Influence of the Heating Rate: Melting and Chemical Reactions - METTLER TOLEDO

Influence of the Heating Rate: Melting and Chemical Reactions

Introduction

Many organic chemicals, and in particular active pharmaceutical drugs, decompose at relatively low temperatures, usually directly after melting. The correct interpretation and proper evaluation of DSC, TGA and DTA measurements is often difficult because several effects may overlap.

Many useful tips and information on curve interpretation have been published in previous UserCom articles [1,2].

This article describes an important method that can be used to separate and identify melting processes and chemical reactions.

The method is based on measuring the substance concerned several times at different heating rates. The heating rate used for each measurement is usually increased by a factor of 2 each time, for example 1, 2, 5 and 10 K/min, in order to make the changes involved clearly visible. 

Simple substances are used as examples to demonstrate the typical appearance of the resulting DSC, TGA and DTA curves. Another example shows, however, that some substances yield unexpected curve shapes.

 

Results 

Melting

Melting is a phase transition in which a substance passes from the solid to the liquid state. In principle, the melting temperature is independent of the heating rate. It is determined as the temperature at which the melting peak begins (extrapolated onset). There are no time-dependent effects other than the heat flow into the sample. 

If the melting points of pure substances do however change with heating rate in practical measurements, then the measuring cell must be recalibrated and if necessary adjusted. Very pure substances are used for this purpose because their melting temperatures are accurately known. They allow instrumental effects such as the heat transfer to the sample to be measured and compensated. Users of thermal analysis and readers of UserCom are no doubt familiar with this from their own experience or from previous articles [3, 4].

Most organic substances do not have the high degree of purity required for performing calibrations. Melting takes place over a temperature range. The purity of a substance can be determined by analyzing the shape of its melting curve [5]. The temperature range of a melting curve depends slightly on the heating rate (Figure 1, upper diagram). The melting temperature (measured as the extrapolated onset), the temperature of the peak maximum and the heat of fusion are, however, independent of the heating rate provided that the material does not decompose (Table 1). Deviations in the results at higher heating rates (from 5 K/min upward) indicate that the melt is not in thermodynamic equilibrium.

Chemical Reactions 

A chemical reaction takes place over a period of time and therefore depends on the heating rate (Figure 1 below). At lower heating rates, the maximum of the reaction peak (maximum reaction rate) is at a lower temperature because it takes more time to reach a particular temperature. This is the basis of methods to describe kinetic behavior using dynamic measurements. For example, model free kinetics requires at least three measurements to be performed at markedly different heating rates.

 

A chemical reaction can therefore be identified by the fact that the DSC (and SDTA and DTG) peaks show a distinct shift along the temperature axis (Table 1). The reaction enthalpy is independent of the heating rate if the reaction mechanism does not change with temperature.

Overlapping of Melting and Reactions 

From the point of view of curve interpretation, the effect of different heating rates means that it is possible to separate melting processes and reactions by varying the heating rate. The temperature of the onset of melting remains unchanged providing the reaction products do not depress the melting point, but the reaction peak shifts. At high heating rates (> 10 K/min), the sample temperature increases so rapidly to higher temperatures that the melting process is completed before the reaction has really begun. 

Conclusions

Measurements performed at different heating rates help to distinguish between phase transitions and reactions in DSC, TGA, DTA and other thermal analysis techniques. In general, melting temperatures are independent of the heating rate. Chemical reactions (decomposition, polymerization, etc.) however show a marked shift to higher temperatures when the heating rate is increased (about 40 K per decade of the heating rate). 

Varying the heating rate can therefore be used to separate overlapping effects (peaks) and improve evaluation. There are, however, situations in which small quantities of decomposition products lower the melting point. The melting process then appears to depend on the heating rate, which can easily lead to wrong interpretation. 

To avoid this, other measurement techniques such as simple visual observations (i.e. thermo-optical analysis) or X-ray diffraction should also be used.

Influence of the Heating Rate: Melting and Chemical Reactions | Thermal Analysis Application No. UC 237 | Application published in METTLER TOLEDO Thermal Analysis UserCom 23