My darkroom has no ventilation (and venting it would be quite expensive) so I strive to keep everything odor-free. I switched from Acetic acid stop to Citric acid stop (15 grams/liter) for printing years ago, and I can't detect any Ammonia smell from Ilford Rapid Fixer at 1+9 in distilled water, but even so I notice mild breathing discomfort after an hour or two of working. A comment by koraks about SO2 evolution at pH<4.5 in post #2 of the "Plain Sodium Sulphite as a paper printing stop bath?" thread caught my eye, and I wondered if perhaps this was my issue - sulfite carried over from the developer could evolve SO2 in the stop, and stop carried over to the fixer could convert some of the fixer's sulfite to SO2 as well. SO2 becomes Sulfuric acid when it hits the moist mucus membrane lining the windpipe, and this could be what was causing my irritation. He also provided a handy link to an online Citrate buffer calculator in post #7 of that thread. Today I tried a pH=5 buffered stop bath and I'm thrilled to say that it seems to have made a big improvement - I experienced none of the mild discomfort that usually affects me every printing session.
Here's what I did. I mixed up a ~0.099M pH=5.0 Citrate buffer solution (that's 0.099 Molar, but it doesn't matter if you don't know what that means) using 17 grams of Sodium citrate and 8 grams of Citric acid in a total volume of 1 liter. I measured a pH close to 5.0 immediately after mixing with a three-color pH test strip. After running 20 8x10-equivalents through that liter I remeasured and found that the pH had risen to 5.5. This is still acidic enough to work but is on its last legs. This tells me that the safe capacity at this strength is around 20 8x10s per liter. In the future I'll simply mix a double batch if I think I'll be printing more than 15 or so 8x10s - both Sodium citrate and Citric acid are relatively cheap and I definitely want to avoid the stop bath crashing, which it will do quickly as it reaches exhaustion.
Wikipedia says 5 different compounds are all (sloppily) referred to as "Sodium citrate". The chemical name for what I got from Artcraft is Trisodium citrate dihydrate, CAS 6132-04-3, and is the version referenced by the online Citrate buffer calculator. I assume you could use any of the other types of citrate by adjusting the formula to correct for molecular weight differences, but it's easier to spend $10 (plus shipping) and buy a pound of the right stuff from Artcraft - Mike's a great guy and can always use the business. A pound of Sodium citrate and half a pound of Citric acid will make around 26 liters of buffered stop bath.
For reference, here's the link to the Citrate buffer calculator: https://www.aatbio.com/resources/buffer-preparations-and-recipes/citrate-buffer-ph-3-to-6-2
Here's what I did. I mixed up a ~0.099M pH=5.0 Citrate buffer solution (that's 0.099 Molar, but it doesn't matter if you don't know what that means) using 17 grams of Sodium citrate and 8 grams of Citric acid in a total volume of 1 liter. I measured a pH close to 5.0 immediately after mixing with a three-color pH test strip. After running 20 8x10-equivalents through that liter I remeasured and found that the pH had risen to 5.5. This is still acidic enough to work but is on its last legs. This tells me that the safe capacity at this strength is around 20 8x10s per liter. In the future I'll simply mix a double batch if I think I'll be printing more than 15 or so 8x10s - both Sodium citrate and Citric acid are relatively cheap and I definitely want to avoid the stop bath crashing, which it will do quickly as it reaches exhaustion.
Wikipedia says 5 different compounds are all (sloppily) referred to as "Sodium citrate". The chemical name for what I got from Artcraft is Trisodium citrate dihydrate, CAS 6132-04-3, and is the version referenced by the online Citrate buffer calculator. I assume you could use any of the other types of citrate by adjusting the formula to correct for molecular weight differences, but it's easier to spend $10 (plus shipping) and buy a pound of the right stuff from Artcraft - Mike's a great guy and can always use the business. A pound of Sodium citrate and half a pound of Citric acid will make around 26 liters of buffered stop bath.
For reference, here's the link to the Citrate buffer calculator: https://www.aatbio.com/resources/buffer-preparations-and-recipes/citrate-buffer-ph-3-to-6-2
