What is a colorless crystalline substance obtained by reduction from phthaleïn, into which it is easily converted by oxidation; hence, any one of the series of which phthalin proper is the type called?

phthalin — A colorless crystalline substance obtained by reduction from phthaleïn, into which it is easily converted by oxidation; hence, any one of the series of which phthalin proper is the type. (Webster's 1913.)

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