C27H30O5S – thymol blue – consists of green crystals that are poorly soluble in water. It is used to determine the acidity or basicity of solutions. Its color depends on its surroundings: it is blue in an alkaline medium, green in a slightly alkaline medium, and yellow in a neutral medium. Further, it is red in a slightly acidic medium, and magenta in a strongly acidic environment. It can serve as a component in universal indicators – strips of paper impregnated with an indicator solution or a mixture of indicators.