Synthetic dyes/stains are generally created so that they are one of the following stain types:
Basic - contains basic groups that have an affinity for acidic tissue elements.
Acidic - contains acidic groups that have an affinity for basic tissue elements.
Basic stains are used to stain nuclei and other basophilic (basic loving) cellular structures in tissues. Acidic stains are used to stain cytoplasm and other acidophilic (acid loving) cellular structures in tissues.
Many biological staining procedures rely on acid-base chemistry.