Significance and Use

The melting temperature range of a compound broadens as the impurity level rises. This phenomenon is described approximately by the van't Hoff equation for melting point depressions. Measuring and recording the instantaneous heat flow into the specimen as a function of temperature during such a melting process is a practical way for the generation of data suitable for analysis by the van't Hoff equation.

The results obtained include: sample purity (expressed as mole percent); enthalpy of fusion (expressed as joules per mole); and the melting temperature (expressed in Kelvin) of the pure form of the major component.

Generally, the repeatability of this test method decreases as the purity level decreases. This test method is ordinarily considered unreliable when the purity level of the major component of the mixture is less than 98.5 mol % or when the incremental enthalpy correction ( c ) exceeds 20 % of the original detected enthalpy of fusion.

This method is used for quality control, specification acceptance, and research.

1. Scope

1.1 This method describes the determination of purity of materials greater than 98.5 mole percent purity using differential scanning calorimetry and the van't Hoff equation.

1.2 This test method is applicable to thermally stable compounds with well-defined melting temperatures.

1.3 Determination of purity by this test method is only applicable when the impurity dissolves in the melt and is insoluble in the crystal.

1.4 SI values are the standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

1.6 There is no ISO method equivalent to this method.

2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

ASTM Standards

E473 Terminology Relating to Thermal Analysis and Rheology

E793 Test Method for Enthalpies of Fusion and Crystallization by Differential Scanning Calorimetry

E794 Test Method for Melting And Crystallization Temperatures By Thermal Analysis

E967 Test Method for Temperature Calibration of Differential Scanning Calorimeters and Differential Thermal Analyzers

E968 Practice for Heat Flow Calibration of Differential Scanning Calorimeters

E1970 Practice for Statistical Treatment of Thermoanalytical Data

Keywords

differential scanning calorimetry; purity; Van't Hoff equation

FIG. 3 Temperature Shift for samples that melt

FIG. 4 Temperature Shift for samples that decompose

ICS Code

ICS Number Code 17.200.10 (Heat. Calorimetry)

DOI: 10.1520/E0928-08

ASTM International is a member of CrossRef.

ASTM E928