I am a chemist by training, though not an expert in photographic chemistry. However, I was reading about properties of developers one day and it occurred to me that an ascorbate/para aminophenol developer adjusted to pH 9 or so should activate the para aminophenol (perhaps weakly) but not the ascorbate, and that the ascorbate should regenerate the para aminophenol under these conditions. Does this sound something like what you are describing?
One more thing. I understand that when ascorbate is oxidized it increases the acidity of the solution. If so would a developer such as I outlined above have a compensating effect by lowering the local pH in the gel in the areas of most active development, thereby inhibiting development in those areas?
Alan
Alan;
Yes, this might work. It is not super additivity but rather probably fits into the realm of the ET (Electron Transport) developer mechanisms. However, if the pH drops then the activity could fall off overall.
There can be no doubt that the phenidone-ascorbate pair is synergistic at pH 8.8 without sulfite. How does one differentiate between the synergism of electron transfer and that of superadditivity? What if I added sulfite to the PC-TEA working solution. Would I expect to find an increase in activity even if I kept pH constant? So far as I know, there is no ascorbic compound equivalent to hydroquinone monosulfonate. I am asking questions, not being derisive. I think one could make a PC-TEA-sulfite working solution, test it, and then make a PC-TEA solution with the same pH by adding a little TEA to the working solution without sulfite. Concentrations of P, C and the pH would be the same in both, the only difference being some amount of sulfite.
You can demonstrate ET effectively by using a very very tiny quantity of one (the most active developer) and a very large amount of the less active or inactive developing agent.
One is quite active by itself, but in too low of a quantity to do more than just barely work and the other does virtually nothing by itself.
This argument is only true at the chosen pH.
That is one method of testing.
It is hard to differentiate between synergy and ET development. In synergy both developers are used at a relatively similar concentration and could do the job alone, so for example many developers with HQ and Metol can be reformulated to use just Metol at the right concentration. And they use each at about the same amount.
For the PC-TEA-sulfite system I developed Delta 100 in PC-TEA 1:100 with added sulfite 0,10,30,70 g/L 17.5min 68F as noted in a previous thread.Sulfite slightly raised the pH to 9.5,but if there was any superadditive effect it was not detected as all the negatives taken at the same EI have the same density by visual observation.
I might add to my previous post that one of the ingredients in an ET developer need not even be a developer. And, actually that was the case in Kodak's first use of this type of development. One reagent did not develop silver under any conditions, but very good development took place when a tiny tiny amount of a developing agent was added at the right pH.
G.W.Crawley investigated the effect of minimal sulfite, about 0-5 g/L, in sulfite metol system on his way to FX-1. The minimal sulfite made a lot of difference here.I wonder if minimal ascorbate has similar effect with metol.
This is the gist of what Mees and James report from experiments many many years ago and what Patrick has excerpted the figure from. They found that any sulfite in the presence of Metol or HQ will speed up their individual reactions due to the fact that Sulfite reacts with the oxidized forms of these developers.
On top of that HQ and Metol are superadditive, and so you have a multiplicity of effects going on.
But Metol and hydroquinone are not superadditive without sulfite. That also is shown in the graph in Mees and James.
The ratio of ascorbic acid to Phenidone in PC-TEA is about 40 to 1 by weight. I don't know the molecular weight of Phenidone but that of ascorbic acid is 176. That was the ratio at which the increase in activity levelled off as ascorbic acid was added to 0.05 moles/liter of phenidone. I dont remember where I got the molecular weight of Phenidone for that experiment, nor what value I used. I am looking at the graph. I will look up the text. I'm sure you know the MW.
I mixed an ascorbate clone of FX-1 where sodium sulfite was replaced by sodium ascorbate: Metol 0.5g ,Sodium Ascorbate 5g ,Sodium Carbonate 2.5g ,water to 1L ,pH about 11. Delta 100 was developed in this 18m 68F. It gave somewhat overdeveloped negs with a high EI.
It is known from Crawley's experiments that if there is no sulfite the action with metol and carbonate alone is very slow.Therefore it follows that both the sulfite in FX-1 and the ascorbate in the clone must be participating in the development,but if by chemically similar reactions seems not to be known.
BTW this clone of FX-1 is similar to a developer listed by Patrick Gainer in his article "Vitamin C developers" at Unblinkingeye.com.
Originally Posted by alanrockwood
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