Medical brain notes 14

  INTRODUCTION Thermoanalytical methods  essentially encompass such techniques that are based entirely on the con-cept of heating a sample followed by well-defned modified procedures, such as : gravimetric analysis, differ-ential analysis and titrimetric analysis. In usual practice, data are generated as a result of continuously re-corded curves that may be considered as ‘thermal spectra’. These thermal spectra also termed as  ‘thermograms , often characterize a single or multicomponent system in terms of :   ( a ) temperature dependencies of its thermodynamic properties, and   ( b ) physicochemical reaction kinetics.   Broadly speaking the thermoanalytical methods are normally classified into the following three categories, namely :   ( i ) Thermogravimetric Analysis (TGA),   ( ii ) Differential Thermal Analysis (DTA), and   ( iii ) Thermometric Titrations.   All the above mentioned techniques shall be discussed briefly with specific re...

  THERMOGRAVIMETRIC ANALYSIS (TGA)   1. THEORY   A large number of chemical substances invariably decompose upon heating, and this idea of heating a sample to observe weight changes is the underlying principle of thermogravimetric analysis (TGA). However, TGA may be sub-divided into  two  heads, namely :   ( a ) Static (or Isothermal) Thermogravimetric Analysis, and   ( b ) Dynamic Thermogravimetric Analysis.   1.1. Static Thermogravimetric Analysis   In this particular instance the sample under analysis is maintained at a constant temperature for a period of time during which any changes in weight are observed carefully.   1.2. Dynamic Thermogravimetric Analysis   In dynamic thermogravimetric analysis a sample is subjected to conditions of predetermined, carefully controlled continuous increase in temperature that is invariably found to be linear with time.     2. INSTRUMENTATION   The essential requirements for an i...

  DIFFERENTIAL THERMAL ANALYSIS (DTA)   1. THEORY   The difference of temperature between the sample under estimation and a thermally-inert reference material is continuously recorded as a function of furnace temperature in differential thermal analysis (DTA). In actual practice both TGA and DTA are regarded as complementary techniques whereby information gathered by the usage of one approach is invariably supplemented and enhanced by the application of the other method. The range of phenomena measurable during a DTA-run is found to be much larger than in a TGA-run.   It is pertinent to mention here that in the course of TGA many vital processes, for instance : crystalli-zation, crystalline transitions, pure fusion reactions, glass transitions, and solid-state reactions devoid of vola-tile components might not be indicated as they happen to cause little change in weight of the sample. TGA invariably describes with ample precision the stoichiometry related to chemical...