Chemicals -- exposed to air

[Michael Ting]

If I leave my chemicals in open tray, exposed to air, will that make the chemical expire/altered ?

[Ralph Barker]

The short answer is yes. Developer will oxidize, reducing its effectiveness. Stop bath and fixer, although affected to a lesser degree, will start to evaporate.

[Donald Qualls]

With fixer, you'll also see oxidation become an issue -- sulfite will oxidize to sulfate, and as the sulfite level drops, thiosulfate will oxidize to sulfate and evolve sulfur dioxide or precipitate elemental sulfur. This does take longer than the failure of developer as the developing agents oxidize.

Tight fitting floating lids in vertical sided trays will help this a lot, as will the greatly reduced air interface area in a slot processor (and in a slot processor this can be helped further by displacing air with another gas like nitrogen, butane, or "canned air" before putting the cylindrical slot covers on, but beware of potential safety issues with these gasses in a closed darkroom). With conventional, slope-sided nesting trays, floating an empty tray on the liquid will reduce the exposed area and slow down the spoilage, possibly enough to get you a second session from a tray full of Dektol.

FWIW, I don't attempt to keep my Dektol from session to session -- I've got gallons of stock solution and my problem will be using it before it goes off even in the airtight, completely full storage jars. I've also been discarding my print fixer after each session, and just had to discard my first printing batch of stop bath, as the indicator was starting to change after developing some 50-60 8x10 RC prints in a single quart.