Hemin comes as a fine black powder; it's not really soluble in aqueous solutions or even miscible in water. For our culture media it's prepared as a sterile suspension by mixing 100 mg of hemin with 100 ml of a solution of 4% triethanolamine (a surfactant), and incubating this at 65°C in a waterbath for 30 min. The hemin forms a black suspension that's, rather surprisingly, both sterile and chemically stable. Sterility is surprising because 30 min at 65°C isn't expected to kill spores, and stability is surprising because, once the hemin is added to culture media it goes off within 48 hr.
What to do? Was the problem the triethanolamine or the hemin, which was also a new bottle (well, a bottle of uncertain age that had never been opened). The new and old hemins were both from Sigma and had identical labels (the old one in a brown glass bottle, the new one in a white plastic bottle), so that's probably not the problem. Our ancient CRC handbook (the 'rubber bible') recently disappeared, so I looked triethanolamine up in the Merck Index. Not much help, so I just left the problem for the next day, thinking I'd email the building to see if anyone had any liquid triethanolamine.
But this morning I've done some Google searching and discovered that liquid triethanolamine is mildly alkaline, but solutions of the hydrochloride are quite acidic. Triethanolamine hydrochloride is used in various molecular biology and microscopy protocols, always with the pH adjusted to 8.0. So this morning I'm going to try adjusting the pH of the 4% triethanolamine before I add it to the hemin.
Later: And here's the result! Raising the pH caused the hemin to form a nice evenly black suspension (bottle on left). The bottle on right has its original low pH and all the hemin has settled to the bottom of the bottle.