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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 Generally, a 1% impurity results in a 0.5
