What is the significance of melting point determination

A solid's melting point may be so reduced by impurity that it becomes a liquid at room temperature. This sort of behavior is not uncommon for solids whose melting points are only marginally higher than room temperature.

The temperature values are uncorrected. Lisa Nichols Butte Community College. Complete text is available online. Identification As a compound's melting point is a physical constant, it can be used to support the identity of an unknown solid. The melting temperature of a crystalline solid is thus an indicator for the stability of its lattice. At the melting point not only the aggregate state changes; quite a lot of other physical characteristics also change significantly.

Amongst these are the thermodynamic values, specific heat capacity, enthalpy, and rheological properties such as volume or viscosity. Last but not least, the optical properties birefringence reflection and light transmission change. Compared to other physical values the change in light transmission can easily be determined and can therefore be used for melting point detection.

Melting points are often used to characterize organic and inorganic crystalline compounds and to ascertain their purity. Pure substances melt at a sharp, highly-defined temperature very small temperature range of 0.

The temperature at which all material of a contaminated substance is molten is usually lower than that of a pure substance. This behavior is known as melting point depression and can be used to obtain qualitative information about the purity of a substance. In general, melting point determination is used in the lab in research and development as well as in quality control in various industry segments to identify and check the purity of different substances.

At the melting point, there is a change in light transmission. Powdered crystalline materials are opaque in the crystalline state and transparent in the liquid state. This distinct difference in optical properties can be measured in order to determine the melting point by recording the percentage of light intensity shining through the substance in the capillary, the transmittance, in relation to the measured furnace temperature.

There are different stages of the melting point process of a solid crystalline substance: At the collapse point, the substance is mostly solid and comprises only a small amount of molten material. At the meniscus point, most of the substance has melted but some solid material is still present. At the clear point, the substance has completely melted.

The melting point measurement is usually performed in thin glass capillary tubes with an internal diameter of 1 mm and a wall thickness of 0. A finely-ground sample is placed in the capillary tube to a filling level of 2 — 3 mm and introduced in a heated stand liquid bath or metal block in close proximity to a high accuracy thermometer.

The temperature in the heating stand is ramped at a user-programmable fixed rate. The melting process is visually inspected to determine the melting point of the sample.

The capillary method is required in many local pharmacopeias as the standard technique for melting point determination. The pharmacopeia's requirements for melting point determination include both minimum requirements for the design of the melting point apparatus and for performing the measurement. For sample preparation, a dry powdery substance is ground in a mortar and filled into the capillaries, which are then inserted into the furnace.

The melting point accessories box contains sets of melting point capillaries, an agate pestle and mortar, tweezers, a spatula b , and 5 capillary filling tools a. Step 1: First, the sample needs to be dried in a desiccator. Then a small portion of sample is finely ground in a mortar. The capillary filling tool perfectly assists the filling as the empty capillaries are securely held in a peg-like grip.

Collecting a small sample portion from a mortar is easily done with the assistance of the tool. Step 3: The small amount of sample at the top of the capillaries is then moved down the capillary by releasing the grip and gently bouncing the capillaries on the table several times.

This action packs the sample tightly down into the bottom of the capillary. The 'bouncing effect' causes tight packing of the substance and avoids the inclusion of air pockets. Step 4: The correct filling height can be checked with the engraved ruler on the capillary filling tool. Generally the filled height should not exceed 3 mm. Along with proper sample preparation, the settings on the instrument are as well essential for the exact determination of the melting point.

Correct selection of the start temperature, the end stop temperature and the heating ramp rate are necessary to prevent inaccuracies due to a heat increase in the sample that is incorrect of too fast:. Melting point determination starts at a predefined temperature close to the expected melting point. Up to the start temperature, the heating stand is rapidly preheated. At the start temperature the capillaries are introduced into the furnace, and the temperature starts to rise at the defined heating ramp rate.

The heating ramp rate is the fixed rate of temperature rise between the start and stop temperatures for the heating ramp. Results depend strongly on the heating rate - the higher the heating rate the higher the observed melting point temperature. For highest accuracy and non-decomposing samples use 0. You then need to get the powder all the way to the bottom of the capillary tube by tapping the closed end multiple times on a hard surface so that the sample is compacted down at the closed end of the tube you could also use a method where you drop the tube multiple times through a glass tube approx.

For further accuracy, you can use a packing wire to further compact the sample. Note that the sample height should be between 2 and 3mm to ensure best results. Always use the same batch of capillary tubes for routine measurements. Not all capillary tubes are equal and using the same batch will ensure repeatable results. Always clean the tube before running the melting point experiment. Failure to do so can result in a low melting point or a wide melting range.

Measuring Melting Point Lab Equipment. Posted by westlabblogcanada Jan 02 0 Comments. Measuring Melting Point The determination of a melting point of a sample is a standard laboratory procedure and is relatively straightforward. If a chemical decomposes before reaching its melting point, decomposition temperature shall be provided instead.

You can often find it in the section 9 of a safety data sheet SDS. You probably do not want to store or transport a solid at a temperature close to or above its melting point in which case melting may cause leaking and severe consequences.

In addition to that, melting point is often used to predict the partition behavior of a chemical between solid and gas phases.