Drying of liquids

The processes of synthesis and isolation of organic compounds generally require the availability of a solution of these compound in an organic solvent. For example, in the case of extractions with aqueous solutions of an organic layer, part of the water is transferred to the organic solution due to the partial miscibility of the organic layer and the water.

It may also happen that a reaction is carried out in aqueous solution and it is necessary to extract the product obtained with an organic solvent in order to isolate and characterize it adequately.

The usual procedure for drying liquids is to treat the organic solution with a desiccant. Desiccants are anhydrous inorganic salts that absorb water until they become hydrated. The most commonly used salts are listed in Table 1.

Table 1: Commonly used desiccants.
CompoundCapacityDrying speedDrying applications
Calcium chloride (CaCl2)HighMediumDrying of hydrocarbons
Calcium sulfate (Drierite®)  (CaSO4)LowFastGeneral purposes
Magnesium sulfate  (MgSO4)HighFastNot applicable to very sensitive acid compounds
Potassium carbonate (K2CO3)MediumMediumNot in acid compounds
Sodium sulfate (Na2SO4)HighSlowGeneral purposes

To dry a liquid, the desiccant is left in contact with the solution in an Erlenmeyer flask, and after a few minutes it is separated by gravity filtration.

Molecular sieves

They are synthetic materials that reproduce the structures of natural zeolites. They consist of aluminum silicates and alkali or alkaline earth metals. They have a high adsorption capacity for small molecules and show a certain selectivity depending on pore size (see Table 2).

Molecular sieves become "active" when water is removed from them as voids in their structure become available. The removal of water of hydration does not cause significant structural changes. Due to these properties they can be used as desiccants or to remove other small molecules that contaminate solvents.

When they no longer absorb any more molecules, they are said to be saturated. They can be regenerated to become active again by simultaneously subjecting them to strong heating and the passage of a gaseous flow of air or nitrogen.

Table 2: Molecular sieve types, selectivity and applications.
Pore size (Å)Does absorbDoes not absorbApplication (drying)
3NH3, H2OC2H6Polar liquids
4H2O, CO2, SO2, H2S, C2H4, C2H6, C3H6, EtOHC3H8 e hidrocarburos superioresNon-polar liquids and gases
5normal (linear) hydrocarbons up to n-C4H10, alcohols up to C4H9OH, mercaptans up to C4H9SHisocompounds or rings larger than C4
8branched and aromatic hydrocarbonsGases
10di-N-butylaminetri-N-butylamineHMPA

The steps to follow to dry a liquid are as follows:

  • The desiccant is added to an Erlenmeyer flask with the solution.
  • The Erlenmeyer flask is shaken by hand for a few seconds, covered and allowed to stand.
  • After a few minutes, the liquid is separated from the solid by gravity filtration.