Drying

Remember the ethyl alcohol — water azeotrope? You might be thinking: If I cannot distill the water out and I want my alcohol anhydrous (dry), because the water will kill my yield, what should I do? You need to dry. Sometimes you will have to dry reagents, sometimes solvents, and sometimes the products themselves.

Baths. Baths can dry many solid substances that do not decompose under heat. Some substances can take more heat than others so a thermometer must be used along with the knowledge of how much heat can be safely used without destroying the product, or changing it into a different substance. The types of baths are many: water, air, toluene, sand, oil, and graphite, but they all have the same general rules. Hot plates and heating mantles must follow these rules also.

1. Always protect the substance you are drying from the water in the atmosphere by fitting a drying tube into the glassware that is holding your substance. The drying tube should be filled with a suitable drying agent.

2. If using a liquid, never allow it to boil.

3. Never use excessive heat for drying. I have heard of nitro propene burning faster than gunpowder due to excessive heat. Personally, I feel this could have been caused by a nearby pilot light that was left burning.

Solids can also be dried at room temperature on filter paper or porous tile. You should protect the substance from dirt and dust by covering with filter paper or a funnel. A vacuum desiccator will greatly speed up the drying process, and should be used on products that are destroyed by the small amount of water in the atmosphere. A vacuum desiccator is shown in Figure 4.

Drying of Liquids. Liquids are usually dried by filtering through or mixing with a solid dehydrating agent. The most common solid drying agents are: calcium chloride, sodium hydroxide, caustic potash, anhydrous sodium sulphate, anhydrous potassium carbonate, anhydrous cupric sulphate, phosphorus pentoxide, and metallic sodium. Now for the bad news, it is essential that the drying agent have no action on the liquid or any substance that may be in the liquid. Great care should be used in the choice of a drying agent, and much research may be required. If you do not find the necessary information call a chemist or some one who knows. I will mention a few rules.

1. Never use calcium chloride to dry alcohols or amines.

2. Never use caustic potash or caustic soda to dry acids, phenols, esters, certain halides, etc.

3. Always use a very small amount of drying agent, otherwise you will lose product by excessive absorption. It is better to use several small amounts than to use one large excessive amount. A useful agent called Blue Drierite can be mixed with the cheaper White Drierite and visually inspected to determine if its absorbing powers are used up. Blue Drierite turns pink when it has no more absorbent power. If you use Blue Drierite directly, you take a chance of contaminating your product with a cobalt, as it was made for use in drying tubes (see Figure 8).

4. To dry a moist solid it is often convenient to dissolve it in ether and dry this ethereal solution with the proper drying agent. Evaporate to retrieve the solid.

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