| | | -
[QUOTE=Ian Grant;809192]
Metabisulphite is used to control the pH, the SO2 forms
sulphurous acid in solution, in combination with Sulphite
it also has a buffering effect.
That assumes some SO2 does form in an alkaline solution.
We are dealing with the alkaline developer FX-15. SO2 will
form if the solution is acidic; first sulfurous acid then as the
solution becomes more acidic, free sulfur dioxide.
The Sulphite/Metabisulphite buffer range is between pH 8
and pH 6.5, becoming more acidic as the proportion of
Metabisulphite is increased,
From one extreme to the other the ph range can be as
great 10 + to near 3. Of course not much buffering
save for up OR down at those extremes.
As a preservative Metabisulphite is many times more
effective than Sulphite., which is why it's used in the
food trade and wine making. Ian
In general acidic environments do preserve better.
Patrick Dignan recommends dissolving phenidone in
a bisulfite solution.
FX-15 is a long way from being acidic. That ONE
HUNDRED grams of sulfite swamps any ph effect by
the ONE HALF gram of bisulfite, or for that matter,
the ONE gram of carbonate.
I'm quite sure the latter two could be left out and
nobody would notice the difference. Dan
-
So you're saying that Crawley and the chemists at Agfa, Kodak & Ilford etc don't know what they are doing when they put such small quantities of Metabisulphite etc in a developer 
Paterson/Crawley put less in one of their other developers, sometimes small quantities have disproportionally beneficial effects.
Acutol-S / FX-15 isn't a normal fine grain film developer, it was formulated specifically to give high acutance which it gives at the expense of a slight increase in grain and loss of tonality. It's no co-incidence that the other developers using these small quantities all give additional acutance.
Take the Metabisulphite & the Carbonate out and you'll have a developer with quite different attributes.
Ian
| |
Adv Reply
