ASTM E928 Purity Determination by DSC Testing

ASTM E928 is a standard test method used to determine the purity of a material with a greater than 98.5 mole percent purity using differential scanning calorimetry and the van't Hoff equation.

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Introduction

This ASTM standard, E928, describes a test method that employs differential scanning calorimetry and the Van’t Hoff equation to determine the purity of materials with more than 98.5 mole % purity. Moreover, it is one of the new methods for purity assessment based on the melting behavior of materials and the released heat flow.

Scope of ASTM E928

The technique estimates the purity of organic chemicals by measuring their melting points and the amount of heat accompanying the melting process. The purity can be judged from the melting range and melting enthalpy from the DSC analysis. The broader the melting temperature range, the higher the increase in the impurity level of a compound. This phenomenon is described approximately by the Van’t Hoff equation for melting point depressions. Furthermore, the ASTM E928 test is utilized in quality control, specification acceptance, and research.

Test Procedure of ASTM E928

Principle of DSC:

The Differential Scanning Calorimetry (DSC) technique involves heating a small sample of the material at a controlled rate. As the material melts, it absorbs heat, resulting in an endothermic peak on the DSC curve. The shape and area of this peak provide information about the melting behavior and purity of the substance. The heat flow into the sample is compared to a reference material, and the difference in heat flow (differential) is recorded.

The ASTM E928 test procedure and other details are as follows:

Particulars	Details
Sample Preparation	A few milligrams of organic chemicals are placed into a DSC pan.
Heating	Heating at a controlled rate (1 to 10°C/min) until melting occurs. The temperatures at which melting begins and when the sample is entirely melted are recorded
Data Analysis	The DSC curve of heat flow versus temperature was analyzed using the onset of melting, peak melting temperature, and heat of fusion. The area underneath the melting peak corresponds to the heat of fusion, which generally relates to sample purity.
Result	Van’t Hoff Equation: This equation connects the depression of the melting point with the sample’s purity. The heat of fusion, in addition to an observed melting range, is considered in this equation. Purity Calculation: The purity of the sample is determined through the melting point depression, that is, the difference between the melting point of the pure substance and the observed melting point.

Specimen Size of ASTM E928

The test sample (liquid or solid) should be mixed before sampling and sampled by removing portions from various parts of the container.
The portions should be mixed well to provide a representative sample for the purity determinations.
Only 1 to 3 mg is required for each analysis. Also, any physical or mechanical treatment of the material should be avoided that will cause chemical changes.

Applications

Organic Chemicals: Purity assessment for research and manufacturing.
Assuring Quality: This technique is used in quality control laboratories to determine if a substance contains impurities.

Conclusion:

ASTM E928 employs a technique similar to DSC for assessing the purity of organic chemicals based on their melting behavior. This test relies on the melting point depression relationship, which can be directly related to purity with high accuracy and efficiency. Furthermore, this standard has widespread application in industries where the purity of the organic compound is of prime importance, such as pharmaceuticals or chemical manufacturing.

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FAQs

What is ASTM E928?

ASTM E928 outlines procedures for determining sound power levels in various environments, ensuring that results are consistent and reliable.

What are the advantages of Purity determination?

The DSC is an accurate and dependable method for purity determination. Only a few samples are needed, and the test can be done in a relatively shorter time.

Why is ASTM E928 important for determining purity?

ASTM E928 provides one accurate, reliable method for determining organic chemical purity. Purity is a critical issue in many industries, as pharmaceutical impurities can affect a product's safety and efficacy.

How long does it take to perform a purity test using ASTM E928?

Depending on the heating rate and the complexity of the analysis, the test can be completed relatively quickly, often within a few hours.

What are the limitations of ASTM E928?

ASTM E928 is an excellent method for assessing the purity of organic compounds; it may not be appropriate for substances with extremely complicated melting behaviors or materials with poorly defined melting points.