Melting Point Instruments​

The determination of the melting point can be performed for powdered crystalline samples. The sample being investigated must be fully dry, homogeneous, and in powdered form. Moist samples must be dried first. Coarse crystalline samples and non-homogeneous samples need to be finely ground in a mortar.

The key to good results is careful and exact sample preparation. Prepare your samples so that the amount of substance is the same across all capillaries. Slight differences in the amounts could cause deviations in the measured melting point temperatures.

Check the capillary fill height using the lines marked on the melting point sample preparation tool. For accurate measurements, the optimum fill height of 3 mm should be observed.

The sample preparation tool offers many possibilities for checking the fill height of your substance. See Fig. 1 for an example of estimating various fill heights. For best results, it is important that the melted substance covers the holes for transmission light detection.

Pharmacopeia requirements for melting point determination at a glance include: 

Use capillaries with outer diameters ranging from 1.3–1.8 mm and wall thicknesses from 0.1– 0.2 mm. Apply a constant heating rate of 1°C/min. If not otherwise stated, the temperature is recorded point C when no solid substance is left (which corresponds to the clear point). Pharmacopeias such as the United States Pharmacopeia (USP) require the determination of the melting range, where points A (corresponds to the collapse point) and C are used for temperature determination. The recorded temperature represents the temperature of the heating stand, which can be an oil bath or a metal block, in which the thermocouple is positioned.

Learn more about how to achieve compliance with local and international pharmacopeias for melting point determination

If you want to make sure that the melting point analyzer is providing the correct results, you need to verify its measurement accuracy. As it is not possible to measure the sample temperature directly using a certified thermometer, the temperature accuracy is checked using reference substances, ideally with certified temperature values. Thus, we can compare nominal values including tolerances with actual measured values.

If calibration fails, which means 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.

The instrument should be adjusted with at least two reference substances that encompass the whole melting range required. The calibration should be performed with at least one reference substance melting point lying within the required temperature range. The new adjustment should be checked with a different reference substance than the one used for adjustment.

We highly recommend that you use METTLER TOLEDO melting point standards for calibration and adjustment purposes of METTLER TOLEDO melting point instruments. Each melting point reference substance comes with a certificate and both nominal pharmacopeia and thermodynamic melting points written on the label. The substances are securely identified with two barcodes showing the filling code and lot number.

Furthermore, METTLER TOLEDO offers a melting point performance verification package, called the MP VPac™, which contains prefilled and sealed capillaries for use in instrument verification.

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 by METTLER TOLEDO contains everything needed for accurate and reproducible sample preparation.

The melting point accessory box is part of the standard delivery of the MP90 and is a highly recommended optional accessory for MP55, MP70, and MP80 instruments.

The melting point accessory boxes are available with standard or USP reference substances. The melting point accessories box contains two sets of 150 melting point capillaries each, three METTLER TOLEDO melting point reference substances or USP melting point reference standards, an agate pestle and mortar, tweezers, a spatula, and 5 capillary filling tools.