Iron, in the parts per billion range in which it occurs in sea water, can be determined spectro-photometrically with 2, 4, 6-tripyridyl-s-triazine (TPTZ). The intense violet compound formed by TPTZ and the ferrous ion is extractable into nitrobenzene if sodium perchlorate be added to the solution. The extraction affords a means of concentrating the iron from a large sample into a small volume of nitrobenzene for spectrophotometric measurement....
Collins, Peter, and Harvey Diehl. 1960. "Tripyridyltriazine, a reagent for the determination of iron in sea water." Journal of Marine Research 18, (3). https://elischolar.library.yale.edu/journal_of_marine_research/952