Saltingout

Salting out is a technique used to decrease the solubility of a solute in an aqueous solution by adding salt, causing the solute to precipitate. It is commonly applied to proteins and some polymers, allowing their separation from other components in a mixture.

The mechanism involves increasing the ionic strength of the solution, which reduces the activity of water and weakens the solvation layer around solute molecules. Ions compete with solutes for water molecules, promoting protein–protein interactions and leading to aggregation and precipitation. The effect depends on the type of salt, the concentration, temperature, and pH. Salts that strongly structure water, known as kosmotropes (for example sulfate), are more likely to induce salting out, whereas some salts can cause salting in at low concentrations and salting out at higher concentrations. The Hofmeister series is often used as a guide to predict salt behavior for a given solute.

Commonly used salts include ammonium sulfate, potassium sulfate, and sodium sulfate, with ammonium sulfate being particularly favored for gentle protein precipitation due to its high solubility and relatively mild effects on many proteins. The process is typically performed by gradually increasing salt concentration until the target solute precipitates, allowing partial purification and enrichment.

Desalting is usually required after salting out, accomplished by dialysis, gel filtration, or other methods to remove excess salt and recover the precipitated material. The approach can also co-precipitate impurities, and not all solutes respond similarly, so conditions must be optimized for each application.

Applications of salting out extend to protein purification and fractionation, enzyme extraction, and the removal of unwanted components in complex mixtures. It is a foundational step in many biochemistry workflows, often used in combination with further chromatography or precipitation methods.