Melting Point Tester: The Complete Guide to Instruments, Types, and Applications



A melting point can immediately identify the material and define its purity. A melting point tester can determine the exact melting point of a material using a heat ramp and recording temperature changes when a solid transitions to a liquid.

This guide covers how a melting point tester works, the different types of melting point testers for labs and research institutions, and the industrial standards of testing.

What Is a Melting Point Tester?

A melting point tester is a lab instrument that you can use to determine the temperature at which a solid substance transitions into a liquid. It is a transition temperature and a physical property that can identify a compound and assess its purity.
A pure substance has a sharp and defined melting point. If any contaminants or unwanted substances are present, the melting point drops and spreads over a range of temperatures. The purity and melting behavior analysis is what makes a melting point tester a crucial instrument in pharmaceutical, chemical synthesis, and material research.
A melting point measuring instrument typically includes a heating element, a temperature sensor, a sample holder, and an observation system, such as a lens or camera.

Working Principle and Testing Method of a Melting Point Tester

The main principle behind the melting point testing machine is heating the solid sample at a controlled rate and recording the temperature. The most important part is detecting the phase change.
Modern melting point testers use a precision melting heating block with a resistance thermometer, usually a platinum resistance thermometer (PT1000 sensor). The block heats the sample at a rate set by the user, and typically the ramp is 0.1°C/min to 1°C/min for accurate results.
Detecting the phase change is done through two ways.
  • Visual Observation: Operator watches through a lens and observes when the sample clears.
  • Photometric Detection: A light source shines through a capillary, and when the sample completely melts, the increase in light transmission is recorded by a photodetector, which starts a temperature log automatically.
When logging the temperature manually or automatically, there are two important temperatures. Onset point: when the sample first begins to collapse or melt, and clear point: when all solid material has completely changed into liquid.
Oil Heating vs Heating Block Testing Methods
Oil bath is an old method of melting point testing done with a Thiele tube. A glass tube filled with mineral oil is heated with an external burner or current. The capillary tube with the sample is immersed in the heated oil alongside a thermometer.
There are many drawbacks to this old method, the most significant one being non-uniform heating. The degree of accuracy is greater than 1°C, and the setup poses a burn hazard because of the oil spillage. Modern melting point test apparatus no longer uses this method.
Heating Block Method
Heating block method is a popular melting point testing method in labs. A solid aluminum block is used because of its high thermal conductivity, and it can distribute heat more uniformly. The heating block is controlled using a small microprocessor temperature controller. Heating blocks can hold heating rates as slow as 0.1°C/min and reach hundreds of degrees in temperature.
Capillary Method vs Liquid Melting Point Testing
Most pharmaceutical labs use the capillary method under USP <741>, Ph.Eur.2.2.14, and JP2.60. A small amount of dry powder is packed into a glass capillary tube with an outer diameter of 1.3 – 1.8 mm and a wall thickness of 0.1 mm. The capillary is filled up to 4 mm and compacted.
The filled capillary is inserted into the heating block vertically. Then there’s a light source that illuminates the sample from below, and a photodetector monitors the light passing through. When the solid melts, light transmission increases sharply, and the instrument can record the onset and clear temperatures.

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