The solvent itself can therefore be another way to manipulate the staining process. The choice of solvent can impact significantly on the activity of the dye.
Solvents are of two kinds, polar and non-polar .
In most cases we would use a polar solvent for staining, although we might adjust its degree of polarity somehow. Non-polar solvents are of little use in staining, and are more commonly used to ensure that the staining process stops completely. Ionic reactions largely cease under such conditions and the preparations are, to all intents and purposes, permanent. The simplest solvent is water, and it is undoubtedly the most used. Other chemicals may be added to it to alter its characteristics in some fashion, the most common being pH. Water is a strongly polar solvent.