熔沸点的测定.ppt

Melting Point and Boiling Point
1. Purpose
Learn the knowledge of melting point and boiling point.
Grasp the methods to test melting point and boiling point with Capillary Tubes.
Grasp the usage of Digital melting-point apparatus.
2. Principle
Melting Point
Definition: The melting point of a substance is defined as the temperature at which the liquid and solid phases exist in equilibrium with one another .
Figure1. Phase change with time and temperature
Sharp increase after melting, so heat slowly
If heat is added to a mixture of the solid and liquid phases of a pure substance at the melting point, ideally no rise in temperature will occur until all the solid has melted and been converted to liquid.
The melting point and freezing point of a pure substance are identical.
A pound have a settled melting point, usually lower than 350℃, with a temperature range of ~1 ℃.
Figure2. The effects of impurities on melting point
According to Raoult's law, at a certain pressure and temperature conditions, if we put solute into a solvent, the partial pressure of solvent vapor will obviously decreased. The solid and liquid phases intersect at points M1, which represents the boiling point pounds containing impurities are lower than that of pounds.
当含杂质时(假定两者不形成固熔体),根据拉乌耳定律可知,在一定的压力和温度条件下,在溶剂中增加溶质,导致溶剂蒸气分压降低,固液两相交点M´即代表含有杂质化合物达到熔点时的固液相平衡共存点,T1为含杂质时的熔点,显然,此时的熔点较纯粹者低,熔点距扩大。
因此,熔点是判断晶体化合物纯度的重要指标之一。
Melting point methods and apparatus
(1) Capillary tubes and sample preparation
Commercially available capillary tubes have one end already sealed and the other opened end to permit introduction of the sample(2-3mm)。
熔点管制备:将毛细管一端用酒精灯烧熔封口,呈45°角,不停旋转。
注意事项:
样品需研细,可填装结实,有空隙则不易传热,影响结果。
样品装实后2~3mm。
样品装填需迅速,防治吸潮。
Samples Preparations
表面皿
桌面
玻璃管
样品(2~3 mm)
样品
样品管
样品只可用一次
(2) Thiele tube apparatus
b形管
石蜡油浴
橡皮圈
熔点管
样品
温度计
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Proper use of the Thiele tube is critical to obtain reliable melting points。
Heating rate. Firstly, the hea