Heating programmers

The temperature at which massive magnesium and its alloys will ignite in air depends on the heating programme and the presence or absence of moisture. 

The furnace must be designed in such a fashion so as to incorporate an appropriate smooth input thereby maintaining either a fixed temperature or a predetermined linear-heating programme (e.g., 10°C-600°C per hour). 

Subsequent to optimisation of the heating programme, the instrument should be calibrated with aqueous standards. When a linear and reproducible calibration curve is obtained, a series of standard additions on the samples should be performed in order to elucidate whether a matrix interference is operating. This is evident when the analyte absorbance is enhanced or depressed in the sample matrix compared to the pure standard. Often however, interference is indicated by the shape of the analyte absorbance peak differing in the sample relative to that in the standard or when the analyte absorbance in the sample is irreproducible or spurious. 

In order to confirm these results, a linear heating programme has been applied where the relatively mild final temperature was reached only at the end of the experiment, not leaving further time at the top temperature to react. The presence of more catalyst has indeed formed more liquid products (Figure 7.4), confirming the above findings. 

This term refers to the determination of kinetic parameters (f(nr) or g(ar), A and ",) for a reactant subjected to a predetermined heating programme, usually, but not necessarily, a constant rate of temperature increase (d77di = P) (Chapter 5). Isothermal data may provide the more sensitive tests for distinguishing the best fit rate equations (g( r) = kt), whereas rising temperature observations may be preferred for the determination of Arrhenius parameters (A and EJ. Reasons for any differences noted in the results of the alternative treatments should be investigated. 

The frequency of modulation is adjusted so that there are many cycles over the duration of the transition, which in this case is the glass transition and is therefore broad. A typical modulation condition is 0.5 C in a 60-s period for studying cure of an epoxy resin (Van Assche et al, 1997). The MDSC instrument provides a modulated heating programme of... 

Differential thermal analysis (DTA) is based upon the measurement of the temperature difference (AT) between the sample and an inert reference such as glass or AIjO as they are both subjected to the same heating programme. The temperature of the reference will thus rise at a steady rate determined by its specific heat, and the programmed rate of heating. Similarly with the sample, except that when an exothermic or endothermic process. occurs a peak or trough will be observed. Typical behaviour is shown schematically in Figure 11.7. 

The use of the HPA system is fairly simple the 30 mL or the 70 mL standard quartz vessels allow intakes of sample equivalent to 100 or 230 mg carbon, respectively, with nitric acid (65 to 69% w/v) volumes of 2 mL per 100 mg C. The heating programme from ambient to maximum temperature is as follows ambient to 250 °C 1 min 250 to 300 °C 30 min. This maximum temperature is maintained for 90 min then turned off. After cooling (approx. 60 min) the samples are made up to volume and treated as required for subsequent determination. In one of the author s laboratory (M.S.) the analyte solutions are treated with a Kl-ascorbic acid mixture for reduction to As(lll) and subsequent FIA-AAS-determination with excellent precision and absolute detection limit (Guo et al., 1990). 

The different experimental conditions in the TG and the SEM can result in minor differences in the reaction temperatures because, in the TG, a continuous heating programme is running, while in the SEM heating takes place in steps. However, these differences will be small because the thermal SEM-EDX analyses are done in a static mode at temperatures intermediate between those of the reactions and these temperatures are sufficiently separated. 

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