If you added the iron and a reductant like Na2S(9H2O) while the medium is
still hot, then FeS can form and precipitate.
I add the filter sterilized Na2S(9H2O) solution to Hungate Roll tubes
prior to use after the medium has cooled to room temperature.
Agar roll tubes are held at approximately 50C in a water bath to keep the
agar molten, and the Na2S(9H2O) solution is injected just prior to
inoculation and rolling.
I have rarely had a black ppt. form in my medium. Please contact me
directly if you have any other questions or problems, and I will be glad
to try to help.
J. Robert Paterek, Ph.D.
Manager, Environmental Biotechnology
Institute of Gas Technology
1700 S. Mt. Prospect Rd.
Des Plaines, IL 60018
paterek at igt.com