steam tap is the lever beneath the reagent shelf, the steam outlet is the jet pointing out the side of the reagent shelf. This jet is aimed toward you. NEVER DISTIL TO DRYNESS. Many organic solvents react slowly with air to form peroxides which may explode violently if overheated. (Ethers are particularly notorious in this regard.) As long as a few drops of solvent remain and the glass is “wet”, it is near the boiling point of the solvent. When it is dry, however, some heat sources can quickly raise the temperature several hundred degrees and any high boiling liquids present are similarly overheated. The use of steam as a heat source largely avoids this problem. ALWAYS USE A BOILING CHIP. Liquids tend to superheat, that is to reach a temperature above the boiling point without boiling. ( A liquid will evaporate from the surface at any temperature. It is boiling when it is being converted to the vapour phase throughout its volume.) When a bubble of vapour forms in such a superheated liquid, its growth rate may be almost explosive. The liquid "bumps" and if it is flammable and an ignition source is near by, a fire can result. A boiling chip or stone has micropores which, especially on heating, emit tiny air bubbles which serve as nucleation centres and prevent superheating and, hence, bumping. Thermometer placement. TECHNIQUE In order to record the true boiling or vapour temperature, the thermometer must be correctly placed. If it is too far down, it may be in vapours of higher temperature than those being collected. If it is placed too high, it will be above the vapours. Either way, the temperature recorded will be incorrect. Generally, the thermometer is placed so that the middle of the mercury bulb is even with the bottom of the side-arm. Distillation rate. - 2 - In theory, the boiling point is independent of heat input but in practice, overheating of liquid and vapour occur if the heat input is excessive. Consequently, a slow, steady distillation rate should be maintained. This rate can vary from a few drops per minute to several millilitres per minute, depending on the degree of separation desired. Slower distillation normally gives better separation. If the thermometer bulb and the distillation head are not always
- 3 - bathed in condensed and returning material, the temperature may fluctuate. On the other hand do not overheat the system.
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