Why melting range

For example the melting point can help qualitatively determine a substances purity. A pure non-ionic, crystalline organic compound should have a narrow melting point range, approximately 0. However a substance which is not pure will have a larger range in comparison and will also likely melt at a lower temperature. A quantitative comparison between a sample and literature value can also be used to identify a compound as pure compounds have characteristic melting point ranges. However it helps to have some idea as to what the compound might be!

The common method for taking the melting point of a solid organic compound is to take a small amount of the compound in a capillary tube and place it in a measuring apparatus. This apparatus gradually heats the compound using a heating bath from a set temperature. There is a lens to view the capillary in the heating bath to allow the scientist to determine when the solid has melted and thus determine the melting point.

It is good practise to record the range the compound has melted — i. Then to carry out at least two further determinations through heating more slowly until two consistent ranges have been obtained. At the melting point, the solid and liquid phase exist in equilibrium. Thus the melting point depends on pressure and usually reported at standard pressure. The apparatus consists of four test tube slots in which substances can be tested as well as a screen where individuals can adjust key criteria with respect to their substance.

First of all, you would take a sample of your solid out of your sample bag or from an unknown sample that has been given to you by your TA, and using the capillary tube, obtain a small quantity of the fine powder by gently rolling the tube through the substance.

If your product particles are too large to fit into the capillary tube, place it in a mortar and crush it into a fine powder. Secondly, to lower the substance to the bottom of the tube, either tap the closed end of the capillary tube vigorously on the bench top, or drop the tube into the long clear plastic tube and allow it to bounce on the bench top.

Ensure that there is approximately 3mm of sample within the tube. Then, press the method icon on the MP50 machine as has been instructed by your TA. Subsequently, insert the capillary tube into the sample holder of the machine. The thermodynamic factor is an empirically determined instrument-specific factor. The thermodynamic melting point is the physically correct melting point.

This value does not depend on heating rate or other parameters. This is a very useful value as it allows melting points of different substances to be compared independently of experimental setup.

Before the unit is put into operation, it is recommended to verify its measurement accuracy. In order to check the temperature accuracy, the instrument is calibrated using melting point standards with exact certified melting points. Thus, the nominal values including tolerances can be compared with actual measured values.

If calibration fails, which means if the measured temperature values do not match the range of the certified nominal values of the respective reference substances, the instrument needs to be adjusted. In order to ensure measurement accuracy it is recommended that the furnace is calibrated with certified reference substances on a regular basis, for example once a month. A three-point calibration with benzophenone, benzoic acid and caffeine is performed, followed by an adjustment.

The adjustment is then verified by calibration with vanillin and potassium nitrate. The reason is that the melting point temperature is not measured directly within the substance, but outside the capillary at the heating block, due to technical reasons.

Therefore, the temperature of the sample lags behind the furnace temperature. The higher the heating rate, the more rapid the rise in oven temperature, increasing the difference between the melting point measured and the true melting temperature. Due to the dependence of the rate of heat increase, measurements taken for melting points are comparable with one another only if they are taken using the same rates. The red solid line represents the temperature of the sample see figure below.

At the beginning of the melting process, both sample and furnace temperatures are identical; the furnace and sample temperatures are thermally equilibrated beforehand. The sample temperature rises proportionally to the furnace temperature. We have to bear in mind that the sample temperature increases with a short delay which is caused by the time needed for heat transmission from the furnace to the sample.

While heating up, the furnace temperature is always higher than the sample temperature. At a certain point the furnace heat melts the sample inside the capillary. The sample temperature remains constant until the whole sample is molten. We identify different furnace temperature values T A and T C which are defined by the respective melting process stages: collapse point and clear point.

The sample temperature inside the capillary rises significantly once the sample is completely molten. It increases parallel to the furnace temperature showing a similar delay as in the beginning. The thermodynamic melting point is the physically correct melting point see figure below. Melting point depression is the phenomenon of reduction of the melting point of a contaminated, impure material compared to the pure material. The reason is that contaminations weaken the lattice forces within a solid crystalline sample.

In conclusion, less energy is needed to break the forces of attraction and to destroy the crystalline structure. The melting point is therefore a useful indicator of purity as there is a general lowering and broadening of the melting range as impurities increase. If two substances melt at the same temperature, a mixed melting point determination can reveal if they are one and the same substance.

The fusion temperature of a mixture of two components is usually lower than that of either pure component. This behavior is known as melting point depression.

For mixed melting point determination, the sample is mixed with a reference substance in a ration. Whenever the melting point of the sample is depressed by mixing with a reference substance, the two substances cannot be identical. If the melting point of the mixture does not drop, the sample is identical to the reference substance that was added. Commonly, three melting points are determined: sample, reference and mixing ratio of sample and reference.

The mixed melting point technique is an important reason why all high-quality melting point machines accommodate at least three capillaries in their heating blocks.

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