CHEM3440Lec14F06 - Thermal Methods We will examine three...

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CHEM*3440 Chemical Instrumentation Topic 14 Thermal Analysis Thermal Methods We will examine three thermal analytical techniques: Thermogravimetric Analysis (TGA) Differential Thermal Analysis (DTA) Differential Scanning Calorimetry (DSC) TGA and DTA have found important roles to play in industrial chemistry, particularly with the analysis and development of polymers. DSC is an important research tool, providing access to accurate thermodynamics data as well as information regarding reactivity and phase transformations. Thermogravimetry Analysis of mass changes in a sample as a function of temperature. Usually observes mass loss: decomposition sublimation reactivity and desorption of products Can observe mass gain: oxidation, forming non-volatile oxides Instrumental Components There are three principal components in a TGA: analytical balance furnace and temperature control system control of atmospheric gases exposed to sample Vacuum, Gases Sample Crucible Oven Controller Analytical Balance
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TGA Data Analysis Thermogram is graph of mass versus temperature. Sometimes given as % of original mass. Mass Temperature Transition temperature TGA Example1 Theophylline (180.17 g/mol) is a pharmaceutical used for chronic asthma treatment. Its stable form is that of the monohydrate (198.18 g/mol). The amount of water in an actual sample is important to measure in order to ensure accurate dosage. Dehydration is complete at 110 ˚C. Complete hydrate should lose 9.1% of mass, based on stoichiometry. Sample A loses only 4.13%. Hence, it is only 45.54% pure hydrate. Sample B loses 8.69% and is therefore 95.79% pure. Note that it loses a small amount of adsorbed moisture before scan even started. TGA Example2 Calcium carbasalate is a salicylate like aspirin with its analgesic effects but seems to do less damage to the stomach lining. The pure, active compound shows a 49.46% weight loss starting at 210 ˚C due to decomposition. A particular preparation of it (called Alcacyl), shows a 5% weight loss, due to moisture and followed by a 24.45% weight loss at the decomposition temperature. This indicates that the original sample was 49.43% carbasalate. TGA Example3
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CHEM3440Lec14F06 - Thermal Methods We will examine three...

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