CAUTION: VAPORS ARE EXTREMELY TOXIC. All work with osmium must be carried out in a fume hood.

Osmium tetroxide (OsO4) is obtained commercially as golden tan to beige crystals sealed in breakable glass ampoules. Prepare stock solution as follows:


  1. Place ampoule containing 1 g of OsO4 into a 100 ml glass stoppered reagent bottle, preferably one with a Teflon-coated rim, and fill bottle almost full with concentrated nitric acid. Rotate and shake gently with the stopper in place to wet the surface of the ampoule and the inner surface of the bottle.
  2. Fill a 100 ml graduated cylinder with concentrated nitric acid and place or dip into it a thick glass rod suitable for breaking the ampoule.
  3. Rinse the graduated cylinder and the glass rod several times with distilled water. Set aside and protect the rod from contact with dirty surfaces or dust.
  4. Rinse the reagent bottle containing the ampoule several times with distilled water, being careful not to touch the ampoule or the inner surface of the bottle or stopper with your hands.
  5. Measure out 25 ml of distilled water with the acid-cleaned graduated cylinder and add it to the reagent bottle.
  6. Break the ampoule with the clean end of the glass rod.
  7. Insert the stopper and let the bottle stand at room temperature until the osmium crystals dissolve, then store at 4oC. NOTE: Because osmium vapors may escape in the refrigerator, it is best to store the reagent bottle in its own air-tight container, such as a coffee can with plastic wrap over the top.
  8. Store osmium in small vials flushed with nitrogen gas and freeze in 1 dram screw-top vials.

Assume all fixatives are at least partially light-active. Protect osmium tetroxide and other fixatives from prolonged exposure to intense light.

Note: Students in the BMZ 5364 class will be able to obtain the osmium tetroxide in 1 dram vials in the freezer in NML 205, premixed. This page is included for future reference in labs where you may have to mix your own.