[cdft]

when the H2S gas is bubbled into the iron chelate solution,there forms some unkown materials insoluble as well as sulphur.what are these things?Ferric hydroxy?or ferrous sulfide?Anyone knows?

[Arkcon]

Yes, this is most likely iron sulfide, this is a common high school assay.  You might get something weirder because of the chelate.  But iron sulfide is most likely what you're getting.

[cdft]

THANKS.Since there exists the chelon which can chelate the ferrous ion in the solution,how and why could the ferrous lose as the FeS precipitation as u said?Such as NTA,it chelates both ferric and ferrous stably in the solution.