Crystal violet

Crystal violet is a versatile triarylmethane dye used across many scientific fields.

It is known by several other names, including gentian violet, methyl violet 10B, or hexamethyl pararosaniline chloride. This substance is highly significant because of its ability to stain biological materials. It is widely used as a histological stain to help scientists see cells under a microscope. It is also a key component in Gram's method, which is a way to classify different types of bacteria. Beyond biology, it serves industrial roles as a dye for paper, textiles, and various inks.

The chemical behavior of crystal violet is defined by how it reacts to its environment. When dissolved in water, the dye typically appears blue-violet. This specific color has an absorbance maximum at 590 nm. However, the color changes based on the acidity, or pH, of the solution. At a pH of +1.0, the dye appears green. In a very strongly acidic solution with a pH of -1.0, it turns yellow. These color shifts occur because the charged state of the dye molecule changes. In the yellow form, all three nitrogen atoms carry a positive charge. The green color appears when only two nitrogen atoms are positively charged. At a neutral pH, the molecule loses extra protons, leaving only one nitrogen atom with a positive charge.

In alkaline solutions, the dye undergoes a different process. Hydroxyl ions attack the central carbon atom of the molecule. This reaction produces a colorless form called triphenylmethanol, also known as the leuco dye. This colorless state is also formed under very acidic conditions when water molecules attack the central carbon. This chemical reaction can cause the yellow color to fade slightly. Because of these shifts, crystal violet is a useful humidity indicator. When added to silica gel, it turns orange when dry. As it absorbs moisture, it transitions through green and eventually reaches a violet color.

The history of this dye involves several important chemists. Charles Lauth first synthesized methyl violet in 1861. Crystal violet itself was first synthesized in 1883 by a firm in Basel. To improve the production process, German chemists Alfred Kern and Heinrich Caro collaborated at BASF. They worked to optimize a difficult synthesis that used the highly toxic chemical phosgene. Kern also discovered that using diethylaniline instead of dimethylaniline could create a related dye called C.I. 42600. In 1884, Hans Christian Gram used a violet stain to develop his famous method for classifying bacteria.

Crystal violet has many specific applications in modern laboratories. In DNA gel electrophoresis, it serves as a nontoxic alternative to fluorescent dyes like ethidium bromide. This is helpful because it does not require ultraviolet illumination. Using UV light can sometimes cause DNA destruction during cloning. When used at a 10 ppm concentration, crystal violet can detect as little as 16 ng of DNA after 30 minutes. It can also be used to stain proteins in SDS-PAGE. In these tests, it has shown a fivefold improvement in sensitivity compared to Coomassie brilliant blue.

It is even used in forensics to develop fingerprints from live human skin.

In the medical field, gentian violet has several important properties. It is antibacterial, antifungal, and anthelmintic, which means it can treat parasitic worms. It is used to treat fungal infections like Candida albicans, which causes thrush. In resource-limited settings, it is used to manage burn wounds and mouth ulcers in children. It is also used in dentistry and to mark skin before surgery. In veterinary medicine, it can treat infections in fish, though using it in fish meant for human food is often illegal. The dye is readily absorbed into fish tissue and is metabolized into a colorless form.

While useful, there are important safety precautions regarding crystal violet. Studies in mice have shown that it may have carcinogenic potential, meaning it could cause cancer. The FDA has determined there is not enough data to prove it is safe for use in animal feed. In 2019, Health Canada reviewed the dye and found it to be carcinogenic when in direct contact with the body. They recommended that people avoid using it on the skin and that pregnant or nursing women avoid it entirely. Because of these risks, many medical drugs containing the dye have been discontinued or recalled in certain regions.