ASTM E928-96

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: E 928 – 96
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Determination of Purity by Differential Scanning
Calorimetry
This standard is issued under the fixed designation E 928; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Summary of Test Method
1.1 This test method covers the determination of purity by 4.1 This test method consists of melting the test specimen
use of differential scanning calorimetry and the evaluation of that is subjected to a temperature-controlled program while
the results using the van’t Hoff equation. recording the heat flow (power) into the specimen as a function
1.2 This test method is generally applicable to thermally of time. The resulting endotherm area is often measured to
stable compounds with well-defined melting temperatures. yield the enthalpy of fusion. The melting endotherm area is
1.3 Determination of purity by this test method is only then partitioned into a series of fractional areas (about ten,
applicable when the impurity dissolves in the melt and is comprising the first 10 to 50 % of the total area). Each
insoluble in the crystal. fractional area is assigned a temperature. A plot of the
1.4 Computer- or electronic-based instruments, techniques, reciprocal fractional areas and their corresponding tempera-
or data treatments equivalent to this test method may also be tures is then produced. The resulting plot is seldom a straight
used. Users of this test method are expressly advised that all line. To linearize the plot, an incremental amount is added to
such instruments or techniques may not be equivalent. It is the the enthalpy of fusion and the fractional areas recalculated and
responsibility of the user of this test method to determine the plotted. The purity level is then calculated from the slope of
necessary equivalency prior to use. this straight line.
1.5 The values stated in SI units are to be regarded as the
5. Significance and Use
standard.
1.6 This standard does not purport to address all of the 5.1 The melting temperature range of a compound broadens
as the impurity level rises. This phenomenon is described
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- approximately by the van’t Hoff equation for melting point
depressions. Measuring and recording the instantaneous heat
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. flow (power) into the specimen as a function of time or
temperature during such a melting process is a practical way
2. Referenced Documents
for the generation of data suitable for analysis by the van’t Hoff
2.1 ASTM Standards: equation.
E 473 Terminology for Thermal Analysis 5.2 The results obtained include: sample purity (expressed
E 793 Test Method for Enthalpies of Fusion and Crystalli- as mol percent); enthalpy of fusion (expressed as joules per
zation by Differential Scanning Calorimetry mol); and the melting temperature (expressed in Kelvin) of the
E 967 Practice for Temperature Calibration of Differential pure form of the major component.
Scanning Calorimeters and Differential Thermal Analyz- 5.3 Generally, the repeatability of this test method decreases
ers as the purity level decreases. This test method is ordinarily
E 968 Practice for Heat Flow Calibration of Differential considered unreliable when the purity level of the major
Scanning Calorimeters component of the mixture is less than 98.5 mol % or when the
incremental enthalpy correction (x) exceeds 20 % of the
3. Terminology
original detected enthalpy of fusion.
3.1 Definitions—The definitions relating to thermal analysis 5.4 This test method is used for quality control, specifica-
appearing in Terminology E 473 shall be considered applicable
tion acceptance, and research.
to this test method.
6. Interferences
6.1 This test method is nonspecific. Many impurities may
This test method is under the jurisdiction of ASTM Committee E-37 on
cause the melting temperature broadening. Thus, it is not useful
Thermal Measurements and is the direct responsibility of Subcommittee E37.01 on
Test Methods and Recommended Practices. in identifying the nature of the impurity or impurities but only
Current edition approved Dec. 10, 1996. Published April 1997. Originally
the total mol percent of impurity present.
published as E 928 – 83. Last previous edition E 928 – 85 (1989)e .
6.2 The van’t Hoff theory assumes the following:
Annual Book of ASTM Standards, Vol 14.02.
E 928
6.2.1 The impurities dissolve in the melt of the major Only 1 to 3 mg is required for each analysis.
constituent forming a solution approximately described by 8.2 Avoid any physical or mechanical treatment of the
ideal solution theory; material that will cause chemical changes. For example,
6.2.2 The solubility of the impurity in the solid of the major grinding the sample for size reduction often introduces such
constituent is negligible; and changes as a result of heat generated by friction.
6.2.3 The major constituent must display a single well-
9. Calibration
defined melting endotherm in the temperature range of interest.
9.1 Accurate calibration of the absolute temperature or
Microscopic investigations of the melt and the solid may help
enthalpy of fusion is not necessary to determine the mol
to establish whether or not solid or liquid solutions have been
percent purity. However, if the accurate melting temperature or
formed.
enthalpy of fusion of the pure compound is desired, calibrate
6.3 In some cases the sample may react with air during the
the instrument in accordance with Practice E 967 and E 968.
temperature cycle, causing an incorrect transition to be mea-
Perform calibrations at the heating rate used for the purity
sured. Where it has been shown that this effect is present,
determination (see 11.7) and in the temperature range of the
provision shall be made for sealing the specimen and running
melting temperature of the pure material.
the test under an inert gas blanket. Since some materials
9.2 When metals are used with aluminum specimen holders,
degrade near the melting region, carefully distinguish between
it is best to use a fresh sample each time as alloying of the
degradation and transition.
metallic standard with aluminum may result on reheating.
6.4 Since milligram quantities of sample are used, ensure
that samples are homogeneous and representative.
10. Procedure
6.5 Sublimation will lead to a different heat consumption
10.1 Caution—Toxic and corrosive effluents may be re-
and, perhaps, a change in composition of the specimen. The
leased upon heating the material. It is the responsibility of the
specimen holder should be examined after the measurement for
user of the standard to take appropriate safety measures.
crystals not part of the resolidified melt.
10.2 Wash the empty specimen container in an appropriate
solvent, such as hexane, then heat to 700 K for 1 min.
7. Apparatus
10.3 Cool the specimen container and store in a desiccator
7.1 The essential equipment required to provide the mini-
until ready for use.
mum instrument capability for this test method includes:
10.4 Weigh 1 to 3 mg of the sample to an accuracy of 0.01
7.1.1 Differential Scanning Calorimeter (DSC), consisting
mg in a pre-cleaned holder.
of:
10.5 Under ambient conditions, hermetically seal the speci-
7.1.1.1 DSC Test Chamber, composed of a furnace(s) to
men container so there will be no mass loss during the scan.
provide uniform controlled heating of a specimen and refer-
Minimize the free space between the specimen and the lid to
ence to a constant temperature or at a constant rate within the
avoid sublimation onto the lid
...