Drying solvents

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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.

General Techniques

Salting out

Main article: 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.

Anhydrous salts / Desiccants

Probably the most commonly used method for removing water from a solvent is by using the anhydrous form of a salt as desiccant. This process, which is useful for both polar and non-polar solvents, involves adding the salt (such as anhydrous magnesium sulfate, sodium sulfate, calcium sulfate, calcium chloride) directly to the solvent. Once the salt has hydrated, it can be removed by filtration or decantation, followed by distillation if necessary.

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.

Chemical drying

In this technique, an appropriate water-reactive substance (a drying agent) is added to the solvent. Usually, drying agent is chosen so that the products of reaction with water are easily removed, eg by filtration or distillation. Substances commonly used for this purpose include magnesium (for alcohols), calcium oxide, calcium hydride, phosphorus pentoxide, and sodium metal (sometimes in combination with benzophenone).

Sodium should only be used in very clean solvents with a low starting water content, otherwise the risk is run of fire or explosion.

Molecular sieves

Main article: Molecular sieve

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.

Azeotropic distillation

Certain solvents can be efficiently dried by azeotropic distillation, sometimes using a Dean-Stark trap.

Determination of water content

In simple (binary) mixtures of an organic solvent and water, it is often possible to determine the approximate water content by measuring the density and consulting an appropriate lookup table/graph. This is somewhat imprecise and normally only usable for mixtures containing >= 1% water. However, the drying techniques presented on this page can usually get solvents much drier than that. For reactions requiring totally anhydrous conditions, it is valuable to have a means of verifying the water content of solvents which have been dried (part-per-million levels of water).

One relatively-simple technique is to use benzophenone and sodium. Sodium can reduce benzophenone to produce the benzophenone ketyl radical, which is a strong blue colour. The ketyl is very reactive and is destroyed by traces of water or dissolved oxygen. Therefore, a persistent blue colouration indicates very low water content in the solvent (typically around 20 to 50ppm). This technique is not compatible with protic solvents (like alcohols) or halogenated solvents, as these will react directly with the sodium. It can be used with non-polar solvents (for example, toluene or hexane) and with polar aprotic solvents, such as diethyl ether.

Quantitation of water levels is more difficult, but possible, using Karl-Fischer titration. This is an iodometric technique based on the reduction of I2 by SO2, which can only take place in the presence of water. Modern KF titration normally uses expensive specialised automated KF titrators, however it is possible to perform KF titration manually using simple equipment.

Solvent-specific recommendations

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
Acetic acid
  • Anhydrous copper(II) sulfate
  • Phosphorous pentoxide
  • Fractionally distillation over 5% acetic anhydride and 2% CrO3.
Acetic anhydride
  • Drying with phosphorous pentoxide then fractionally distill over potassium carbonate
  • Sodium wire (some sources)
Acetone
Acetonitrile
  • Drying with calcium chloride, followed by either distillation or decantation; Calcium chloride can also be used to keep the solvent dry
  • Drying with anhydrous potassium carbonate, then 3A molecular sieves, followed by distillation
  • Drying with phosphorous pentoxide (5% w/v), followed by distillation (tip: avoid this method if using in reactions with acid sensitive substrates)
Aniline
  • Distillation over potassium hydroxide
  • Drying over calcium hydride, followed by distillation.
  • Drying with barium oxide, decantation or filtration, followed by distillation over fresh barium oxide.
Anisole
  • Drying with calcium chloride/sulfate, another drying with sodium wire, followed by distillation over fresh sodium wire
Benzene
  • Drying with anhydrous sodium sulfate, followed by either decantation or distillation.
  • Drying with phosphorous pentoxide, sodium, lithium aluminum hydride, calcium hydride, 4A molecular sieves, calcium sulfate, followed by decantation or filtration, usually with a fritted funnel
  • Drying with alumina, calcium hydride or 4A molecular sieves, followed by distillation.
  • Store over molecular sieves or sodium wire.
Butanol
  • Drying with magnesium sulfate, calcium oxide or potassium carbonate, followed by distillation over magnesium/iodine, sodium or calcium metal
  • Drying with 3A moloecular sieves (powder)
sec-Butanol
  • Drying over potassium carbonate or calcium sulfate, followed by fractional distillation.
  • Distillation over calcium oxide, then over magnesium.
  • Distillation over calcium hydride.
tert-Butanol
  • Drying with calcium hydride (5% w/v), followed by distillation.
  • Drying with calcium oxide, potassium carbonate, calcium sulfate or magnesium sulfate, followed by fractional distillation.
  • Distillation over magnesium/iodine or calcium, sodium, potassium metal.
  • Storage over 3A molecular sieves.
Carbon tetrachloride
  • Anhydrous calcium chloride, magnesium sulfate, 4A molecular sieves
  • Refluxing with phosphorus pentoxide, followed by distillation.
Chloroform
  • Distillation over anhydrous magnesium sulfate
  • Drying with phosphorus pentoxide followed by distillation.
Dichloromethane
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; molecular sieves alone can be used for a few days
  • Calcium hydride, calcium oxide, barium oxide, drierite, dry activated alumina can also be used to achieve near-dry DMSO, though further drying using molecular sieves and vacuum distillation is required to remove the last traces of water.
  • Storage in the presence of dry 4A molecular sieves to keep it dry[1]
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.
  • Phosphorus pentoxide
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.
Methyl ethyl ketone
  • Drying with calcium sulfate, magnesium sulfate, sodium sulfate or potassium carbonate.
  • Drying over boron trioxide
Nitromethane
  • Low alkaline 4A molecular sieves
  • Anhydrous neutral magnesium sulfate, sodium sulfate.
Pyridine
  • Refluxing with calcium hydride or oxide, followed by distillation
  • Activated 3A molecular sieves
  • Anhydrous magnesium sulfate, sodium sulfate, potassium carbonate, CaO, BaO, NaOH, KOH.
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.
Triethylamine
  • Addition of sodium metal, followed by distillation
  • Drying with calcium hydride followed by distillation.
Xylene
  • Drying with molecular sieves followed by distillation or decantation
  • Distillation over sodium metal.

Desiccant compatibility

While most desiccants can be used safely to dry most common solvents, some cannot be used as they will either react with the said solvent or dissolve in it.

Desiccant Compatible Incompatible Notes
Alkali metals Alkanes, arenes, ethers Acetone, alcohols, halogenated solvents, DMSO, nitromethane Reacts violently with halogenated solvents, less so with alcohols and DMSO; air sensitive
Alkali metal hydroxide Amines and pyridines Acids, base-sensitive solvents, nitromethane Reacts with acids releasing water
Alkaline earth metals Alcohols, alkanes, arenes, ethers Acetone, alcohols, halogenated solvents, DMSO Reacts violently with halogenated solvents, less so with alcohols and DMSO; Reaction with alcohols give their respective alkoxides, that can be regenerated back to alcohol by adding water; air sensitive (except magnesium)
Alkaline earth metal oxides Alcohols, alkanes, arenes, basic solvents, ethers, halogenated solvents Acetone, esters, dipolar aprotic solvents May react with alcohols in excess, will cause aldol condensation with ketones; not useful with dipolar aprotic solvents
Alumina Alcohols, alkanes, arenes, esters, ethers, halogenated solvents Acetone, acids Reaction with acetone and acids
Boron trioxide Acetone, acetonitrile, esters, ethers, halogenated solvents Alcohols, basic solvents Reacts with alcohols and basic solvents
Calcium chloride Alkyl and aryl halides, esters, ethers, halogenated solvents Acetone, acids, alcohols, aldehydes, amines, carbonyl compounds Reaction of CaCl2 with acetone forms an addition compound
Calcium hydride Alcohols, alkanes, amines, DMF, ethers, HMPA, pyridines Esters, acids, nitromethane Reacts with esters; air sensitive
Calcium sulfate Acetone, alcohols, aldehydes, halogenated solvents, ketones and pretty much all solvents Inert Drying may be strongly exothermic
Cement (Portland) Alcohols, alkanes, arenes Acids, esters, nitromethane Reacts with acids, esters; after hydration results in a very hard mass
LiAlH4 Alkanes, arenes Alcohols, esters, halogenated solvents, nitromethane Reaction with alcohols, esters, halogenated solvents
LiBH4 Alkanes Alcohols, halogenated solvents, esters, nitromethane Reaction with alcohols, halogenated solvents, esters
Lithium chloride Alkanes, arenes, halogenated solvents Butanol, propanol, methylformamide, hydrazine Dissolution in alcohols, methylformamide, hydrazine
Magnesium sulfate Acetone, alcohols, aldehydes, alkanes, arenes, esters, ethers, halogenated solvents, ketones, pretty much all solvents Inert May cause small traces of aldol condensation with acetone
Molecular sieves Alcohols, ethers Acetone, acids Will cause aldol condensation of acetone; reacts with acids
Phosphorus pentoxide Halogenated solvents Alcohols, amines, organic acids and carbonyl compounds; HMPA, DMSO, acetone Reaction; decomposition
Potassium carbonate Acetone, alcohols, aldehydes, halogenated solvents, various ketones, methyl ethyl ketone Acids Reaction with acids releases carbon dioxide and water
Silica gel Alkanes, arenes, esters, ethers Acetone May cause self-condensation with acetone
Sodium sulfate Acids, alcohols, esters, ethers, halogenated solvents Acetone, ketones Will cause some aldol condensation with ketones

References

Relevant Sciencemadness threads

  • http://www.gaylordchemical.com/wp-content/uploads/2015/07/GC-Literature-109B.pdf