Increasing pH of commercial fixer to reduce odour

john_s said:

That would require an acidic fixer, perhaps with hardener, wouldn't it?

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Hi, it was a long time ago, and my memory is pretty fuzzy, but I think it was most likely a low-pH non-hardening fixer. That machine ran almost exclusively a textured double-weight paper which was dried emulsion-up, against the canvas apron. Everything on it was from an internal copy & restoration group with several dozen employees. Most were "work prints" that an artist would retouch or otherwise "repair," maybe adding color per a customer's description. These work prints were then photographed, and final customer prints were made. The processor was set up with an optional toning step, using Kodak Polytoner. So depending on what the C&R customers had ordered, they were toned, or not. (Those Kodak machines had moveable lower rollers, attached to long rods clamped to a rack on top of the machine; by lifting the "rods" completely up the operator could bypass the Polytoner.)

Rudeofus said:

This is what the chart posted by me clearly shows: sulfite going going going, and as soon as it's gone, the fixer sulfurs out.

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Right, the only part I took issue with was that the thiosulfate concentration was increasing. This is something I never observed in lots of years of dealing with fixers and paper blix. So I'm guessing this was an odd occurrence somehow related to the non-typical mixture.

As a note, it occurs to me that what was reported as sulfite was, at that pH, largely bisulfite ion. I'm guessing that they stated that in the text, mentioning that it would be reported as equivalent amounts of sodium thiosulfate. This is of no great significance, for the most part, just saying that there is probably more in the text that explains more thoroughly.

One additional comment on the possible collapse of thiosulfate, not related to the technical paper. We ran large amounts of paper blix, around 3,000 gallons per day. (This was the first version of RA4 blix, same as used for Ektaprint 2/3). After electrolytic desilvering the "bleach" part is essentially de-aerated. So it needs a large amount of aeration to make it viable again. This aeration effectively depletes the remaining sulfite, so the thiosulfate is no longer protected. Sulfite is added back in at the mixing stage, but there is a period of time with virtually none. But there is no obvious large-scale collapse of thiosulfate. It seemes likely that this is a consequence of being non-acidic - the pH of that blix would be a little over 7.0. This is not by any means conclusive, simply an observation.