Chemistry Question: Xray processing chemicals

I clean our x-ray processor at work so I was a little curious about the chemicals for developing and fixing films. It’s no big deal to get a little developer in the fixer but if you get fixer in the developer it’s shot and depending on how much gets in there, you could get a really, really bad smell. (I’m sure the same applies to developer in fixer if enough gets mixed together, it’s just that the fixer still works unless badly contaminated.)

A couple times a co-worker accidentally added fixer to the developer reservoir and bad things happened. The first time the company that supplies the chemicals switched the color of the jugs and the co-worker did not read the label. A very strong ammonia smell resulted and he called me about it. I knew the jugs were different colors and advised him to drain the tanks and air the place out. The next time it happened (same co-worker) he again did not read the labels and the chemicals were back in the correct colored jugs. He immediately realized what he had done when he smelled the ammonia smell and called me again so I could walk him through draining the tanks. Yes, he has now learned to always read the label and even asks me if the label is worn and he’s not sure which it is.

I heard a story about another clinic that mixed their chemicals and ended up with such bad fumes they had to evacuate their clinic. So I got curious about just how dangerous these fumes might be.

Developer contains:
Sodium Sulfite
Potassium Hydroxide

Fixer contains:
Ammonium Thiosulfite
Acetic Acid
Aluminum Sulfate

I did not write down the percentages, I hope it’s not needed. Yes, ammonium thiosulfite is in the fixer but it does not smell strongly of ammonia unless it is mixed with the developer. So can anyone tell me what chemical reactions result from mixing these chemicals and how bad it is for humans?


I think the smell just comes from mixing a strong base with an ammonium salt. This produces ammonia. It could be really irritating, but not a likely health hazard in the concentrations I think you will produce it.

Another issue would be both the sodium sulfite and the thiosulfate portion of the ammonium salt. These will react with acid to make sulfer dioxide, which could be a hazard. Other clinics may have slightly different formulations that make this more of an issue.

I don’t know for sure why a little bit of fixer ruins the developer. The fixer may have a high concentration of silver ions and this may get reduced if you mix it with the developer. Does the developer change color if fixer gets in it?

I hadn’t noticed if it changes color but I’ve never mixed the chemicals. Old developer does turn a very dark brown.