My developer went dark in a few days, without using it... I recently change my storing pots. Is it the air or the darkness where I failed?
There are many possibilities, so I'll go through them all:
- Some developers simply go dark, even if well stored. Ansco 130 is prone to this, as is Rodinal and its cousins.
- Most developers need to be stored away from oxygen when possible. This is usually done by keeping bottles full. You can keep the bottle full by using a collapsible bottle, decanting the liquid into smaller bottles, inserting relatively inert nitrogen gas into the jar, or inserting glass marbles into the developer jar to displace the air space. The more air that's in a developer's bottle, the more likely it will oxidize. (Many developers go dark when this happens, although some, like XTOL, do not. Some developers, like PMK and Rodinal, do not suffer from reasonable exposure to air.) Also note that some plastic bottles have leaks, and you will get more air interchange as well if the cap is not tightly closed.
- Developers do not like light. Direct light, especially sunlight, will degrade many developers.
- Developers do not like heat. They should be stored in as reasonably cool conditions as is practical, but above freezing. Anything in the 10-to-20-degree range is fine. Life will be shorter if you store at warmer temperatures.
- Developers exhaust. They have a finite capacity. If you use them to their limits, they will darken as you go. The developer's instructions should give you a sense about capacity. Note that capacity varies by dilution, if the developer is diluted from a stock solution.
- Although this wasn't a factor in your situation, age darkens developers. The longer you store them, even if perfectly, the darker they will tend to get (with some exceptions, again, like XTOL).
You didn't specify what developer you lost, or how you stored it, so it's difficult to give you more specific information at this time.
Benzene was the first organic compound produced by man in a laboratory; it was derived from coal. You need to know that most all developing agents are spinoffs of benzene.
Now to answer your question: Developing agents are organic compounds that have an affinity for oxygen and they are a reducing agent able to liberate metals from their salts. Most developer solutions usually contain two developing agents. The developing solution is mainly water and water naturally dissolves gasses. Thus the developer contains dissolved oxygen. When a developing agent comes in contact with oxygen, it is destroyed and reverts to coal tar. Hence oxygenated developer turns black.
Now all developing solutions contain the preservative, sodium sulfite. The job of the preservative is to slow down the oxidation process of the developer and to react with the breakdown products that otherwise will stain film or photo paper. Should the preservative become exhausted, the oxidation rate upsurges.
We try to preserve the developer by limiting its contact with air during storage. We have some tricks up our sleeve. We can use a squeezable plastic bottle and squeeze the air out, and then cap the bottle. We can blow into the bottle. The theory is, our breath has a low oxygen content. We can add marbles to the bottle; these displace some liquid reducing the airspace in the bottle. Old time darkroom workers poured hot paraffin wax in the bottle to act as a floating lid. Plastic floating lids are available for some containers. Floating plastic chips also reduce the surface area of fluid to air contact. Photofinishers agitate, not by shaking or mixing, but by bubbling nitrogen into the developing tank.
The key points for you: Use non-porous flexible bottles designed to store developer. Use brown or green bottles that shield the content from light. Use one-time mixes made from concentrates and then discard. Look at the solution before use; it should be transparent, however most have the color of tea.