Metüülpunane

Metüülpunane, also known as methyl red, is a pH indicator that exhibits a color change from red at acidic pH values to yellow at alkaline pH values. Its transition range typically falls between pH 4.4 and 6.2. This color change occurs due to a structural rearrangement of the molecule in response to the concentration of hydrogen ions. In its protonated form, which predominates in acidic solutions, metüülpunane absorbs light in the green region of the spectrum, resulting in a red appearance. In its deprotonated form, found in alkaline solutions, it absorbs light in the blue region, appearing yellow.

Metüülpunane is commonly used in laboratory titrations, particularly for acid-base titrations where the equivalence point falls within its pH transition range. It is also employed in microbiological tests, such as the methyl red test, which is used to detect the production of mixed acids by bacteria during carbohydrate fermentation. The synthesis of metüülpunane involves the diazotization of *o*-toluidine followed by coupling with *N,N*-dimethylaniline. It is a synthetic organic compound and is generally stable under normal laboratory conditions. While effective, other pH indicators may be preferred for titrations outside of metüülpunane's specific pH range.