Correction in DTA purity determination

An equation for calculating the melted fraction (F) in purity determinations by DTA has been derived, which makes a correction for the thermal lag of the sample cell during the melting process. A set of samples of benzoic acid containing different quantities of anthracene has been tested. There are two peaks in the melting thermograms and the calculated impurity contents in these samples without using the correction are less than those prepared. This binary system is assumed to form an eutectic; its composition and melting point are estimated as X = 0.056 mole fraction of anthracene and T = 117.8°C respectively. Results obtained using naphthalene containing 0.415 mol% of fluorene show that the correction for the thermal lag of sample cell is necessary. In addition, a linear correction in the F calculation for the melting of eutectic and premelting of the main component is also desirable, giving a good straight line of the melting temperature T versus 1/F and a reasonable purity estimation.