Fast Blue B Salt, also known as Methylene Blue, is a synthetic dye with a chemical formula of C16H18ClN3S. It belongs to the class of thiazine dyes and is recognized by its deep blue color. This versatile dye finds extensive applications in various industries, including textiles, microscopy, and the medical field. In the textile industry, Fast Blue B Salt is used for dyeing fabrics, giving them a rich blue hue. Its high solubility and colorfastness make it a popular choice for achieving vibrant and long-lasting shades in clothing and other textile products.

In microscopy, Fast Blue B Salt plays a crucial role as a biological stain. It is utilized to stain cell nuclei, making them more visible under a microscope. This staining technique aids researchers and medical professionals in studying cell structures and diagnosing various medical conditions. Additionally, Fast Blue B Salt has applications in the medical field as a medication. It is used as a medication under the name Methylene Blue for various purposes, including treating methemoglobinemia (a blood disorder) and as a diagnostic tool in certain medical procedures. Its ability to interact with biological molecules and its distinctive blue color make it an essential compound in both laboratory research and medical practice.