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Think Melting Points
Sample Preparation Visual Observations
Preparing the Sample Onset Point
Careless sample preparation is the leading cause of The onset point, also called the collapse point, is considered
inaccurate and irreproducible results. Substances must the start of the melt. Liquid clearly appears for the first time
be fully dry, homogeneous and in fine powdered as a separate phase in coexistence with the crystals. The
form. Moist samples must be dried first. Coarse crystalline sample has changed appearance and collapsed away from
and non-homogeneous samples must be crushed into a the walls. It must not be confused with the "sintering point"
fine powder in a mortar. which corresponds to just isolated drops due to a few surface
crystals melted (often on the capillary walls).
Filling the Capillary Meniscus Point
Press the open end of the capillary into The meniscus point is when a liquid meniscus becomes
the substance several times until the clearly visible. There is solid phase at the bottom and a clear
capillary holds 2-3 mm of sample. For liquid phase on top with a complete and well defined
consistent results, it is very important meniscus. The meniscus point is often listed in European
that the capillary is not over or under melting point tables and is the preferred value of British
filled. Pharmacopeia.
Packing the Sample Clear Point
Push the powder to the closed bottom The clear point is when the substance becomes completely
of the tube by repeatedly tapping the liquid and there is no solid remaining. The clear point is more
bottom of the tube against a hard dependent on the ramp rate than the onset point. The clear
surface. More sample can be added if point is the temperature most often listed in U.S. based
the level is too low. Tight packing and melting point tables. When a single temperature is listed,
a fixed level of sample are very it is usually the clear point.
important for optimum results.
Ramp Rate Mixtures
Thermodynamic Melting Point Melting Point Depression
The temperature obtained for the clear point is dependent Mixtures of substances, whose components are insoluble in each
on the ramp rate. This is because the transition from solid to other in the liquid phase, display a melting point depression and,
liquid does not take place instantaneously, it requires a finite amount instead of a sharp melting point, a melting range. The size of the
of time. This time depends on the (1) heat of fusion of the sample, (2) the melting point depression depends on the composition of the mixture.
thermal conductivity of the sample, (3) the thermal conductivity of the glass
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