Developer Color Problem

I've been mixing my developers from scratch for years without problems, other than stupidity. The last two batches had a common problem that I've never seen before. I mix two liters of D-72 at a time and store it in 500 ml brown plastic bottles. The last two times, after standing tightly capped with no air layer for a week or so, the developer had developed a decidedly brown color. Like it had been standing overnight in an open tray. Checking it by developing a print, it seemed to have normal activity.

Both times it was in the same three of the four bottles that color appeared, but not the fourth. There were no new chemicals used that have not been used many times before. Checking further, I realized the three bottles with the stain were recycle number #7 - "Other". The one without the color was #2 - "HDPE". Looking into the neck of the bottles there was a black layer on the innner surface. The black layer would not rub off. I don't know if it was on the surface the first time I used them.

Of course the solution is easy. Toss them out, which I'll do after I take them to the photo store and see if they have ever heard of this before. But I would like to know what is causing this.

glass brown bottles are the solution although there are some good plastic ones

Ryuji Suzuki wrote this piece on the merits of various types of plastic bottles for developer storage. His list doesn't go up to #7, though, just #1 through #4. In sum: Stick to #1 (PET), #3 (PVC), or glass for developers, and #2 (HDPE) or glass for acids.

Glass has class. I like amber because I know its a chemical rather than a drink. I know, the label tells you that. But, if the label falls off or someone doesn't read it.

I used to work at a plastic sheet manufacturer. I think I'll take one of the bottles to the lab guy for a quickie analysis. If it is a plastic with high oxygen permiability that probably explains the problem.