Osmium Tetroxide: Storage Conditions

This is a subject that is hotly debated and there is probably no one "best" approach to storage of osmium tetroxide. It is just about the strongest oxidizer known to mankind. One should take very seriously the recommended safety precautions for use. And one should take very seriously the recommendations for storage.

We discriminate between osmium tetroxide in the "as received" and unopened ampoules vs. the aqueous stock solution that is mixed by dissolving the contents of an ampoule in water.

As-received ampoules:

Although in theory, one could store the as-received ampoules at room temperature, we would recommend refrigerated storage just the same. We would recommend storage in the original SPI-Chem shipping vials and outer white packaging. While it is highly unlikely, with a possibility approaching zero (at least for SPI-Chem ampouled osmium tetroxide), that an ampoule will leak during these conditions, in the event that should happen, it would be immediately obvious as a result of a blackening of the outside of the white shipping box. Should this blackening be observed, the entire box should immediately be removed for further storage and disassembly to a properly operating fume/exhaust hood.

The outside of the refrigerator used for such storage should be clearly marked "No Food or Beverage" and "EM Lab Chemical Storage Only". Be sure to check on your institution's safety policies as to whether a more ordinary household refrigerator could be used for this purpose or whether a special made-for-laboratory-use refrigerator should be used.

Already mixed stock solution, such as 4% aqueous:

Typically, the stock solution is made up by taking a 1 gram ampoule, scoring and then snapping it open, and then dropping the entire contents and ampoule into the bottom of an Erlandmeyer flask (or equivalent) containing 25 ml deionized and distilled water (to make a 4% stock solution). The crystalline osmium tetroxide is quite slow to dissolve in water at room temperature and patience is needed. We would recommend this be interpreted as meaning that an overnight dissolution time might be quite necessary. Before going away and leaving it to dissolve by itself, the following procedure should be followed:

The mixture of partially dissolved osmium tetroxide and water, are poured into some kind of a glass-stoppered reagent bottle (leaving the glass bits and pieces behind in the flask), the top is wrapped with Parafilm^® which in turn itself is placed inside of a second bottle with a screw on cap. The larger second bottle should contain something to "cushion" the interior bottle from banging (with the risk of breaking) on the bottom of the outer bottle. While some laboratories have been using cotton for this purpose, we would recommend something that is not prone to lint, thereby becoming a possible source of contamination. Instead we would recommend our own SPI-Wipers lint free cotton wipers.

The lint-free cotton has the advantage of not only being lint free but also in the event of any dripping or spillage, it will act as an excellent absorbent, and since the osmium tetroxide will eventually react with the organic cotton matrix, it will also be reduced to the less hazardous osmium dioxide, OsO². The second bottle top should also be wrapped with Parafilm^® in order to provide yet a further seal against leakage into the environment.

We would recommend carrying the safety concern one step further by placing the just described outer glass bottle inside of a third bottle, this one being of opaque plastic construction. Lint free cotton SPI-Wipers should also be placed on the bottom of this now third and outermost plastic bottle to act as a cushion.

The three container assembly should be left in the hood, at room temperature, for a minimum of twelve hours to dissolve.

And after the passing of twelve hours, and once it is clear that dissolution is complete, the entire three bottle assembly can be placed in refrigerated storage. And the stock solution is ready to use.

The outside of the refrigerator used for such storage should be clearly marked "No Food or Beverage" and "EM Lab Chemical Storage Only". Be sure to check on your institution's safety policies as to whether a more ordinary household refrigerator could be used for this purpose or whether a special made-for-laboratory-use refrigerator should be used.

Osmium tetroxide is very volatile and also reactive, and despite what you believe to have been perfect stoppering of the inner most bottle, you may find that by the next morning, the Parafilm has turned black. This is not all that unusual. You might even find that after some several days, the second bottle's Parafilm is starting to turn black. This too is not at all unusual. When you switch to the refrigerated storage of the solution, and apply new Parafilm wrapping, you will still find that the Parafilm will eventually start turning black. But since everything is at a lower temperature, this process should be occurring much more slowly.

But by following these instructions, and by replacing the Parafilm wrappings on a regular basis, you should be able to be successful at keeping osmium tetroxide vapors from leaking out into the environment and turning the entire refrigerator black. Now this has not ever happened to us in our own laboratory, but we have heard of it happening to others. So apparently is can happen. Therefore it is important that you have set up, as part of your safety procedures, the constant checking of your stock solution bottle to make sure that nothing is leaking out into the refrigerator or the environment.

Be sure to consult the MSDS sheet for osmium tetroxide for further information about safety precautions and procedures.