Methyl yellow

Methyl yellow, or C.I. 11020, is a chemical compound which may be used as a pH indicator.

In aqueous solution at low pH, methyl yellow appears red. Between pH 2.9 and 4.0, methyl yellow undergoes a transition, to become yellow above pH 4.0.

Additional indicators are listed in the article on pH indicators.

As "butter yellow" the agent had been used as a food additive before its toxicity was recognized (Opie EL)