How does a Melting Point Machine Work?

Melting point is the equilibrium temperature of the solid and liquid state of a substance under atmospheric pressure, and is usually measured by the temperature at which a crystalline substance warms up and transforms from a solid to a liquid state, which is recorded as the melting point. Melting point is an important method for recognizing substances and determining their purity. Therefore, melting point machine has an important position in chemical industry and pharmaceutical research. It is a necessary instrument for the production of drugs, spices, dyes and other organic crystalline substances, and is also one of the basic instruments commonly used in the laboratory. Next, let us understand the principle of melting point machine and the use of precautions.

Definition of Melting point

MP (Melt Piont) is defined as the temperature at which a substance changes from a solid to a liquid. During the melting of a purely crystalline substance, all the energy injected into the substance is consumed in the form of heat of fusion. The body temperature of the substance remains constant until all the substance has melted from the solid phase to the liquid phase (i.e. the melting process is complete). Substance melting is a highly controllable process and the melting point of a substance is a parameter that can be calibrated with high precision.

Pure solid substances generally have a fixed melting point and a short melting range, generally no more than 0.5 - 1 °C, through which the purity of the substance can be determined and the unknown substance identified. The melting point is determined by mixing the unknown substance with a known substance. If the melting point does not change and the melting range does not exceed 1 ℃, it can generally be considered that the two are the same substance, this method is called mixed melting point method.

Why measure melting point?

Melting points are commonly used to characterize organic and inorganic crystalline compounds and to determine their purity. The melting points of pure substances are very well defined (very small temperature range of 0.5 - 1 °C), whereas the melting points of impure contaminated substances are usually larger. All the material of the contaminated substance melts at a temperature that is usually lower than the melting point of the pure substance. This behavior is called melting point depression and can be used to obtain qualitative information about the purity of the substance.

In general, melting point determination is used in laboratory research and development as well as in quality control in various industry sectors to identify and check the purity of different substances.

Principle of Melting Point Measurement

At the melting point, the light transmittance changes. Compared to other physical values, the change in light transmittance is easy to determine and can therefore be used for melting point detection. Powdered crystalline materials are opaque in the crystalline state and transparent in the liquid state. This apparent difference in optical properties can be measured, and the melting point can be determined by recording the percentage of light intensity (transmittance) passing through the substance in a capillary tube as a function of the measured furnace temperature.

The melting point process of a solid crystalline substance is divided into different stages: at the collapse point, the substance is mostly solid and contains only a small amount of molten material. At the bending point, most of the substance has melted, but some solid material is still present. At the clearing point, the substance has completely melted.

The choice of melting point machine:

The melting point instrument on the current market is a variety of types and brands, how to choose a suitable melting point instrument can refer to the following aspects:

1) The test conditions of the sample: Powdered samples can be used to capillary melting point determination based on the melting point instrument. According to the melting of the sample state is transparent, the sample initial melt, melting range and other specific indicators to determine whether the silicone oil method of visual melting point apparatus or automatic melting point apparatus. For flake or block samples can be used microscopic melting point machine.

2) Test accuracy, repeatability of test results and test range, are the choice of melting point machine reference basis. According to the actual demand, to meet the measurement range, the higher the accuracy and repeatability, the better.

Precautions for the use of melting point machine:

1. Melting point machine placed in a dry, dust-free, ventilated laboratory environment. Avoid excessive humidity as well as dust contamination, reducing the service life of the melting point instrument. The instrument should be kept away from acid, alkali and other corrosive liquids;

2. Capillary sampling process, pay attention to avoid capillary breakage, injury to hands. If the sample to be tested after the decomposition of toxic gas, the instrument should be placed in the ventilation kitchen test. When testing, avoid touching the heating part to prevent burns;

3. Melting point machine before use test test tube sleeve and furnace body assembly; check whether the light source output is good.