Ian how to decompose it metabisulfite to bisulfite?
Na2S2O5 + H2O = 2NaHSO3. In words one meta molecule is
composed of two bisulfite molecules minus one molecule of
water. The formation of one over the other depends upon
the temperature of crystallization.
They are NOT equivalent by weight by one molecule of
water. They are though so nearly the same as to be
interchangeable; at least for our purposes. Neither
has an advantage over the other. Consider the
two to be equivalent for all purposes.
Add water to the meta. The fully hydrated bisulfite
will form. Dan
If you want to be totally accurate in your measurements, 0.913g or metabisulfite can be substituted for 1g of bisulfite (or is it 0.931g? I cant remember)
As to their interchangeability, I do not know. Common sense would dictate to use the one that is called for, because there is a reason it is called for; and if it was either/or, then the recipe would probably say so. However, a lot of sense in photography is uncommon, so I can't make any definitive statements. I don't know much about chemistry any more...and whatever I did know at some point was not general, but specifically related to the performance one particular job I had. I just follows da destructions and things come out OK...
However, I just bought the bisulfite, as well as the metabisulfite, from Photographer's Formulary, so there is your source. Although I believe that "di" is the modern-day "official" term for "bi", it is listed in their catalog as *bi*sulfite.
I use the disulfite as a buffer; for contrast control in continuous-tone litho film developer formula LC-1. I use the metabisulfite in reversal processing according to Ilford. The source I used for the LC-1 recipe states that the bisulfite solution (Part B of the LC-1 formula) is used to cut the alkalinity of the developer solution (Part A), so that would make it acidic, right?
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2F/2F
Last edited by 2F/2F; 05-14-2008 at 05:08 AM.
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As to their interchangeability, ...
The source I used for the LC-1 recipe states
that the bisulfite solution (Part B of the LC-1 formula)
is used to cut the alkalinity of the developer solution
(Part A), so that would make it acidic, right?
As I and others have said they are interchangeable.
That one additional molecule of H2O does not alter
the chemical's reactions.
The developer does not become acidic. It becomes
a little less alkaline. The formula is not unlike that of
D-76. Hydroquinone is not an active reducing agent
in that formula especially at the lower ph.
D-25 may do as well as LC-1. It is without the
hydroquinone and works at a low ph. Even D-23
at 1:7 dilution I'd think would be a good
substitute. Dan
Sodium Bisulphite and Sodium Metabisulphite aren't always interchangeable. The mistake is to assume that when dissolved the Metabisulphite instantly forms Bisulphite, in fact it's an equilibrium reaction, some of the Metabisulphite will in fact form SO2 and H20 in some solutions.
So while you may be OK substituting Metabisulphite for Bisulphite, the other way around is not really advisable. Metabisulphite is a stronger anti-oxidant.
The developer does not become acidic. It becomes
a little less alkaline.
I was speaking of part B; not the working solution.
So, we are in agreement. To quote myself: "the bisulfite solution (Part B of the LC-1 formula) is used to cut the alkalinity of the developer solution (Part A)"
To make myself clear, I am not looking for a substitute for LC-1. From what you have said, the killing of the hydroquinone as alkalinity decreases is probably exactly what makes LC-1 great. You WANT the hydroquinone there so that you can tell it how active to be in relation to the metol. The two solution nature of LC-1 is why I use it. This gives it almost unlimited versatility, and very good repeatability, even when reused, which most film developers do not. With the wide range of various contrasts required for various different types of contact printing emulsions, I appreciate LC-1s flexibility and ease of use. Two stock solutions, one of which develops to high contrast if left to its devices, the other of which lets you selectively (and finely) tame the contrast.
To me this works much better than trying various dilutions of prepackaged film developers along with very small amounts of powdered bisulfite additives to suit the situation. I know it is the same thing, but I simply find LC-1 easier, more consistent, and a ton cheaper. I will often print the same neg. on regular paper and cyanotype, or pull out old negs that were shot for silver prints, but which I now want to print as cyanotypes. This means that I often need to do a lot of contrast adjustment from the original neg. to get the litho that suits the particular emulsion I'm using. LC-1 lets me do this cheaply and easily.
[QUOTES=Ian Grant;629761]
"Sodium Bisulphite and Sodium Metabisulphite aren't always
interchangeable. The mistake is to assume that when dissolved
the Metabisulphite instantly forms Bisulphite, in fact it's an
equilibrium reaction, some of the Metabisulphite will in fact
form SO2 and H20 in some solutions."
A mistake to assume "instantly". Less of a mistake to assume
there be some little time until "equilibrium" is reached. As species
several forms will coexist at any instant in time. The warmer
the solution the greater the population of the meta.
"So while you may be OK substituting Metabisulphite for Bisulphite,
the other way around is not really advisable. Metabisulphite is
a stronger anti-oxidant."
How can you say that? The meta molecule has 2/3 more oxygen
than the bisulfite molecule. Dan
[QUOTES=Ian Grant;629761]
How can you say that? The meta molecule has 2/3 more oxygen
than the bisulfite molecule. Dan
Yes but the Oxygen isn't "free" O2, and at no point in a Bisulphite or Metabisulpite reaction is Oxygen ever liberated.
In fact just the opposite Metabisulphite is used as an Oxygen scavenger in industry. Try a Google search for "Metabisulfite Oxygen scavenger" and you'll find plenty about it.
Metabisulphite is a very powerful reducing agent and is used to convert solutions of Gold Chloride back to fine Gold powder, which precipitates out rapidly.
So in some photographic applications Metabisulpite is used as a preservative, because it absorbs any dissolved oxygen in the solution. It does this as it disassociates into Bisulphite also forming SO2 + H2O. The hydrated (dissolved) S02 is just as important.
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