Neutral Red

NEUTRAL RED, neutralrot, or toluylene red, is a dye in the form of a black-green powder, which gives red solutions in water, and brownish-red solutions in alcohol. In very highly diluted organic acids, the red color changes to fuchsine-red, while in highly diluted alkalis or even in plain water, it changes to yellow. This sensitivity of the dye served as the basis for its introduction into practice, mainly bacteriological, for the purpose of determining fermentation processes and their intensity. In histological technique, Neutral Red is used as a nuclear-staining and fat-staining dye. Furthermore, Neutral Red generally stains basophilic elements of protoplasm well, for example, Nissl granules, mucin; Neutral Red reveals well the structure of striated muscle fibers. The weak toxicity of Neutral Red made it possible to introduce it into the practice of vital staining, subcutaneously, intravenously, or per os, as well as in the determination of total blood mass. Tissues are stained red in this process (namely, lymph, tissue fluid, blood plasma); living cells and their granules do not take up the dye. Neutral Red is used for vital and supravital staining of blood, for example, by covering a thin layer of blood with a glass slide on which a solution of the dye has been previously dried; in this way, the basophilic granulation of erythrocytes and the neutrophilic granules of leukocytes are stained; later, the nuclei, especially of lymphocytes, are stained. As a basic dye, Neutral Red stains bacteria well, and moreover, earlier and more intensely than cell nuclei, which is convenient, for example, for staining gonococci. A 1:10,000 solution of Neutral Red in physiological saline stains dysentery amoebae intravitally, for which the dye is mixed directly with the suspicious material. The endoplasm of the amoebae is pink; the ectoplasm remains colorless. Neutral Red stains fatty acids, soaps, and phosphatides red; neutral fats are not stained (see also Lipoids). Neutral Red is used as an indicator in acidimetry and alkalimetry (0.1% solution in 70% alcohol). Its transition interval from yellow to red corresponds to pH 6.8–8.0. A mixture of two parts of Neutral Red with one part of Neutral Blue (oxazine dye, syn. naphthol blue) forms the so-called neutral violet extra, a greenish-colored dye that dissolves easily in water and alcohol and strongly irritates the respiratory tract. This dye, in a 0.5% aqueous solution, allows for the identification of various components of tissues depending on their reaction, reducing or oxidizing capabilities.