naphtholsulfonic acid
noun
naph·thol·sul·fon·ic acid
variants:
or chiefly British naphtholsulphonic acid \ -ˌsəl-ˌfän-ik-, -ˌfōn- 
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Medical Definition of naphtholsulfonic acid
: any of several sulfonic acids derived from the naphthols and used as dye intermediates