Dyes are invariably applied in solution. 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 a polar solvent for staining would be used, although its degree of polarity might be adjusted somehow. Non-polar solvents are of little use in staining, and are more commonly used to ensure that the staining process stops completely. That is why resins dissolved in xylene or toluene (both are non-polar solvents) are used as mounting media. Ionic reactions largely cease under such conditions and the preparations are, to all intents and purposes, permanent.
Water
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. It is this property that makes it so valuable. Polarity derives from the uneven distribution of charges in the water molecule. The electrons of the two hydrogen atoms are displaced as far from the oxygen atom as possible. This caused one side of the molecule to be more positively charged and the other more negatively charged. Consequently, the water molecules attract negatively charged ions to one side and positively charged ions to the other, enhancing solution.
Ethanol
Ethanol is the next most commonly used solvent. It is similar to water in that it is polar, but to a significantly lesser degree. So it is that although ionic staining can be accomplished from ethanol solution, the results are often paler and take longer to accomplish.
Sometimes ethanol and water are mixed to adjust the polarity so that a particular type of staining can be done more effectively. Ionic bonding is accomplished more effectively from strongly polar solvents. In those cases where a dye can bond both ionically and through dipole interactions, reducing the polarity of the solvent can decrease the ionic bonding and accentuate the dipole-dipole interactions. A case in point is the demonstration of amyloid with congo red used from alcoholic solution.
