Drying solvents
Having properly functioning solvents is an absolute necessity for fine home chemistry work, especially for those interested in organic chemistry. Removal of water and other impurities from these which is often required for store-bought or homemade solvents, can be accomplished by many means, depending on the solvent.
Contents
Drying with an anhydrous salt
Probably the most commonly used method for removing water from a solvent is by using the anhydrous form of a salt as desiccant. In this process, which is useful for both polar and nonpolar solvents, involves adding the salt (such as anhydrous magnesium sulfate, sodium sulfate, calcium sulfate, calcium chloride or compounds that react with water forming precipitates such as calcium oxide, calcium hydride) directly to the solvent, followed by one or more careful distillations to avoid drawing water out of the hydrated salt. If water is present, a finely powdered anhydrous salt tends to "clump up" upon absorbing the water. Desiccants used for this process typically need to be checked beforehand to ensure that they will not react with the solvent in any way. Though it isn't a salt, sodium metal is often used in the form of shavings in the same manner with nonpolar organic solvents due to its extreme reactivity towards water. Sodium should only be used in very clean solvents with a LOW water content, otherwise the risk is run of explosions and fire.
Salting out
Salting out is a technique that takes advantage of one solvent's reduced solubility in a solution of some compound relative to its solubility in pure water. This is mostly used for polar solvents that are miscible or highly soluble in water, especially when normal distillation produces an azeotrope. This technique is often used prior to distilling with a salt, as it cannot remove all of the water, but is a convenient way to remove most of it without having to use any anhydrous compounds, and it is one of the only ways to break an azeotrope outside of vacuum distillation. Examples of salting out include the separation of ethanol and water using potassium carbonate, the removal of isopropanol from water using sodium chloride, sodium hydroxide, or a mix of the two.
Molecular sieves
Molecular sieves are precise tools for the removal of water or other liquid components of a mixture. Using precisely sized pores in a material such as silica, clay, or alumina, they selectively trap molecules of a certain size by adsorption. While they may not be particularly cheap and can be difficult to re-dry after their use, molecular sieves have the advantage of being able to remove a significant amount of water from a solvent without introducing any of their own impurities. A disadvantage of molecular sieves, however, is the long amount of time they must be given to complete the water-removal process, often in excess of 24 hours.
Solvents and their principal method(s) of water removal
This section includes a commonly used method for drying a solvent that is sufficient enough for that solvent's typical use, either for solvation or as a reagent. This is not an exhaustive list by any means, and other methods can most likely be used.
| Solvent | Method |
|---|---|
| Acetone | Salting out with sodium acetate, simple distillation to remove it from water followed by distillation over anhydrous calcium sulfate or magnesium sulfate. Boron trioxide can also be used. |
| Acetonitrile | Drying with calcium chloride, followed by either distillation or decantation; Calcium chloride can also be used to keep the solvent dry. |
| Benzene | Drying with anhydrous sodium sulfate, followed by either decantation or distillation. |
| Chloroform | Distillation over anhydrous magnesium sulfate or phosphorus pentoxide. |
| Dichloromethane | Drying with anhydrous sodium sulfate or calcium hydride, followed by distillation; Phosphorus pentoxide can also be used. |
| Diethyl ether | Dried using anhydrous calcium chloride, molecular sieves, followed by distillation; for higher purity, sodium metal and benzophenone are used. |
| Dimethyl sulfoxide | Distillation in partial vacuum over molecular sieves. Calcium hydride can also be used, though it's somewhat less effective. |
| Dimethylformamide | Drying with barium oxide or molecular sieves, followed by partial vacuum distillation. |
| Ethanol | Distillation of the 95% azeotrope followed by salting out with anhydrous potassium carbonate and a second distillation over anhydrous calcium or magnesium sulfate. Direct drying can be done with magnesium metal, followed by distillation or filtration. |
| Ethyl acetate | Drying with anhydrous sodium sulfate, ethanol removal can be facilitated by salting out using potassium carbonate, followed by distillation of the ethyl acetate layer over anhydrous magnesium sulfate. |
| Isopropanol | Salting out to 91% using sodium chloride, followed by salting out to 100% using sodium hydroxide. Once sodium hydroxide has been added and the two layers separated, the entire mixture should be cooled as low as possible to precipitate/freeze sodium hydroxide, which will hurt glassware in a distillation. The isopropanol layer can be poured off the frozen sodium hydroxide solution and distilled over anhydrous magnesium sulfate, preferably with a small amount of sulfuric acid present to mitigate glassware damage from hot sodium hydroxide. |
| Methanol | Salting out with potassium carbonate or simple distillation to remove from water with or without a desiccant. Direct drying can be done with magnesium metal turnings, followed by distillation or filtration. |
| Tetrahydrofuran | Distillation over molecular sieves; Adding metallic potassium under inert conditions for complete removal of water, followed by distillation. |
| Toluene | Adding sodium metal with benzophenone, followed by distillation; azeotropic distillation with benzene, molecular sieves. |
| Xylene | Drying with molecular sieves followed by distillation or decantation, or distillation over sodium metal. |
Incompatible desiccants with solvents
There are many desiccants that cannot be used to dry many solvents as they will either react with the said solvent or dissolve in it.
| Desiccant | Solvent | Effect |
|---|---|---|
| Alkali metals | Alcohols | Reaction |
| Alkali metals | Halogenated solvents | Reaction |
| LiAlH4 | Alcohols | Reaction |
| LiAlH4 | Halogenated solvents | Reaction |
| LiCl | butanol, propanol, methylformamide, hydrazine | Dissolution |
| Phosphorus pentoxide | Alcohols, amines, organic acids and carbonyl compounds; HMPA, DMSO, acetone | Reaction; decomposition |
References
- http://ccc.chem.pitt.edu/wipf/Web/Solvent_Drying.pdf
- http://www.chem.ucla.edu/~bacher/General/30BL/tips/dryingofsolvents.html
- http://chemwiki.ucdavis.edu/Reference/Lab_Techniques/Distillation/Drying_Solvents
