Thermogravimetric analysis: temperature limits and rate of heating

In the case of the decomposition of carbonates and hydrates, the Polanyi-Wigner equation often applies. A mathematical expression relating the temperature at a given stage of the reaction to the rate of decomposition, fraction of reactant left, initial weight and area of the compound and finally the rate of heating has been derived from the above equation. For a given thermobalance the minimum detectable rate of decomposition and fraction of reactant left are fixed so that the observed final decomposition temperature is a function of the rate of heating for a constant weight and area of material. Using this equation, values for the final decomposition temperatures have been calculated and compared with data found in the literature for the decomposition of calcium carbonate and for the dehydration of potassium chrome alum and orthoboric acid. The equation also has been applied to the dehydration of 5-nitrobarbituric acid trihydrate as determined by us.