One of the mysteries of the English language finally explained.
Any of a group of orange and yellow sulphonated azo dyes, some of which are used as pH indicators. Frequently with capital letter, numeral, etc. as a postpositive, denoting a particular dye of this group, as "tropaeolin O".
Late 19th century; earliest use found in Chemical News. From tropaeolum) + -in, after German Tropäolin. Apparently so called from the resemblance of the colours of the dyes to those of the flowers of some species of Tropaeolum.
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