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.
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.
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 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.
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.
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.
|Methyl ethyl ketone||
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.
|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|