF PHARMACY ANURADHA NAGAR, SAKEGAON ROAD, CHIKHLI. DIST- BULDANA (M.S.) 443201

2. C ontent Introduction Definition Types Principle Instrumentation Applications

3. Introductio n The term “thermal analysis” incorporates those techniques in which some physical parameter of the system is determined and recorded as function of temperature. When matter is heated it undergoes certain physical and chemical changes .These changes that take place when an unknown sample is heated provide us with information that enables us to identify the material. Types of Thermal Analysis: I ) Thermo-gravimetric ( Change in Mass) II) Differential Thermal Analysis (Change in Temperature) III) Differential Scanning Calorimetry (Compensation of temperature)

4. Definition An Internationally accepted definition of thermogravimetry is as follows: “ It is a technique where by weight of substance in an environment heated or cooled at a controlled rate, is recorded as a function of time or temperature. ”

5. Types Of Thermogravimetry Isothermal/Static Thermogravimetry : In this technique the sample weight is recorded as a function of time at constant temperature. Quasistatic Thermogravimetry : In this technique the sample is heated to constant weight at each of the series of increasing tempearture . Dynamic Thermogravimetry : In this technique a sample is heated in an environment whose temperature is changing in predetermine manner generally at linear rate.

6. Principle In thermogravimetric analysis, the sample is heated in a given environment (air, N2 , CO2 , He, etc.) at controlled rate . The change in the weight of the substance is recorded as a function of temperature or time. The temperature is increased at a constant rate for a known initial weight of the substance and the changes in weights are recorded as a function of temperature at different time interval . This plot of weight change against temperature is called thermogravimetric curve or thermogram , this is the basic principle of TGA.

7. Example: TGA Curve for AgNO3 The diagram indicates the TGA curve for AgNO3 . The horizontal portion of the curve indicates that, there is no change in weight (AB &CD) and the portion BC indicates that there is weight change. The weight of the substance (AgNO3 ) remains constant upto a temperature of 473°C indicating that AgNO3 is thermally stable upto a temperature of 473°C. At this temperature it starts losing its weight and this indicates that the decomposition starts at this temperature. It decomposes to NO2 , O2 and Ag. AgNO3 → Ag + NO2 + O2

8. Instrumentation/ Block Diagram of TGA

9. Instrumentation The apparatus required for TGA analysis are (a) A furnace which can be heated so that the temperature gives linearity with time . (b) A furnace controlled thermobalance A known weight of the sample is taken in a crucible(c), which is enclosed by a furnace(F). The furnace(F) temperature is raised slowly, the temperature of the sample and the corresponding weight are taken . A platinum/platinum rhodium thermocouple is used to measure the sample temperature, and the change in weights are found out by finding the beam deflection on adding a known weight to the pan.( i.e ) the change in the weight are recorded from the beam deflection . Recorder: A recorder records the change in weight in y axis and w.r.to temperature on the x-axis. We get a thermogram .

10. Sample Preparation Sample preparation has a significant effect in obtaining good data. It is suggested that maximizing the surface area of the sample in a TGA pan improves resolution and reproducibility of weight loss temperatures. The sample weight affects the accuracy of weight loss measurements. Typically 10-20mg of sample is preferred in most applications. Whereas , if the sample has volatiles 50-100mg of sample is considered adequate. It is to be noted that most TGA instruments have baseline drift of ±0.025mg which is ±0.25% of a 10mg sample.

11. Experimental Conditions Heating Rate Samples are heated at a rate of 10 or 20°C/min in most cases. Lowering the heating rates is known to improve the resolution of overlapping weight losses. Purge gas Nitrogen is the most common gas used to purge samples in TGA due to its inert nature. Whereas, helium provides the best baseline . Air is known to improve resolution because of a difference in the oxidative stability of components in the sample . Vacuum may be used where the sample contains volatile components, which helps improve separation from the onset of decomposition since the volatiles come off at lower temperatures in vacuum. e.g. oil in a rubber tire product.

12. Factors affecting the TG curve The factors which may affect the TG curves are classified into two main groups. (1) Instrumental factors (2) Sample Characteristics ( 1) Instrumental factors (a) Furnace heating rate (b) Furnace atmosphere ( 2) Sample characteristics includes (a) Weight of the sample ( b) Sample particle size

13. Instrumental factors Furnace Heating rate: The temperature at which the compound (or sample) decompose depends upon the heating rate. When the heating rate is high, the decomposition temperature is also high. A heating rate of 3.5°C per minute is usually recommended for reliable and reproducible TGA . Furnace atmosphere: The atmosphere inside the furnace surrounding the sample has a profound effect on the decomposition temperature of the sample. A pure N2 gas from a cylinder passed through the furnace which provides an inert atmosphere.

14. Sample characteristics (a)Weight of the sample: A small weight of the sample is recommended using a small weight eliminates the existence of temperature gradient throught the sample . (b) Particle size of the sample: The particle size of the sample should be small and uniform. The use of large particle or crystal may result in apparent, very rapid weight loss during heating.

15. Application of TGA TGA can be used to evaluate the thermal stability of a material . TGA is used in the analysis of ceramics and thermally stable polymers . In an overview of thermal analysis testing it is always preferable to do a TGA experiment on unknown samples before doing a DSC experiment (especially for pharmaceuticals ). From TGA, we can determine the purity and thermal stability of both primary and secondary standard. For studying the sublimation behaviour of various Substances.

16. Reference Gurdeep R. Chatwal , Sham K. Anand , Instrumental Method Of Chemical Analysis, 5 th edition, Himalaya Publishing house,Page No.-2.701. Sharma B.K. Goel Publishing House “Instrumental Methods of Analysis” page No.- 234-237

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