formalein

Formalein is a commonly used term for a solution of formaldehyde in water, often with methanol added as a stabilizer. The standard commercial grade is about 37% formaldehyde by weight, with roughly 10–15% methanol and the remainder water. It is a colorless, pungent liquid that forms a homogeneous mixture with water and is widely used in laboratory and clinical settings.

Formalein is typically stored in tightly closed containers, kept in a cool, well-ventilated area away from heat and flames. It can slowly polymerize to paraformaldehyde if stored improperly or for extended periods, especially in the absence of stabilizers. Methanol in stabilized formulations helps reduce polymerization. Exposure to formaldehyde vapors or solutions can cause irritation of the eyes, nose, and throat, and prolonged or high-level exposure may have more serious health effects. Inhalation of formaldehyde is a recognized health hazard, and safety measures such as adequate ventilation and appropriate personal protective equipment are advised. Formaldehyde is classified as a known or probable carcinogen in humans by international authorities, so exposure should be minimized.

Formalein has a variety of uses, most notably as a tissue fixative in histology and pathology, where it preserves biological specimens by cross-linking proteins. It is also used in embalming fluids, as a disinfectant and preservative in some settings, and as a chemical intermediate in manufacturing processes. Because of its toxicity and environmental hazards, handling, storage, and disposal are subject to regulatory controls and guidelines, and it is generally not intended for consumer use.