[pure-silver] Re: Vitamin C and Hydroquinone?

----- Original Message ----- 
From: "titrisol" <titrisol@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, January 28, 2005 10:22 AM
Subject: [pure-silver] Vitamin C and Hydroquinone?


> Hi all,
> I've been following the ascorbic developers attently, for 
> a
> while, and have tried "perverting" rodinal succesfully.
>
> I've seen that most developers are either Phenidone-C or 
> Metol-C
> but I haven;t seen any Hydroquinone-C developers.
> Can someone clarify why? or guide me to a resource on this
> topic?
> - Is there anatagonism between these 2 agents?
> - Do they act similarly?
> - Is ascorbate enough with phenidone?
>
> Thanks
>
  Hydroquinone and Ascorbic acid have similar functions in 
developers. Together with Metol or Phenidone they act in a 
way that is known as superadditive. The combined developers 
are more efficient and long lived than either alone. The 
chemical relationship is complex but even where the 
Hydroquinone is inactive as a developing agent, as in D-76, 
it still acts to regenerate the Metol. Hydroquinone and 
Ascorbic acid are not antagonistic but do not mutually 
support each other to become superadditive. Also, neither is 
a good general purpose developing agent on its own. The only 
developers using Hydroquinone as the exclusive agent are 
extremely high contrast developers used for photomechanical 
work.
   I am not sure what you mean in the last question. If you 
mean does the combination of Phenidone and Ascorbic acid 
form a good, practical, developer, the answer is yes, and it 
does not need the addition of a third agent.
   Metol is a good developing agent on its own, as in D-23 
or D-25, but D-23 has a more limited capacity than D-76 
where Metol is combined with Hydroquinone. Phenidone is a 
very low contrast developer on its own as in POTA. It needs 
the help of another agent like Hydroquinone or Ascorbic acid 
to become useful as a general purpose developer.
   p-Aminophenol, the active ingredient in Rodinal, is 
similar to Metol but inferior to it as a sole developer. It 
is used in Rodinal because it is soluble in sufficient 
quantity to make highly concentrated stock solutions. It has 
not been exploited in non-concentrated developers because 
other agents, particularly Metol, are superior to it. About 
the only non-concentrate formulas are substitutes for Metol 
where there is a concern about Metol poisoning. Those with a 
sensitivity to Metol will do better with Phenidone and 
Ascorbic acid developers, which are not sensitizing and are, 
in general, better developers.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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