Chemical composition of methyl orange determined by IR spectroscopy

Methyl orange (also known as helianthine or acid Orange 52) is a synthetic azo dye, commonly used as a pH indicator in titrations. Its chemical formula is C14H14N3NaO3S, with a molecular weight of 327.33 g/mol. Its structure includes an azo group (-N=N-) linking two benzene rings, one of which is substituted with a sulfonic group (-SO3Na) and the other with a dimethylamino group (-N(CH3)2).

Methyl orange is soluble in water and exhibits a significant color change depending on pH: it turns red in acidic medium (pH < 3.1) and yellow in basic medium (pH > 4.4), with a color transition between these values. This property makes it useful in acid-base analysis, especially in titrations of weak acids with strong bases.