Ascorbate Developers
From Silvergrain Labs
A photographic developer is usually an aqueous solution of reductants to selectively reduce sufficiently exposed silver halide crystals having latent image center, to metallic silver grains. Although there have been many reductants used in photography, for many practical reasons, including stability, economy, and ease of preparation, compositions of commercially available developers have been almost exclusively metol and hydroquinone combination or phenidone and hydroquinone combination. These combinations have superadditive property and are usually preferred. However, more recently, there is rising concern for toxicity and environmental load of hydroquinone and other dihydroxybenzene compounds. Therefore, photography industry has attempted to replace hydroquinone with more environmentally sound compounds.
See US Patents 2688549, 3942985 and references therein for details. Dozens of US Patents discuss ascorbic acid based photographic developers, mostly dated in 1980s and 1990s. These are mostly concerned with production of developer concentrates for volume processing of industrial and medical photographs (e.g. radiography) rather than artists.
Image quality
Ascorbate film developers are capable of excellent image quality, with significant improvement from MQ or PQ formulations. For example, Eastman Kodak product called XTOL is a film developer producing negatives similar to those produced by D-76 developer, except that the image is sharper and finer grained. (US Patent 5756271) However, XTOL has a few serious problems, at least one of which remains today.
Ascorbic acid, its isomers, derivatives and salts are in some aspects similar to hydroquinone in developing power and pH region where reaction is activated, among others. However, this similarity is very superficial.
Sharpness of hydroquinone developer is not very high, because hydroquinone in MQ or PQ system reduces adjacency effect. This is probably because the oxidation products of hydroquinone (ones that are exhausted after developing reaction) is alkaline. This may accelerate development in the area surrounding areas of intense development reaction. On the other hand, the oxidation products of ascorbates are acids, potentially inhibiting development in areas nearby the site of intense reaction. This means that ascorbates are more desirable when adjacency effect and compensation effect are sought.
The nature of the oxidation products also affects tonality. For example, when PQ developers were formulated, it was necessary to give great care to prepare buffered low pH solution to avoid the unnatural highlight tones, but because of the difference descibed above, ascorbate-phenidone developers are expected to give better tonality with relatively wider tolerance in preparation. Furthermore, this combination gives better sharpness and granularity compared to PQ counterparts.
Ascorbate print developers can be readily formulated for neutral and coldtone development, and the image quality is excellent. When properly formulated, the image tends to accept toners very well. Slightly warmtone ascorbate developers can be formulated, although hydroquinone and catechol are better suited for very warmtone developers.
Ascorbic acid has also been employed as a developing agent in scientific studies of development because its chemical behavior is simpler than hydroquinone, and thus more suitable for some studies.
Challange in ascorbate developers
Ascorbates present a major challange when it comes to formulating practical developers. It is the problem of stability, or shelf life, of the stock solution.
When a very pure ascorbate solution is prepared, the rate of aerial oxidation is known to be very slow and practically negligible. However, when a developer is mixed with ascorbate in darkroom, many stock solutions do not last long, and the solution may or may not discolor as the solution deteriorate. It is thought that a trace amount of impurities such as transition metal ions present in such working solutions is acting as an oxidation catalyst, which greatly accelerates the rate of aerial oxidation. Analogous problems are seen in food, beverage, and cosmetics industries.
One problem with impurities present in photographic developers is that, the "photographic grade" chemicals allow somewhat large amount of impurities. Such amounts of impurities are not a problem with most MQ or PQ formulae, but they are too much for ascorbate formulae. However, it is impractical to call for purer chemical stocks because they are much more expensive. Also, typical darkroom practice does not use enough precaution to prevent contamination of impurities from apparatus and water source, even if purer chemical stock were used. Therefore, a method of inactivating transition metal catalysts is necessary before realizing a stable, practical photographic developers using ascorbate.
Note: the above paragraph is often misquoted to mean that distilled water will solve the problem. This is untrue. Impurities can come from the chemical stoc as well as the water supply. ALL sources of impurities must be removed for this approach to work.
Non-discoloring problem
One frequent complaint against XTOL and other ascorbate based developers is that the developer solution does not discolor when it is dead. (Most MQ and PQ developers turn orange brown or dark brown when they are dead.) This problem is solved in DS-10 and its successors.
The right bottle is filled to the top, while the left bottle contains a small amount of DS-10. Both bottles were closed on 30 Oct 2005 and the photograph was taken on 9 April 2006.
Chelating iron
DTPA and polyamine polycarboxylic acids
Eastman Kodak used chelator called DTPA (diethylenetriaminepentaacetic acid) in an attempt to chelate the transition metal ions. The DTPA cannot inactivate the redox catalysis of iron regardless of the amount added. However, if the ration of DTPA:iron is very high, the kinetics of the overall process is slowed down compared to the solution without any DTPA. One problem is that DTPA binds with other metals, including calcium and magnesium found in water supply, and therefore the effective amount of DTPA present in the working developer solution can be significantly lowered if hard water is used to mix the developer.
Other common chelating agents, such as EDTA, NTA, citrate, etc. have the same problem. They may complex iron, but they do not inactivate iron's redox catalysis. (In fact, many ligands potentiate iron's catalyst power.)
At that time, my preferred chelating agent was triethanolamine and diethanolamine. These are more effective than those above, and they can be used as a buffering agent, but their efficacy in stabilizing developers was unpredictable depending on the formula, until much later time.
Phosphates
In the summer of 2002 I realized that ascorbic acid print developer lasted much longer if the solution contained phosphate and borate. Also, I found that ascorbate developers lasted longer when the solution pH was higher, which is opposite to most other cases. At higher pH, iron forms ferric hydroxide, which has low solubility, therefore reducing the amount of iron accelerating the aerial oxidation. With the presence of phosphate, iron binds to phosphate, making also very insoluble compound. Therefore, it is thought that higher pH or presence of phosphate effectively reduced active iron, and thereby prolonging the developer's life.
However, there are some other problems associated with this approach, and this approach is abandoned.
Salicylate
In the summer of 2003, I found a relatively recent research that salicylic acid is an effective iron chelator, and this form of iron is inactive as the oxidation catalyst. I compounded a few ascorbate developers incorporating salicylic acid, and observed how long the solutions lasted. The result was very promising.
Results of cyclic voltammograph indicates that salicylic acid completely inactivates Fenton reaction in water containing iron, and this may be a good evidence why salicylate was effective in stabilizing ascorbate developers. However, the puzzle did not end. In terms of stability constants for iron-sal complex, iron and salicylate forms stable complex in very low pH (Fe(II)-sal) or very high pH (Fe(III)-sal), and all the results I saw cannot be adequately explained by complexing mechanism.
One possibility is that salicylate may be forming a metastable compound with iron. Another possibility may be that the salicylate may be acting as a radical scavenger. All of these possibilities are not mutually exclusive, and so the answer might be all of the above. It's just that there is no good evidence to dissociate multiple hypotheses.
tannic acid and other polyphenolic compounds
Tannic acid makes complex with iron (II) and they are redox-inactive. Therefore, hydrolyzable tannic acid and related polyphenolic compounds were tested in developers. The solutions were relatively stable, but there were also some undesirable consequences with the use of these agents. Therefore, alternative agents are currently sought.
other radical scavengers
Sorbitol and other sugar alcohols are known to be a mild radical scavengers. Sorbitol in developer was a mixed results. It is probably that the sorbitol sometimes potentiated iron's catalyst activity. Another problem was that developers containing sorbitol tended to form more bubbles during inversion agitation.
Polycarboxylic acids, such as citric acid, tartaric acid, and the like are another group of inexpensive and readily available radical scavengers. They are somewhat effective, depending on the solution pH, in increasing the developer's keeping property, but the benefit is usually small. See Kunz, Pfaff, and Roman (1972) below.
compounds under current testing
Compounds that are currently tested include: heterocyclic nitrogen compounds (imidazole, tetrazole, pyridine, pyrrolidione, pyrimidine, histidine, uric acid, etc.), heterocyclic sulfur compounds, polymer macromolecules incorporating one or more of the above.
What can be done with ascorbate
Once the stability problem is completely solved, we will have a nontoxic photographic developer that is also lighter in environmental load, providing image quality exceeding those obtainable with ordinary MQ or PQ developers. One major disadvantage is increased cost, since hydroquinone is much cheaper than ascorbates. However, for fine art photographers, small increase of cost is not a major problem.
Ascorbate is useful in all types of b&w film developers except: classic lith developer and tanning developer. For example, ascorbate can be used in place of dihydroxybenzene agents in extremely fine grain developers, general purpose developers, push developers, accutance developers, graphic art developers (non-lith type) and x-ray developers.
Ascorbate is useful in all types of b&w print developers except ones that use hydroxybenzene compounds as the sole developing agents for extreme warmtone effect.
Patent Review
Kunz, Pfaff, and Roman (1972)
US Patent 3658527 disclosed a photographic developer, wherein one embodiment of the invention used a phenidone-ascorbate developer with no sulfite. They claim that addition of citrate or tartrate, preferrably in conjunction with nonreducing oligosaccharide such as sucrose and raffinose. In their Example 1, there is clear difference in the developer activity 24 hours after preparing the solution between with and without tartrate (both contained sucrose).
Tartrate (50g/l) may be acting as an iron chelator, but it may also be acting as a radical scavenger. Ascorbic acid itself is an excellent radical scavenger, and also an iron chelator. The fact that 50 g/l of tartrate is necessary to show this level of improvement is already indicating that it requires a much superior agent to solve this problem. (Of course, in their application of diffusion transfer process, sulfite can't be used, and it is already more challenging than conventional b&w developers.)
Parker, Lannon, Webb and Long (1994)
US Patent 5376510 disclosed a concentrated ascorbate developer in pH range from 9 to 11 (upon diluting 1+9) and comprising 60-110g/l ascorbic acid, at least one carbonate (150-220g/l), sulfite (100-200g/l) or hydroxide, wherein alkali metal cations are 20:80 to 0:100 in Na:K. The preferred solvent system is water. This invention is hardly nonobvious, since the invention does not go beyond a substitution of hydroquinone with ascorbate in existing PQ concentrated developers. The developer does not have any active means of stabilizing the solution, besides 68ml of 37% Na5DTPA (2.5 g/L in working solution). The developer in their examples 1 and 2 appear to be a typical neutral tone print developer. The patent of Parker et al is only a small annoyance in practice, since in most applications, it is easy to avoid these ranges of ingredients, and it is also easy to make Na:K to be 25:75, for example.
In the detailed description of Parker et al. (1994), they describe the invention to have the Na:K ration to be 50:50 to 0:100, but in their claim 1 (the only independent claim) the ratio is between 20:80 and 0:100. In the abstract, they describe the concentration of ascorbic acid to be 50 to 150 g/l but in their claim 1, the range is 60-110g/l. These are some traces of their claim being narrowed in the process of examination.
Okutsu (1996)
US Patent 5503965 discloses ascorbate developer for replenished machine use, where the pH of the replenisher is at least 0.5 pH unit higher than that of the developer (claim 1). In their claim 2, the replenisher pH is at least 0.3 unit higher and contains 0.5M carbonate or more. It is highly questionable whether claim 1 and 2 are considered nonobvious. Other dependent claims are also not worthy of discussion, except claims 3 and 6, where two families of antistaining agents are used in combination. One family is 2-mercapto-6-hydroxypyrimidine and its derivatives where 4 and/or 5 positions are substituted with usual hydrophobic groups. Another family is 2-mercaptobenzimidazole and its derivatives, wherein the benzene ring is sulfonated at at least one place. These are very interesting agents to use as an antistaining agent, but there is no mention of previous work related to these families of antistaining agents. I personally believe that the value of this patent is in these agents, and their claims 3 and 6.
Fitterman, Brayer and Rachel (1998)
US Patent 5738979 is probably the Kodak reaction to Okutsu (1996) from Fuji, and it discloses a similar replenished system. In this invention, the replenisher has only 0.1 to 0.25 pH unit higher than that of the developer (claim 1) but the concentration s of major ingredients in the replenisher are up to 50% more than those in the developer. In this patent, the preferred chelating agent is DTPA (column 5), and they claim that the silver deposit is less if chelating agent is added to the replenisher. This is rather uninteresting patent.
Opitz and Zawadzki (1998)
US Patent 5756271 discloses XTOL developer and its variants. Example 1 is XTOL provided as two parts dry powder kit:
| US Patent 5756271, Example 1, Part A | |
|---|---|
| sodium sulfite | 10g |
| DTPA | 1.0 g |
| sodium metaborate (8 mol) | 4.0 g |
| (tetrahydrate in modern nomenclature) | |
| Dimezone S | 0.2 g |
| US Patent 5756271, Example 1, Part B | |
|---|---|
| sodium sulfite | 75g |
| sodium metabisulfite | 3.5g |
| sodium isoascorbate | 12g |
Dimezone S is packed with metaborate, making pH of 10.19 when part A alone dissolved in 850ml of water. This makes dissolution of Dimezone S very easy. When both parts are dissolved in one liter of water, the resulting pH is 8.20. This is same as XTOL.
Examples 2 to 5 are ascorbate developers of pH 8.0, but of varying compositions and buffering agents. Examples 2 to 5 are not prepared as dry powder kits or liquid concentrate, and they are shown as ready-to-use developer formulae. Furthermore, developers shown in Examples 2 to 5 are not covered by their claims at all.
Fitterman and Dickerson (2000)
US Patent 6110655 is another ascorbate developer, but it is proposed as a kit, together with a non-rapid acid fixer and x-ray film coated on both sides. The developer part specifies pH of 9 to 12, free of dihydroxybenzenes, ammonium ions and comprises 0.1 to 0.3M ascorbic acid, 0.15 to 0.4M sulfite and 3 to 15mM of phenidone-like or other auxiliary developing agent. (Claim 1a) The above developer is uninteresting. On the other hand, claims 2 and 7 call for glycine (aminoacetic acid) up to 0.25M, and they used this weak developing agent to reduce the amount of Dimezone S as shown in Developing Compositions I and II in Table III.
This invention is extremely focused on medical imaging material and processing, to minimize discharge of ammonium ion, hydroxybenzene compounds, 3-pyrazolidone compounds, silver ion, borate, and possibly other elements that may be harmful to the environment.
Haye, Huston, Roussilhe and McGuckin (2002)
US Patent 6489090 discloses a stabilized ascorbate developer comprising 0.01 to 0.6M ascorbic acid, 0.01 to 1M sulfite, and alpha-ketocarboxylic acid wherein the molar amount of alpha-ketocarboxylic acid is 0.125 to 1.0x that of ascorbate, and 0.08 to 4.0x that of sulfite (claim 1). Claim 6 further specifies said alpha-ketocarboxylic acid to be pyruvic acid, oxalacetic acid or a salt thereof, in concentration of 0.05 to 0.25M.
Their example results show that the developer containing about 10g/L of pyruvate exhibited significantly less yellowing over the course of a week, compared to control developers containing glycine, alanine or no stabilizing agent.
Although it is interesting to see lower degree of yellowing in developers containing pyruvate, it is also seen in Table V that the percentage of remaining ascorbate is practically the same after 4 days of accelerated oxidation condition (70.1 to 71% in control and 73.4 to 75.8% with pyruvate). This is 0.01M difference in ascorbate concentration in a developer that contained 0.2M ascorbate. Is it worth it?
More interesting question is that, what is the mechanism of action of pyruvate in this invention. There is no detailed data on pH drop, but they state that "the pH of each solution was observed to decrease by 0.01-0.02 during the standing time" which is 7 days. It is because of high level of buffering that makes it difficult to observe the generation of acid from oxidation of ascorbate by the pH measurement. Since there is no photographic test result in this patent, it is difficult to exclude the possibility that the addition of pyruvate simply reduced generation of colored compounds during the course of ascorbate oxidation pathway.
Haye, Huston and Roussilhe (2004)
US Patent 6673528 discloses ascorbate developers containing mono- or disaccharide as a stabilizing agent. Such sugars include ribose, fructose, lactose, glucose and galactose. Ribose is preferred in 0.05 to 0.25M concentration. Although they did not name the same kind of sugar, Kunz et al (1972) already mentioned the use of sugars as a stabilizing agent, and this patent is not adding much to our knowledge at all. In particular, their claim 1 is extremely broad and noninformative. The protective effect of these sugars is also not worthy of many words.
Schwartz and Flavin (2004)
US Patent 6686135 discloses ascorbate developer prepared in split stocks: part A is pH from 3 to 6, and comprising 0.05M or more of ascorbic acid, 0.0005M or more of 3-pyrazolidone developing agent, dissolved in a water-miscible hydroxy-containing organic solvent having a molecular weight of 50 to 200, where the weight ratio of said solvent to water is 0.05:1 to 8:1. Part B is an aquaous solution having a pH from 8 to 12 comprising a borate buffer and at least 0.05M of sulfite. The detailed description of this patent also describes a single part prepareation, which is dropped from the claims (it is probably rejected during examination). As you see above, the key ingredient of this invention is use of diethylene glycol (or similar glycols or alcohols) in combination with typical ascorbate developer formulation. There is hardly anything novel or nonobvious in this invention.
