Salting out

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

Soap

The oldest historical application is the salting out of glycerine with sodium chloride in soap production . The glycerine forms a new phase that can easily be separated from the soap core.

Salts

The added salt must be more soluble than the one to be precipitated. An example would be the formation of sodium benzenesulfonate from benzenesulfonic acid and sodium chloride. The purpose is to separate and purify the benzenesulfonic acid from the synthesis mixture.

Proteins

The most common salts for salting out proteins are ammonium sulphate in the course of ammonium sulphate precipitation and potassium phosphate (see also: Hofmeister series ), as these strongly dissociated compounds result in highly charged ions that build up a large hydrate shell. By dissolving the salt, the ammonium and sulfate ions bind a large number of water molecules. The more salt is dissolved, the fewer water molecules are available to interact with the proteins. When the salt concentration increases, different proteins can no longer be kept in solution at a certain point and form a precipitate. Salting out is therefore partly equivalent to dehydration .

Since proteins differ in their solubility properties, they can be separated from one another using this method (salt fractionation). However, certain proteins can easily be denatured if salt is added too slowly or too quickly . In addition to the amount of salt, the speed of dehydration also plays a decisive role. In practice, the salt is often added in dry form and in small quantities. Only after the salt has completely dissolved is a new portion of salt added.

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

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

Organic compounds

Another application in organic chemistry is adding salt when shaking out in a separating funnel . In cases of poor separation of the organic and aqueous phases, this leads to better separation.

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Beyer, Walter: Textbook of Organic Chemistry , S. Hirzel Verlag Stuttgart, 22nd edition (1991)