Synthetic dyes/stains are referred to as either "acid" or "basic." These terms do not describe the hydrogen-ion concentration of the dye solutions, but describe the dye solution's affinity.
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 (base-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.