Thermogravimetric analysis

modern pharmaceutical analytical technique,pharmaceutics

1. Thermogravimetric analysis-principle, factors affecting the results, advantageS, disadvantages M. Simrana fathima M.Pharmacy (pharmaceutics) 1 st year 1

2. Introduction Thermogravimetric analysis is an evaluation technique that measures masses of different Substances at temperature change or at a constant temperature over a given time Thermogravimetric analysis provides a quantitative measure of any weight change associated with thermal induced transition 2 TYPES OF TGA DYNAMIC TGA – sample subjected to increase temperature with time ISOTHERMAL/STATIC TGA- sample subjected to constant temperature for a period of time during which any change in weight is noted 2

3. Principle Principle can be illustrated by weight loss curve of hypothetical compound MCO 3 2H 2 O. This curve is quantitative, in the calculation that can be made to determine the stoichiometry of compound at given temperature . 3

4. The water is evolved at the beginning at A (minimum weight loss temperature) A break is obtained at B. further heating results in the formation of anhydrous MCO 3 weight levels from C and D. The drying temperature of MCO 3 should therefore be placed somewhere between C and D, the value of C and D depends on the heating rate of the furnace, a slow heating rate will shift these temperatures to lower values. At point D the compound MCO 3 starts to evolve Co 2 giving MO weight level from E to F. The thermal stabilities of original compound, the intermediate compound and the final compound can be ascertained from the examination of various region in the curve. 4

5. METHODS Thermogram of Calcium oxalate monohydrate Curve is known as decomposition curve. The plateau in the curve indicates constant weight, represent stable phase over particular temperature interval Inflection in curve represents A) formation of intermediate compounds B) Absorption of volatile compound in the new solid phase. 5

6. OTHER IMPORTANT EXAMPLES PRECIPITATED SILVER CHROMATE : filtered in gooch crucible is heated in a thermobalance, Curve is obtained From 92 °C the weight remains constant up-to 3 12°C. Oxygen is liberated between 3 12°C – 540 °C and loss in weight. So decomposition curve appears as 2Ag 2 CrO 4 2O 2 + 2Ag + Ag 2 Cr 2 O 4. Therefore the residue contains a mixture of silver and silver chromite. 6

7. MERCUROUS CHROMATE Decomposition occurs after 256°C.But stable between 52°C – 256°C Hg 2 CrO 4 Hg 2 O + CrO3 the Mercurous oxide is lost by sublimation and the chromium trioxide remains at constant weight above 671°C. 7

8. CALCIUM OXALATE AND MAGNESIUM OXALATE The decomposition of calcium oxalate takes place in 2 steps CaC 2 O 4 CaCo 3 + Co CaCo 3 CaO + Co 3 The decomposition of magnesium oxalate takes place in 1 step only MgC 2 O 4 MgO + Co 2 + CO The calcium oxalate and magnesium oxalate remains stable from 228°C - 398°C and 233°C - 397°C CaCo 3 is stable from 420°C - 660°C MgO and CaO are stable above 480 °C and 842 °C AT 500°C - CaCo 3 and MgO are stable AT 900 °C - both calcium oxalate and magnesium oxalate exist as simple oxide. 8

9. FACTORS AFFECTING THE RESULTS There are two important factors upon which the TGA depends INSTRUMENTAL FACTORS EFFECT OF HEATING RATE EFFECT OF FURNACE ATMOSPHERE GEOMETRY OF SAMPLE HOLDER 2. SAMPLE CHARACTERISTICS AMOUNT OR WEIGHT OF SAMPLE SAMPLE PACKING SAMPLE PARTICLE SIZE METHOD OF PREPARATION 9

10. 1.INSTRUMENTAL FACTORS Various factors under instrumental factors are furnace heating rate, recording chart speed, furnace Atmosphere, geometry of the sample holder, geometry of furnace, sensitivity of recorder, recording balance and composition of sample container. EFFECT OF HEATING RATE The decomposition of polystyrene in an atmosphere of nitrogen: for 10% decomposition of polystyrene the temperature are 375°C and 394°C for a heating rate of 1 °C and 5°C per minute. For reversible reaction, the rate of reaction has little or no effect but position of the intermediate compound may be affected. 10

11. For example: At a heating rate of 2.5°C/min, the TG curve of NiSo 4 7H 2 O indicates one curve breaks and causes NiSO 4 1H 2 O, but if NiSO 4 7H 2 O is heated at a heating rate of 0.6°C/min , the TG curve shows various breaks such as NiSO 4 6H 2 O, NiSO 4 4H 2 O, NiSO 4 2H 2 O, NiSO 4 1H 2 O. FURNACE ATMOSPHERE Decomposition of calcium carbonate takes place at a much higher temperature, if is carried out in an atmosphere of CO 2 instead of nitrogen Some reversible and irreversible reactions takes place A) X(solid) Y(solid) + Z(gas) X(solid) Y(solid) + Z(gas) C) X(solid) + Y(gas) Z(solid) + Z(gas) 11

12. The function of inert gas in the furnace is to remove the gaseous product liberated during thermogravimetric analysis in reaction (A) and (B) and to stop reaction (C) from taking place The reaction (A) and not (B) is affected if the atmosphere in the furnace has the same gas evolved during TGA. If the composition of gas (Y) in reaction (C) is changed then the effect of reaction will be dependent upon the nature of the gas in the atmosphere. It is desirable to maintain the nature of atmospheric gas as constant as possible throughout the experiment In TGA 3 main furnace atmosphere are used they are static air, dynamic air, inert atmosphere. 12

13. GEOMETRY OF THE SAMPLE HOLDER Flat plates, crucible of various capacities have been used as sample holder and are made of glass, Alumina, ceramic, metals and metallic alloys. A shallow dish is used because of rapid exchange of gaseous material between sample and surrounding atmosphere in the furnace The slope of TG curve is not affected by the geometry of sample holder when the atmosphere in the furnace is same as the gas evolved in the TGA FOR EXAMPLE : In the decomposition of CaCO 3 in CO 2 atmosphere, the geometry of container has no effect on TG curve but If the Decomposition is carried out in dry oxygen free nitrogen atmosphere then the shape of the curve depends on the shape of sample holder. 13

14. 2.SAMPLE CHARACTERISATION The factors that affect the TG curve may include Amount of sample, solubility of evolved gas, sample particle size, heat of decomposition of reaction, sample packing, nature of sample, and thermal conductivity of sample. AMOUNT/WEIGHT OF SAMPLE If a large amount of Sample Is used, a derivation from the linearity with the rise in temperature takes place in exothermic reaction EXAMPLE : liberation CO during the decomposition of calcium oxalate to calcium carbonate CaC 2 O 4 CaCO 3 + CO CaCO 3 CaO + CO 2 14

15. The presence of intermediate compound can be detected by making use of small sample in preference to the larger ones. EXAMPLE : 1800mg of CuSO 4 .5H 2 O, when studied thermogravimetrically does not indicate the plateau corresponding to CuS0 4 3H 2 O, but the plateau corresponding to intermediate compound appear in TG curve when 0.426mg of CuSO 4 .5H 2 O sample is employed. SAMPLE PARTICLE SIZE The rate of reaction and shape of TG curve is altered by using sample of different particle size. The decomposition takes place much earlier and at low temperature with the sample having particle size of smaller dimension. 15

16. Decomposition temperature has been found to decrease with decrease in sample particle size which in turn decrease in temperature at which complete decomposition takes place SAMPLE PACKING A compressed sample decomposes at a higher temperature. If the decomposition is exothermic, sample temperature leads the furnace temperature but if the decomposition is endothermic, sample temperature lags behind the furnace temperature METHOD OF PREPARATION Magnesium hydroxide prepared by precipitation method and naturally occurring has Different temperature of decomposition . The TGA studies indicate that source of method of formation of sample should be ascertained before going for TGA. 16

17. ADVANTAGES OF TGA Any type of solid can be analyzed With minimal sample preparation (at least 0.1mg) eg : powders , pellets, flakes TGA has high accuracy of balance used as well as precise control of heating/cooling rate and atmospheric condition TGA may be convenient and time saving, performance of technique does suffer due to construction requirements Easy sample changing and easy change of sample holder Fast heating rate with good resolution can be maintained In TGA one can hold the furnace at 1000°C without any balance drift, which is not balanced in other thermobalance 17

18. Cooling time is very short therefore Thermogram can be recorded TGA is a rapid process. DISADVANTAGES OF TGA Solid sample only must be used in quantitative and qualitative analysis Data interpretation is not always straight forward Very small Amount of samples are used but non-homogeneous material cannot be tested Sensitive to heating rate and sample masses results in shift in temperature Limited to sample which undergoes weight change. Melting, crystal phase change etc. Cannot be studied Complex thermal traces are often difficult to interpret. 18

19. REFERENCE Instrumental method of analysis by Willard 7 th edition page(767-768). Instrumental method of analysis by Chatwal page(2.712-2.719). Instrumental method of analysis by B. K. Sharma page(232-239). THANK YOU. 19