The phenolphthalein indicator solution is primarily used in chemistry and biochemistry to determine the pH value in various solutions. It is particularly suitable for titration procedures in which the neutralization reactions of acids and bases are investigated. The solution consists of phenolphthalein dissolved in a mixture of ethanol and aqua bidest / ultrapure water. Phenolphthalein is a pH indicator that is colorless in acidic medium and pink in alkaline medium. It shows a color change between pH 8.2 (colorless) and pH 10 (pink). The chemical reaction that causes this color change is the de- and re- protonation of phenolphthalein depending on the pH of the solution. The function of the indicator solution is based on this ability to serve as a visual indicator of pH, making it an indispensable tool in various scientific and industrial applications. In addition, phenolphthalein can find application in biomedical research, for example in the determination of acid-base balance in biological samples.