Melting point apparatus

A: Stuart supplies three types of capillary tube for melting point determination. SMP10/1 is possibly the most convenient as it is sealed at one end and ready to use. SMP1/4 needs to be sealed at one end in a Bunsen flame before use and SMP2/1 is sealed at both ends and can therefore be snapped in half to produce two tubes. However with the SMP30 it is necessary to use full length capillaries due to the depth of the heating chamber.

Q: How do I prepare my sample for melting point determination?

A: This is explained in detail in Protocol P07-003A: Preparation of samples for melting point determination. Briefly, the sample should be dry and ground to a fine powder. Moist and coarse samples will not give accurate or reproducible melting point determinations.

Q: How much sample should I put in the capillary tube?

A: As a guide, around 2 to 4mm in height. For efficient heat transfer, ensure the sample is well packed in the bottom of the tube by tapping on a hard surface. Prepare all samples to the same height for reproducibility. Too much sample will take more heat to melt and could show a wider melting range than smaller samples.

Q: Which capillary tubes should I use with my melting point apparatus?

Q: What ramp rate should I use?

A: Generally 1-2°C/minute is recommended. Heat the sample to a plateau at around 5-10°C below the expected melting temperature then apply a slow ramp rate until the melt is complete. If the melting temperature is unknown, first perform a rapid melt with a ramp of up to 20°C/minute. Once the approximate melting point is known, a more appropriate plateau can be set and a slower ramp rate used.

Q: What are the stages of melting?

A: The melting of a substance is defined by three stages. Firstly, the start or onset of the melt is defined as the temperature at which the substance is observed to collapse definitely against the side of the tube. This represents the low temperature of the melting point range. Secondly, during the melt itself, the meniscus point can be observed. This is the point where a definite meniscus is visible together with equal volumes of solid and liquid in the capillary. Finally there is the clear point at which the substance becomes completely liquid. This is the high temperature of the melting point range. Pure substances usually melt in a range of two degrees or less. If a sample is impure, its melting point will be lower or wider than the expected published value.

Q: What is a mixed melting point?

A: Impurities in a substance will lead to a depression in melting point. This phenomenon can be used therefore to determine whether two substances are identical. The two substances (one reference and the other unknown) are ground together in equal proportions and the melting point determined. If they are the same substance then the melting point will be the same as the published value of the reference. If the two substances are not identical, then the melting point of the reference will be depressed. Where three capillaries can be viewed at the same time, it is possible to view the melting of the unknown in one tube, the reference in the second and the mixture in the third.

Q: How does the SMP40 automatically detect a melt?

A: The SMP40 measures the intensity of the sample image. At the start of the run, when the sample is solid, the intensity is high. As the melt progresses the intensity decreases until the sample becomes clear. During the melt the status of the sample is displayed on the screen: solid: no changes have been detected in the sample's physical form; melting: the sample is showing the first signs of a change in its physical form; liquid: the change in the sample's physical form is complete; finished: once a capillary tube displays "finished" the melt determination is complete. The end point will automatically be detected, however the user can also indicate the other important points in the process manually, either during the run or afterwards when reviewing the video.

Q: What is the memory capacity of the SMP30 and SMP40? How many results can be saved?

A: The SMP30 will only store the results of the previous run. This will be over-written when the next run is started. The SMP40 can save up to around 200 results files on the unit itself but it is also possible to transfer them to a USB memory stick for transfer to a PC for viewing.

Q: Does the SMP40 have PC software?

A: The SMP40 is a standalone automatic melting point instrument. It cannot be connected directly to a printer or PC and there is no PC software. The results files (including videos) can, however, be transferred from the SMP40 to a PC using a USB memory stick where they can be stored, viewed and printed. Details are given in P07-002A: SMP40 results file transfer which can be found in Melting point application notes.

Q: Can I measure boiling points in Stuart melting point apparatus?

A: Different techniques are required to measure boiling points. We have not validated any of the Stuart models for measuring boiling point.

Q: Can I measure the melting point of fats and waxes in Stuart melting point apparatus?

A: Unfortunately none of our melting point equipment is designed for measuring the melting point of fats, waxes and oils; they are designed for the measurement of crystalline solids which have a narrow, defined melting point. There are special methods for fats which use a different technique such as the drop point. Another reason for the unsuitability is the difficulty in introducing a sample into the bottom of the capillary tube. Usually the sample is dried, ground into a fine powder and introduced into the tube at the open end and tapped down into the sealed end. Obviously it is not possible to do this with fats and waxes.

Q: Can I measure the melting point of polymers in Stuart melting point apparatus?

A: The melting of polymers is not the same as that of normal crystalline compounds; there is not a solid to liquid transition but a crystalline to solid amorphous phase which is called the crystalline melting temperature. We have not validated any of the Stuart models for this type of application.

Q: Do you have IQ/OQ documentation for the melting point apparatus?

A: Yes, we have this for the SMP40. The documentation should be ordered together with the unit using part code SMP40/IQOQ; it cannot be purchased retrospectively.

Q: How do I check and calibrate my melting point apparatus?

A: Stuart recommends that the performance of the melting point apparatus is checked at regular intervals using certified melting point standard/s or a calibrated temperature probe. Certified melting point standards can be purchased from the larger chemical supply companies (such as Sigma, Thermo Fisher, VWR etc.). If the instrument needs to be recalibrated, the procedure to do this depends upon the model of instrument. Both the SMP30 and SMP40 can be calibrated by the user using certified melting point standards. Instructions for the SMP30 can be found at SMP30 Calibration procedure and instructions for the SMP40 are given in the user manual. For other models please contact our Service Department.