Thermogravimetry apparatus

Later developments by the Mettler Company include a desktop thermogravimetry apparatus, the TA 4000. This system consists of a computer processor (TCIO A) and the thermobalance, shown in schematic cross section in Fig. 7.4. The readability of the balance is 1 ng. The electrical range of mass compensation is from 0 to 150 mg, and the overall capacity of the balance is 3,050 mg. The temperature range is room temperature to 1,250 K with heating rates of 0 -100 K/min. The TA 4000 system also includes tabletop modules for DSC and TMA, all coupled to the same data processor. 

Electrobalances suitable for thermogravimetry are readily adapted for measurements of magnetic susceptibility [333—336] by the Faraday method, with or without variable temperature [337] and data processing facilities [338]. This approach has been particularly valuable in determinations of the changes in oxidation states which occur during the decompositions of iron, cobalt and chromium oxides and hydroxides [339] and during the formation of ferrites [340]. The method requires higher concentrations of ions than those needed in Mossbauer spectroscopy, but the apparatus, techniques and interpretation of observations are often simpler. 

Thermogravimetry consists of the continual recording of the mass of the sample as it is heated in a furnace, and a schematic diagram of a TG apparatus is given in Fig. 14. The weighing device used in most systems is a microbalance, which permits the accurate determination of milligram changes in the sample mass. The... 

Fig. 4.20. Schematic diagram of apparatus suitable for the measurement of thermogravimetry.

The present work demonstrates the application of thermogravimetry for vapor pressure determination62,63. The optimum use of the apparatus by automation of the measuring program, collection of the data and possible sources for errors will be discussed with some examples. 

A technique, developed by Rouquerol [1] and known as constant rate thermogravimetry requires more sophisticated controllers to adjust the furnace temperature in such a way as to maintain a constant preselected value of do/dt over the major portion of the reaction. This increased complexity of the apparatus results in a simplification in the data treatment. 

Most of the techniques described in Chapters 2 to 6 have been applied to the study of dehydrations. The fractional reaction, a, has often been measured using thermogravimetry, preferably with continuous evacuation, or in a strongly flowing carrier gas, to ensure rapid removal of product water vapour. Measurements of the pressure of evolved water vapour in an initially evacuated constant volume apparatus have also been used [10], which enables the influence of the reverse (hydration) reaction to be determined. Microscopic observations [11-13] continue to be an important approach to the investigation of the formation and the growth of... 

Thermogravimetry is an attractive experimental technique for investigations of the thermal reactions of a wide range of initially solid or liquid substances, under controlled conditions of temperature and atmosphere. TG measurements probably provide more accurate kinetic (m, t, T) values than most other alternative laboratory methods available for the wide range of rate processes that involve a mass loss. The popularity of the method is due to the versatility and reliability of the apparatus, which provides results rapidly and is capable of automation. However, there have been relatively few critical studies of the accuracy, reproducibility, reliability, etc. of TG data based on quantitative comparisons with measurements made for the same reaction by alternative techniques, such as DTA, DSC, and EGA. One such comparison is by Brown et al. (69,70). This study of kinetic results obtained by different experimental methods contrasts with the often-reported use of multiple mathematical methods to calculate, from the same data, the kinetic model, rate equation g(a) = kt (29), the Arrhenius parameters, etc. In practice, the use of complementary kinetic observations, based on different measurable parameters of the chemical change occurring, provides a more secure foundation for kinetic data interpretation and formulation of a mechanism than multiple kinetic analyses based on a single set of experimental data. 

In thermogravimetry the variation of the mass of a sample with temperature is monitored in a thermobalance comprising a microbalance, a furnace and a temperature controller. This apparatus allows the calculation of the fractional loss of mass and thus the stoichiometry of a solvate by TGA (Caira, 1998) (Figure 10-6). 

It should be mentioned that thermogravimetry versus T, or p o suffers from many potential sources of error. This includes buoyancy effects (especially of T and p J, dissolution of H in balance materials such as Pt wires, adsorption of H2O on balance parts (especially cold counterweights) and slow changes and equilibria because H2O and D2O adsorb on apparatus and tubing walls and are changed/exchanged slowly. 

Very often thermogravimetry alone cannot give enough information about the reactions being studied. Other measurements often add to the knowledge gained. These ancillary techniques may be applied at the same time as the TG measurement is being applied and are then referred to as simultaneous and are discussed in Chapter 6. Alternatively other measurements may take place in separate experiments in separate apparatus. This is referred to as combined measurements. Simultaneous measurements include differential thermal analysis (DTA) and evolved gas analysis (EGA). These are explained in later chapters. 

Paulik F, Paulik J, Erdey L (1958) The Derivatograph. I. An Automatic Recording Apparatus for Simultaneously Conducting Differential Thermal Analysis, Thermogravimetry, and Derivative Thermogravimetry. Z anal Chem 160 241-252. For standardization, quasi-isothermal and isobaric analyses and some example research with the Derivatograph see also (1966) Anal Chim Acta 34 419-426 Paulik F, Paulik J (1973) J Thermal Anal 5 253-270 (1975) 8 557-576. 

The extent of dehydration during temperature treatment of tin(IV)oxide powder is Mcertained by thermogravimetry (Simultaneous Thermal Analysis STA 409, Netzsch). Surface are l8 are determined by nitrogen adsorption using a standard volumetric BET apparatus. The tin(IV)oxide content of the loaded crtrriers and the catalysts is quantified gravimetrically. The platinum content of the catalysts is obtained by a spectrophotometric method which has been described elsewhere (11). 

In order to increase the resolution of TG curves, it is necessary to change the heating rate in coordination with the decrease in mass. This technique is called controlled rate thermogravimetry (CRTG). Several kinds of technique for controlling the temperature, such as step-wise isothermal control, dynamic rate control and constant decomposition rate control, are employed. The above technique is mainly achieved using software with commercial TG apparatus (2). 

Behrens, Jr., R., (1986), A New Simultaneous Thermogravimetry and Modulated Molecular Beam Mass spectrometry Apparatus for Quantitative Thermal Decomposition Studies , Rev. Sci. Instrum., 58, pp. 451- 461. 

In the direct calorimetric determination, (- AH =f nn)T), the amount adsorbed ( ta) is calculated either from the variations of the gas pressure in a known volume (volumetric determination) or from variations of the weight of the catalyst sample in a static or continuous-flow apparatus (gravimetric determination). In a static adsorption system, the gas is brought into contact with the catalyst sample in successive doses, whereas the catalyst is swept by a continuous flow in a dynamic apparatus. Comparative calorimetric studies of the acidity of zeolites by static (calorimetry linked to volumetry) and temperature-programmed (differential scanning calorimetry linked to thermogravimetry) methods of ammonia adsorption and desorption have been performed [23]. 

Note I—This procedure is a modification of the original method and apparatus for carbon residue of petroleum materials, where it has been demonstrated that thermogravimetry is another applicable technique. However it is the responsibility of the operator to establish operating conditions to obtain equivalent results when using thermo-gravimetry. 

Being dynamic techniques, both thermogravimetry and differential thermal analysis depend much on careful selection of apparatus and on reproducibility of technique (Mackenzie and Mitchell [1957, 1970a, 1970b] Mackenzie [1946b] Coats and Redfern [1963]). Some consideration of both aspects is necessary before considering the thermal reactions of individual minerals in order to assess what can and what cannot be determined under specific conditions. 

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