Why Capillary Tube is Used in Determination of Melting Point?

4 min readFeb 27, 2024

Why Capillary Tube is Used in Determination of Melting Point

Introduction:

The determination of melting point is a crucial technique in various fields such as chemistry, pharmaceuticals, and materials science. It involves measuring the temperature at which a solid substance transitions from its solid state to its liquid state. One common method used for determining the melting point of a substance is the capillary tube method. In this blog post, we will explore the reasons why capillary tubes are used in the determination of melting point and how this technique works.

Importance of Melting Point Determination:

Before delving into the specifics of the capillary tube method, it is important to understand why the determination of melting point is significant. The melting point of a substance is a physical property that can provide valuable information about its purity, identity, and quality. A pure substance typically melts at a specific temperature, known as its melting point. Any impurities present in the substance can alter its melting point, either by lowering or broadening the temperature range over which melting occurs. Therefore, melting point determination serves as a reliable method for assessing the purity and quality of solid substances.

Capillary Tube Method:

The capillary tube method is a widely used technique for determining the melting point of solid substances. It involves placing a small sample of the substance into a thin glass capillary tube, which is then inserted into a melting point apparatus equipped with a heating source and a temperature sensor. The temperature is gradually increased, causing the solid sample to heat up and eventually melt. The temperature at which the first drop of liquid appears is recorded as the melting point of the substance.

Reasons for Using Capillary Tubes:

There are several reasons why capillary tubes are used in the determination of melting point:

Uniform Heating: Capillary tubes have a small diameter, which allows for uniform heating of the sample. This ensures that the entire sample reaches the same temperature at the same time, leading to accurate and reproducible results.
Observation of Melting Behavior: The narrow bore of the capillary tube allows for easy observation of the melting behavior of the sample. As the temperature increases, the solid sample begins to liquefy, and the melting process can be visually observed through the transparent glass tube.
Thermal Conductivity: Glass capillary tubes have low thermal conductivity, which helps to minimize heat loss during the melting process. This ensures that the temperature gradient along the length of the tube remains small, resulting in more accurate temperature measurements.
Compatibility with Melting Point Apparatus: Capillary tubes are compatible with standard melting point apparatus, making them convenient and practical for routine melting point determinations in laboratories.
Small Sample Size: The small diameter of capillary tubes allows for the use of a minimal amount of sample, which is particularly advantageous when working with expensive or limited quantities of substances.

Procedure for Capillary Tube Method:

The capillary tube method typically follows these steps:

Preparation of Sample: A small amount of the solid sample is finely powdered and placed into a clean, dry capillary tube. Excess sample is removed by tapping the tube gently to ensure a uniform and compact sample.
Insertion into Melting Point Apparatus: The filled capillary tube is carefully inserted into the melting point apparatus, ensuring that it is positioned correctly for heating and observation.
Heating and Observation: The temperature of the melting point apparatus is gradually increased at a controlled rate, while the sample is observed through a magnifying lens. The temperature at which the first drop of liquid appears is recorded as the melting point of the substance.
Confirmation and Verification: The melting point determination may be repeated multiple times to confirm the accuracy and reproducibility of the results. Comparison with known reference substances can also help to verify the identity and purity of the sample.

Conclusion:

In conclusion, the capillary tube method is a reliable and widely used technique for determining the melting point of solid substances. By using capillary tubes, researchers can achieve uniform heating, observe the melting behavior of the sample, minimize heat loss, and work with small sample sizes. Melting point determination plays a crucial role in assessing the purity, identity, and quality of solid substances, making it an indispensable technique in various scientific disciplines.

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