Dynamic purity determinations. I. Location of the solidus by nuclear magnetic resonance

Abstract

The error in purity determinations by differential scanning calorimetry has been studied on two different kinds of systems, metallic and organic, the latter with different types of impurities added. The organic systems have been corrected for solid-solubility error by a nuclear magnetic resonance technique.The impurities chosen for the organic systems were either quite dissimilar to the matrix or similar enough to render solid solution probable. This solid solution leads to deviations from the equation used for dynamic purity determinations. The nuclear magnetic resonance determination of the solidus establishes the proper starting temperature for area summation on a DSC curve.Thermoanalytical techniques are so generally useful that it is not at all difficult to establish new applications or deduce relationships on the basis of some experimental data-then find that there is substantial limitation on that application or concept. This senior author (PDG) has sought, from time to time, to examine some concepts that have developed in thermal analysis or are testable by thermal analysis to prove or disprove [1][2][3][4][5][6][7][8] or to delineate their limits [9, 10, this work].This program of research was undertaken to evaluate the potential error inherent in the purity determination methods which depend upon the assumption of ideal liquid solution but zero solid solution. Impurities are highly probable in real systems and in any real system the most probable impurities are materials closely related in chemical structure to the matrix material because of their origin or processing.The well known effect of small amounts of impurities on melting behavior of crystalline compounds, i.e., lowering of their melting points, provides an excellent basis for purity determinations by various methods (time dependent and time independent). There are three main advantages of these methods: first, simplicity and speed of some of them, viz. the time dependent ones; second, sensitivity which is of such magnitude that it can be compared with the best methods known, for instance, high temperature mass spectrometric analysis [10]; third, a firm base upon thermodynamic principles which means that no reference materials need to be applied. Experimentally, in purity determinations based on lowering the melting point

Dynamic purity determinations. II. Some additional systems

“…The true solidus was obtained by first equilibrating the NMR tube at a temperature below the eutectic temperature, then advancing the voltage input to the heater in small increments, obtaining a plot over a principal peak of the major component after each equilibration. The area at several temperatures were determined and finally, from the intercept of the two segments [1 ], the solidus for that system was found.…”

“…The data indicate that the measured impurity is dependent upon not only the concentration but also the nature of the impurity. The determination of the actual beginning of melting by nuclear magnetic resonance results in more precise purity measurements.In the previous paper [1 ], it was shown that one of the attributed causes of error in the determination of purity by differential scanning calorimetry or differential thermal analysis-lack of thermal equilibrium-is not a principal source of error. Further, the utility of determination of the solidus by nuclear magnetic resonance [2] was established.…”