noun
Usage: sometimes capitalized M&O
: a basic azo dye (CH 3 ) 2 NC 6 H 4 N . NC 6 H 4 SO 3 Na which is made by coupling diazotized sulfanilic acid with dimethylaniline, is used chiefly as an acid-base indicator, and whose dilute solution is yellow when neutral and pink when acid — see dye table I (under Acid Orange 52 ); compare helianthin