Posted 04 October 2004 - 03:36 PM
The purpose of adding salts is to neutralize the charge on the sugar-phosphate backbone of the DNA. For a precipitation, you need to form ion pairs between the polyanion (DNA) and the cation (usually sodium or ammonium). The DNA and counterions are more or less in the free ion form rather than the ion pair form in dilute aqueous solution, and are surrounded by one or more layers of water molecules. Adding ethanol decreases the dielectric constant of the solution. The anions and cations form ion pairs and this results in precipitation of DNA.