A new theory on why developers go bad

[Denis K]

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?

[Lab Magician]

Hi Denis,

In pro labs, if air is used to agitate the dev in processing machines then it goes off rather rapidly. If nitrogen is used then it doesn't. Also, I've used nitrogen to exclude the air from bottles of infrequently used special devs. Filling with nitrogen will allow the evaporation cycle you describe, but does prolong the keeping properties.

Some formulations of developer appear to be less stable than others and still go off eventually.

Baz

[Rob C]

I think it´s still only oxidation, even if the physical actions you describe happen, but I do agree that temp variations could speed it along, probably why these things are usually recommended to live in a constant, low temperature...

Inert floating lids are all very well in tanks (not wood, I´d think), but hardly bottle-friendly (as far as I know!). But shhhhsh, some digital zealot will use this as ammunition!

Rob C

[Denis K]

I think it´s still only oxidation, even if the physical actions you describe happen,  ...snip

Rob C

Then where does all the oxygen come from?

There is just not enough oxygen in a half empty liter bottle of developer to do that kind of damage (IMO).  Dry air only contains 21% oxygen (it's 78% nitrogen) and moist air even less.  So a half liter bottle of developer only contains about 1/10 of a liter of oxygen.  Whop-De-Do.  I would sooner believe that the oxygen comes from the dissolved air in the water used to prepare the stock solution.  But if this is the case then filling the bottle with nitrogen will do nothing.

Your explanation also does nothing to explain the glass/plastic issue, although, I could agree that that data might just be wishful thinking.

Denis K

[Eric Myrvaagnes]

There is just not enough oxygen in a half empty liter bottle of developer to do that kind of damage (IMO).

Why not?


On what scientific principle are you basing your assertion that it takes at least X units of oxygen to cause Y amount of damage to developer? The typical developer has several ingredients, some in quite small quantities. If the oxygen were to "attack" only the smallest essential ingredient, I see no reason it would take more than a little oxygen to kibosh the developer's effectiveness.

[Denis K]

Why not?


On what scientific principle are you basing your assertion that it takes at least X units of oxygen to cause Y amount of damage to developer? The typical developer has several ingredients, some in quite small quantities. If the oxygen were to "attack" only the smallest essential ingredient, I see no reason it would take more than a little oxygen to kibosh the developer's effectiveness.

EricM,  I think your challenge to me demonstrates the importance of me getting out the calculator before I start making theories.  If I do a little figuring I now estimate that a half liter of air contains about 1.2 grams of oxygen whereas the half liter of developer would contain in the order of magnitude of 2 mg of dissolved oxygen.  So it would seen that I am wrong on two counts.

Now if we take D-76 developer as an example, you are indeed correct as it would contain about 1 gram of Metal and 2.5 grams of Hydroquinone per half liter.  So you are 100% correct that the developer would contain critical components in almost the same portion as the oxygen.  I'm sure the molar quantities are somewhat different but the important thing is that they would be in the same order of magnitude.  I think I'm not only wrong, but dead wrong.  It could indeed be the air that is the problem.

Denis K

[Jonathan Ratzlaff]

Some chemicals in developers are not particularly stable and in the presence of oxygen may decompose and when this happens the oxygen is released to react again.  So even if there is a small amount of oxygen in the air, it can cause a larger problem than what you would expect.