Salting out

Salting out is a process in which water-soluble substances can be displaced by adding a salt from the aqueous phase. This can be done by precipitation ( to form a solid phase) or by formation of a liquid organic phase. The basic effect is to increase the ionic strength of the aqueous phase and the consequent reduction in the solubility of many of covalent or ionic compounds.

Soap

The oldest historical application is the salting of glycerol with sodium chloride in soap production. The glycerin forms a new phase, which can be easily separated from the core soap.

Salts

The added salt must in this case be more readily soluble than the be precipitated. An example would be the formation of sodium benzene from benzene and sodium chloride. The purpose is the removal and cleaning of the benzene from the synthesis mixture.

Proteins

The most commonly used salts for salting out of proteins are ammonium sulfate and potassium phosphate ( see also: Hofmeister series ), as these strongly dissociated compounds highly charged ions arise that make up a large hydration shell. By dissolving the salt, the ammonium and sulfate ions bind a large number of water molecules. The more salt is dissolved, the less water molecules available for the interaction with the proteins. With increasing salt concentration, different proteins can no longer be held in solution and form a precipitate after a certain point. The salting corresponds partly to dehydration.

Because different proteins in their solution properties, they can be separated by this method (salt fractionation). However, certain proteins can be easily denatured at too slow or too rapid addition of salt. In addition to the amount of salt that is the speed of dehydration plays an important role. In practice, the salt is often added in a dry form and in small quantities. Only after the salt has completely dissolved, a new salt portion is added.

For small sample volumes, the salt is added in form of a saturated ammonium sulfate solution was added dropwise. However, it should be noted that the volume of the sample solution increases significantly. At a 50 % saturation, the sample volume is doubled.

The precipitated proteins can be isolated by centrifugation. The added salt may be removed by dialysis again downstream.

Organic compounds

Another application in organic chemistry is the addition of salt during shaking in a separatory funnel. This leads in the case of a bad separation of the organic and aqueous phase in a better separation.

Credentials

• Beyer, Walter: Textbook of organic chemistry, S. Hirzel Verlag, Stuttgart, 22 edition (1991 )

• Separation processes