Salt Saturation Test- Definition, Principle, Procedure, Result, Uses


Salt Saturation Test is a test for the precipitation of proteins and their differentiation from other proteins by using the salting-out technique. Salting out is the process of precipitation of proteins by increasing the concentration of salt in the solution. The salts used in this technique are mostly neutral mineral salts like MgSO4, Na2SO4, and (NH4)2SO4. The salt solubilization test also helps in the differentiation of proteins as some proteins precipitate under lower salt concentrations while others require higher concentrations. The reagent used for the test might differ depending on the preference with salts like ammonium sulfate being used due to their increased solubility. The salt saturation test can be performed either as half saturation or full saturation depending on the type of proteins being detected. In the half-saturation, the solution is just half saturated with the salt, but in full saturation, the solution is completely saturated with the salt.

The salt saturation test is based on the principle that proteins are colloidal in nature and have a shell of hydration around them. When mineral salts are added to the solution, they compete with the proteins for water molecules and reduce the solubility of the proteins. The proteins then aggregate and precipitate out of the solution. The amount of salt required for precipitation depends on the size and charge of the protein molecules.

The salt saturation test can be used for various purposes such as:

  • To detect the presence of proteins in a given sample.
  • To differentiate between proteins like albumin and globulin.
  • To purify and fractionate large batches of proteins based on their solubility.

The salt saturation test is a simple and inexpensive method that can be performed in a laboratory with minimal equipment and reagents. However, it also has some limitations such as:

  • The formation of lumps or clumps of solid ammonium sulfate in the solution.
  • The contamination of low-grade mineral salts with heavy metals that interfere with the precipitation process.
  • The acidification of the solution by ammonium sulfate that slows down the precipitation process.

These limitations can be overcome by using proper techniques and precautions such as:

  • Using mortar and pestle to crush solid ammonium sulfate before adding it to the solution.
  • Using online calculators to determine the exact amount of solid ammonium sulfate required for saturation.
  • Adding Tris buffer to maintain a neutral pH in the solution.

The salt saturation test is a useful technique for studying proteins and their properties. It can provide valuable information about the nature and composition of proteins in a sample. It can also help in isolating and purifying specific proteins for further analysis.