[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