Thermobalance Factors

Experimental factors. In the previous section it was stated that the precise temperature regions for each reaction of the thermal decomposition of copper sulphate pentahydrate is dependent upon experimental conditions. When a variety of commercial thermobalances became available in the early 1960s it was soon realised that a wide range of factors could influence the results obtained. Reviews of these factors have been made by Simons and Newkirk30 and by Coats and Redfern31 as a basis for establishing criteria necessary to obtain meaningful and reproducible results. 

Some factors that must be considered in the construction or purchase of an automatic thermobalance have been given by Lukaszewski and Redfern (1) ... 

The transition used to calibrate the temperature scale of a thermobalance should have the following properties [1] (i) the width of the transition should be as narrow as possible and have a small energy of transformation (ii) the transition should be reversible so that the same reference sample can be used several times to check and optimize the calibration (iii) the temperature of the transition should be independent of the atmospheric composition and pressure, and unaffected by the presence of other standard materials so that a multi-point calibration can be achieved in a single run and (iv) the transition should be readily observable using standard reference materials in the milligram mass range. Transitions or decompositions which involve the loss of volatile products are usually irreversible and controlled by kinetic factors, and are unsuitable for temperature calibration. Dehydration reactions are also unsuitable because the transition width is strongly influenced by the atmospheric conditions. 

Buoyancy effect This refers to apparent gain in weight that can occur when an empty and thermally inert crucible is heated. The effect is due to complex interaction between three factors (i) the decreased buoyancy of the atmosphere around the sample container at higher temperatures, (ii) the increased convection effect and (iii) the possible effect of heat from furnace on the balance itself In most modern thermobalances, attention to design factors has made the buoyancy effect negligible. However, if necessary a blank run with an empty crucible can be carried out over the appropriate temperature range. The resultant record can be used as a correction curve for subsequent experiments. 

One of the factors limiting the accuracy of earlier models of thermobalances was the fact that the agate knife-edges and bearing surfaces on the balances were not sufficiently hard to withstand the continuous operation demanded. This led to the development of the Chevenard type of thermobalance, shown diagrammatically in Figure 4, where the agate system is replaced by a four-wire suspension (Duval [1963]). 

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