Mr. Scudder - thank you very much for your comments. I have read many of your posts over the years, and appreciate your expertise. I'm going to treat your points at length, if you don't mind. These are all points that I have considered, so I have something to say on each, but I did not want to include them in my "short description" document as it would no longer have been "short." Most practitioners would not have the chemical background necessary for the discussion of these points, nor even if they did an interest in discussing them.
the testing you do seems inadequate, to me at least, to authoritatively indicate that, 1) complete fixation is occurring (i.e., residual silver levels in the film after fixation)
Yes, I agree, I need to do more testing to confirm some things I believe to be true, but have not verified. My testing thus far has been to measure clearing times vs. concentration, and also I did some exhaustion tests, again measuring clearing time at high dilutions, but where I used repeated sections of film in the same solution until I achieved the same ratio of film to solution as you would see for a full roll of 36 exp. 35mm or 12 exp. of 120 film in the same volume. I have not yet done residual silver tests on the fixed film to confirm that the undeveloped silver salts have been removed to desirable low levels, but I agree that should be done. I wanted to leave that as a final step in development, before buying the reagents. I have the formula for Kodak Residual Test Solution ST-1 in my copy of Publication J-1. I was thinking of using that, instead of the selenium toner variant, since I have no need of selenium toner and selenium is a bit more hazardous to handle and dispose of than sodium sulfide. If you have any advice on ST-1 vs. the selenium toner technique, please let me know.
that fixing bath 2 really stays at a very low level of dissolved silver (e.g., under approx. 5 g/l) after a long period of replenishment (i.e., testing dissolved silver levels in bath 2 over a long period of time).
I agree that testing residual silver in bath F2 after repeated cycles would be a good confirmation that I'm not getting build up of too much soluble silver. I could do that with a hypo-check solution, and do plan to do so. However, I've done some estimates that indicate build of of soluble silver to significant levels would be very unlikely. Here is my logic, I would appreciate your comments. One roll of 36 exp. 35mm film, or 12exp 120, is about 80 in2 of film. Per information I've found from multiple sources (including past comments of PE on forums), regular b&w negative film may have 3-5 grams of silver per m2 of film. Let's take 4 g/m2 as representative. For an 80 in2 roll, this is 0.21 g of silver. Let's assume on average 50% of the silver forms the image (reduced to metallic silver in the developer), so that 50% is undeveloped and needs to be removed by the fixer (I've also seen multiple sources with this estimate). This means about 0.10 grams of silver will be dissolved out of the emulsion by the fixer, per roll. I further assume at least 90% of the silver is removed by the fixer at 1.0X clearing time (again, I have references for this in the literature; actually the value is usually higher than 90%). This leaves 10% unreacted, to be removed in my second fixer bath F2. So, for every roll that enters F2, it brings 10% x 0.10 g/roll = 0.01 grams of silver that becomes soluble silver in F2. Let's further say that I am using a 500 ml working volume of F2, so that for every roll of HP5 I process I take 100 ml of the 500 ml (1/5 or 20%) out of the F2 bottle at the beginning of processing and use that 100 ml to prepare my dilute F1 solution. This "flush" of F2 removes 20% of the soluble silver content of F2. I now return F2 to the full 500ml with fresh 1+9 solution (which contains no silver). After this first "flush" I have remaining in the bottle of F2 80% x 0.01g or 0.008 grams Ag. Then, the second roll brings in another 0.01 grams of silver, combined with the 0.008 grams from the first roll leaves 0.018 g Ag. You can do some algebra to show (or do a simple spreadsheet and copy 20 lines or so to see the convergence) that this repeated process will converge to a constant value, and F2 will never contain more than 5X the amount of silver brought into it by one roll of film (again, assuming bottle volume of 500 ml and removal of 100 ml per roll to make up F1). In other words, because of the regular "flush" and replenishment that I have designed into the method, you can never have more than 0.05 grams of silver in the 500 ml volume of F2, when 100 ml is removed and replenished for each roll. On a concentration per liter basis, (since my bottle is 500 ml or 0.5L) there can never be greater than 0.1 gram of silver per L of solution in F2, which is quite a low value. Now to examine the silver content of the first bath, F1. The estimate of the silver content in F1 is 0.1 g/L x (100/1000) = 0.01 g Ag in 300 ml, or 0.033 g/L to begin, before the roll is processed (if using a 300 ml tank), and 0.033 g/L + 0.10 g/roll x (1000/300) = 0.37 g/L of Ag in F1 when it is discarded. Compare these silver levels to the 8-10 g/L Ag level that can be tolerated "without serious effect" on fixation that has been reported in the literature, or the 5 g/L you cited, or the 1-2 g/L that has been suggested as a very low maximum silver content in the fixer to assure archival fixation. If my assumptions and math are valid (and I welcome your critique), it seems to me that it is very likely that F2 can be relied upon to achieve archival quality fixation, no matter how many cycles I go through. However, as you suggested, it would be advisable to confirm these estimates with tests of residual silver levels in both F2 and in the film itself.
Incidentally, I also calculated the stoichiometric requirements of ammonium thiosulfate in the solutions given the amount of silver that needs to be complexed, and found that in the dilutions I am using I still have a good margin above the stoichiometric requirement in F1 when it is discarded. This is tentative, since there is no simple formula given the complex chemistry and multiple complexes that can be formed, but I used one proposed formula. While this is of course necessary, it is not sufficient, due to the "ionic pressure" point you mentioned, which means you need a quantity in excess of stoichiometric to drive the formation of the desired complexes.
more dilute fixer works as long as fixing times are adjusted accordingly to a certain point. After too much dilution, the ionic pressure (or something like that; I'm remembering here, not looking things up) becomes too low for the thiosulfate ions to react adequately with the silver compounds in the media (film/paper) and fixation is not able to proceed through the various steps needed to result in soluble compounds. Maybe your second bath, which is stronger, will do the job
Yes, I've read some of the literature on this point. I mentioned this explicitly in the document I asked you all to read. This is a major reason that I decided to use the second fixer bath at a substantial concentration of 1+9. While 1+9 is only half the concentration of the 1+4 recommended by Ilford for film, I know some people (such as yourself) have routinely used 1+9, and sometimes a bit more dilute, fixer baths for film and paper with apparently good success. I believe the term you were looking for here is ionic strength, but pressure is a good analogous term. A minimum level of ion concentration is needed to form substantial concentrations of certain complexes, and if you go too dilute the concentration of those complexes will be too low. If you don't form the desired complexes, the silver doesn't wash out of the emulsion to the desired degree. It is my supposition that 1+9 is sufficient to form the silver thiosulfate complexes that are highly soluble and so easily removed into the fixer or during the wash step. I don't know this for certain, but since 1+9 is recommended for paper I feel it is a reasonable assumption, pending confirmation as you suggested by measuring residual silver in the film after washing. Alternatively, if it proves that 1+9 was not sufficient for good complex formation, I could go to using 1+4 as F2. Since F2 will be nearly like fresh fixer for every roll, 3:00-5:00 minutes in 1+4 F2 would actually give complete fixation in of itself (i.e., 3-5 minutes in fresh 1+4 fixer is what Ilford recommends for a single bath fixer), so I thought this a bit extreme, though obviously it would work. Using 1+4 F2 would mean i would be flushing only 50 ml out of that bottle per roll (to make 300 ml of F1), which is 1/10 flush, so my residual silver levels could rise to 10X the amount carried in by one roll, or 10 x 0.01 = 0.1 grams in the 500 ml bottle, which is 0.2 g/L. Still a rather low value, should not be a concern to insure archival processing. However, I was more comfortable using the 1+9 dilution for the higher volume of "flush" that it implies.
Maybe the chemists here on the forum can contribute some testing suggestions and point out other possible foibles, which I have certainly overlooked, as well.
I am not a chemist, but I am a chemical engineer (BSE Princeton, MSChE MIT) with over 40 years experience in doing chemical research, and even longer (50 years) doing darkroom work. I've had several years of academic training in chemistry, however, I am not a photographic chemist, so I may be making some errors and would be glad to hear anyone's suggestions. Thank your very much for your excellent comments, and I look forward to further comments from you, and from others here as well. - Richard.