WARNING – THE FOLLOW COULD ALL BE BUNK - WARNING
Let me throw out a hypothesis about why film developers lose potency during storage.
We are always told to make sure that bottles containing film developer stock are completely filled. The popular wisdom is that oxygen adsorption causes the developer to gradually degrade. This seems unlikely because even if a one liter bottle is only half full it doesn’t seem to me that the half a liter of air could be responsible for the chemical reactions necessary to degrade half a liter of developer. So, what else could be the problem?
Suppose the real problem is associated with the constant evaporation/condensation cycle that takes place in the closed container of developer. You can see this taking place in a partially filled and capped bottle of soft drink setting on a table. If you let it sit for a day you usually see droplets of water sticking to the inside of the container.
My hypothesis is this, as the developer evaporates a phase change takes place as the energy is added to force the change between a liquid and a gas. As the developer undergoes this phase change my hypothesis is that some destructive chemical reaction takes place at the phase change boundary that causes the developer to slowly lose potency. The resulting water-gas (I hate to call it steam) builds up until the dew point is reached within the air void of the container, at which point a condensate develops which flows back into the developer to complete the cycle. As the weeks go by this slow chemical reaction sped up by the evaporation leads to the loss of potency.
Squeezing all of the air out of a container would of course mitigate this, not by limiting oxygen, but by minimizing the volume of the container available for the gas condensate cycle described above. This might also explain the experience some folks have by storing developer in glass bottles instead of plastic. It wouldn’t be too big of a stretch to imagine that different substances have different properties when it comes to supporting an evaporation/condensation layer. A wetting surface for example may be much less supporting of this cycle than a non-wetting layer. Perhaps glass is a wetting surface and plastic is not.
If this is really the culprit, and remember this is merely a hypothesis, I can think of a number of ways to mitigate evaporation that are easier than always having to have a full bottle (e.g. floating 1-hexadecanol or a block of wood on the surface). The only problem is how to construct a test of these methods that would be able to distinguish between oxygen as the cause and my hypothesis being the cause. And yes, I realize that floating something on the surface is already a technique to prevent decomposition, but as far as I can find nobody says what mechanism they feel this prevents.
So, what do you think?