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Preservation of Ascorbate Developers by Patrick Gainer Probably the first thought this title brings to mind among photographers who do their own processing is “sulfite”, followed by “air tight containers”, followed by “marbles.” Developing agents are, after all, reducing agents that become oxidized by reducing silver halides to metallic silver, and are also subject to aerial oxidation. Sodium and potassium sulfites are oxygen scavengers that are supposed to take up oxygen in the solution before it can oxidize the developing agents. It also makes sense to keep as much air out of the container as possible. If I tell you that’s all wrong when it comes to ascorbic acid, you will say I’ve lost my marbles. Why Ascorbate Based Stock Solutions Fade Away Ascorbic acid is a powerful scavenger of oxygen. In its role as antioxidant, ascorbic acid takes in an oxygen ion, gives it two hydrogen ions (protons), and spits out a molecule of water. The ascorbic acid does not gain an atom of oxygen by being oxidized in this manner, but it loses two protons, becoming dehydroascorbic acid. (It’s a bit like calling a minus-blue filter yellow.) This reaction occurs even in acidic solutions. The resulting dehydroascorbic acid is an oxidizer, at least in a certain pH range. In animals it is capable of crossing the blood-brain barrier while ascorbic acid is not. On the brain’s side of the barrier there are enzymes that reduce the dehydroascorbic acid to ascorbic acid. It cannot pass back through the barrier until it has been oxidized. Dehydroascorbic acid in the body has a half life of about 6 minutes. If it is not immediately reduced to ascorbic acid it decomposes into diketogulonic acid and becomes useless. I do not know if that decomposition occurs in photographic developers. I have read that it is much slower at low pH. We do not have the body’s enzyme(s) in our stock solutions. The dehydroascorbic acid that forms in an acidic stock solution may keep for a long time, but it will begin to decompose as soon as the working solution is prepared, due to the increased pH. Worse yet, it has been oxidized and now seeks to be reduced. (Pardon the anthropomorphizing.) It is possible that phenidone will reduce the dehydroascorbic acid to ascorbic acid and be oxidized in the process. Phenidone is worth about 40 times its weight of ascorbic acid in a superadditive combination.. That is, if a solution contains 1 gram of phenidone and 50 grams of ascorbic acid, only about 40 grams of the acid will make any appreciable difference. The rest will not be noticeable at the pH we use in working solutions. Oxidizing 0.1 gram of phenidone will have about the same effect as oxidizing 4 grams of ascorbic acid. Supposing that 1 mole of oxidized ascorbic acid can reduce 1 mole of phenidone, a stock solution of 40 grams ascorbic acid and 1 gram phenidone would be reduced to about 1/5 of its original strength after oxidation of 1/40 of the ascorbic acid. This is not the way superadditivity is supposed to work! Certain metals in solution act as catalyst for the oxidation of ascorbic acid. The most difficult to get rid of or thwart seems to be iron, which is very common in tap water. Ryuji Suzuki (see Ascorbate Developers) has found that salicylates will sequester the iron, making it inactive as a catalyst. Add iron catalysis to headaches and heartaches as things aspirin will cure. Oil of Wintergreen, which you find in liniments, is Methyl salicylate. Neither sulfite nor bisulfite are sure-fire protectors of phenidone or ascorbic acid against oxidation. Perhaps the worst part of this problem is that most ascorbate developers do not give fair warning by changing color before they go really bad. Why then fool around with ascorbates? As we have seen, the byproducts of oxidation of ascorbates are acidic and act as an inhibitor that increases in regions of greatest development activity. Ascorbate developers are thus naturally high acutance, compensating developers. Furthermore, when used without sulfite, they are direct developers with little or no tendency toward solution physical development. I do not know if my explanations are correct. I do know that the presence of water is what allows these and other possible reactions to take place. That is why we try to keep our dry chemicals dry. Then we put them in water and expect them to keep to themselves until we feed them some exposed film. I believe I would not have to try to explain why ascorbate developers fade away if we could keep water out of the stock solutions. A Clue to the Cure When Kodak introduced HC110, I do not recall that there was any publicity about the keeping qualities of HC110 concentrate, but everyone who uses it regularly has come to know of them. Besides the long shelf life of its concentrate, there is another characteristic one would not expect from such a concentrated solution. It will not develop film. A thoroughly exposed piece of film placed in HC110 concentrate will not blacken in the time used for normal development with dilution B. Try that with Rodinal concentrate! This unexpected lack of activity of the HC110 concentrate is due to the fact that the solvents that hold the ingredients in the concentrate are organic liquids that do not hydrolyze and do not cause the ingredients to ionize. It is as if each molecule of the ingredients were in its own tiny, inert, water soluble capsule. Without water, the ingredients are protected from each other and from dissolved oxygen. Add water to make the working solution and the walls of the “capsules” are dissolved and dissipated. The ingredients can act as they would have if originally dissolved in water, but the solution has to maintain its efficacy only long enough to do a roll of film. We have been protecting our stock solutions against the wrong thing. Air is not the culprit. You thought that the alkali part of the developer was the activator? Not at all. Water is the activator. No water, no action. I have found that phenidone and ascorbic acid are sufficiently soluble in hot propylene glycol to make a concentrated stock solution containing 100 grams of l-ascorbic acid and 2.5 grams of phenidone per liter. There is no precipitation upon cooling. This stock has very great resistance to aerial oxidation. It is photographically inactive until you add water and alkali. Diluting one part of it with 50 parts of a 0.5% solution of sodium carbonate in water makes a very good developer. The question as to what protects what from oxidation is avoided. There is no sulfite in the stock solution. There is no need for any in the working solution. It is a strong surface developer as well as a direct developer with little or no possibility of solution physical development. It provides excellent sharpness, fine grain and good gradations. Formulas applying this idea to get sulfite-free stock solutions of other developing agents, including catechol, hydroquinone and pyrogallol are to be found in my article entitled “The Role of Antifreeze in Photographic Science”
in Photo Techniques Magazine of March/April 2004. |
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