Sodium metabisulfite = sodium bisulfite

haryanto · May 12, 2008

i hard to find sodium bisulfite, is there someone knew that's sodium metabisulfite is sodium bisulfite, many thanks

gainer · May 13, 2008

The Darkroom Cookbook says one can be substituted for the other weight by weight. My issue of the CRC Handbook of Chemistry and Physics does not show a metabisulfite that I can find. It only shows Sodium Hydrogen Sulfite, and no common name. My 81 year old brain doesn't remember why this is so. Most cases of it's use are not so critical, and IIRC there is some reason why it all tends to be the same thing soon after the atmosphere gets to it anyway.

haryanto · May 13, 2008

thanks , your 81 year old brain still great, to share lot of knowledge here, many thanks for that, i learn a lot from you

jochen · May 13, 2008

Hello harryanto,
sodium bisulfite is an old name for sodiumhydrogensulfite NaHSO3 and does not exist in solid form. In the solid form the formula is Na2S2O5 which is Sodium disulfite and this has the old name sodium metabisulfite. So if in your formula sodiumbisulfite (in solid form) is required, take the metabisulfite. In aqueous solution (this form is readily commercially available, in German the name is "Bisulfitlauge") there exists the NaHSO3. In old formulae you will more often find the potassium salt K2S2O5.

Ian Grant · May 13, 2008

Jochen, bisulphite & disulphite are the same, the bi- or di- just means two. Similarly Sodium Bichromate and Sodiun Dichromate are identical.

Sodium Bisulphite/Disulphite is not the same as Sodium Metabisulphite, the Metabisulphite is a far better anti-oxidant so is preferred as a preservative in developers. In fact it's also used to preserve peeled potatoes and stop them going brown, and used in the wine industry to kill wild yeasts

However in practice once dissolved Metabisulphite decomposes to the Bisulphite. The Bisulphite is rarely sold in solid form now, but I do have some on my chemical shelves.

Many formulae require Metabisulphite because it gives off SO2 in solution which forms a weak acid.

Ian

haryanto · May 13, 2008

thanks Jochen
Ian how to decompose it metabisulfite to bisulfite?

Ian Grant · May 13, 2008

It decomposes in water, most people just use the same weight of metabisulphite.

Ian

jochen · May 13, 2008

Hello Ian,
you are absolutely right "bi"means the same as "di". But the substance you have in the shelf in solid form under the name "bisulphite" is Na2S2O5 and that is the same as "metabisulfite".

Ian Grant · May 13, 2008

No Jochen, I have Sodium Bisulphite, as well as Sodium & Potassium Metabisulphite. I'm well aware of the chemical differences.

Usually what is sold as solid Bisulphite isn't pure and contains some Metabisulphite. but fresh Metabisulphite gives off far more SO2 in solution and has better preservative properties.

Ian

Ian Grant · May 13, 2008

To clarify, a manufacturers MSDS sheet states:

Sodium Metabisulphite

Composition/Information on Ingredients
Ingredient CAS No Percent Hazardous
Sodium Metabisulfite 7681-57-4 100% Yes
Sodium Bisulfite 7631-90-5 < 0.001% No

while:

Sodium Bisulphite

Composition/Information on Ingredients
Ingredient CAS No Percent Hazardous
Sodium Bisulfite 7631-90-5 58 - 99% Yes
Sodium Metabisulfite 7681-57-4 1 - 42% Yes

So if a Formula specifies Metabisulphite then you shouldn't use Bisulphite.

Ian

dancqu · May 13, 2008

haryanto said:
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

mikemarcus · May 13, 2008

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)

haryanto · May 13, 2008

nice discussions, thanks to u all

john_s · May 14, 2008

Ryuji Suzuki has some explanation on his site. Go to the page below and choose "bisulfite."

http://silvergrain.org/wiki/Special:Allpages

2F/2F · May 14, 2008

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?

dancqu · May 14, 2008

2F/2F said:
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

Ian Grant · May 14, 2008

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.

Ian

2F/2F · May 14, 2008

dancqu said:
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.

dancqu · May 14, 2008

[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

Ian Grant · May 15, 2008

dancqu said:
[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.

Ian

gainer · May 15, 2008

The molecule of sodium bisulfite according to the CRC Handbook is NA2S2O5. Add one molecule of H2O and you get 2 molecules of NaHSO3. What is all the fuss? It appears that metabisulfite is anhydrous sulfite as far as molecular weight is concerned. If the solution itself is supposed to "know" the difference, I don't see how. Maybe if you had plain water and added a molecular weight of the metabisulphite the solution would say to itself "Hey, I'm missing a mole of water!" whereas it would see its volume increase if you fed it 2 moles of sodium bisulfite.

Ian Grant · May 15, 2008

Using Metabisulphite as opposed to Bisulphite is important in some commercial applications, and also in photographic solutions that typically specify 20g - 50g per litre Metabisulphite.

The reason is simple, Metabisulphite in solution liberates SO2. In low concentrations the equilibrium between Metabisulphite and Bisulphite is almost totally towards the Bisulphite. This changes in acidic solutions.

In higher concentrations the equilibrium is more biased towards Metabisulphite.

Interestingly most of the Kodak formulae I have in my database use small amounts of Bisulphite, less than 10% and most are alkali, so it would make little difference whether Metabisulphite or Bisulphite is used. Kodak packaged and sold Bisulphite which was a mix of the Bisulphite and the Metabisulphite

So using pure Metabisulphite is more efficient as anti-oxidant/preservative properties because of the SO2 liberated. Commercial Bisulphite can contain as little as 2% Metabisulphite. It's far less important when your adding Metabisulphite or Bisulphite to an acidic solution.

Over the years I must have uses many metric tonnes of Metabisulphite, I needed maximum SO2 generation. Using commercial Bisulphite would have been inefficient.

If a formula or process specifies Metabisulphite it usually because of the greater purity and efficiency.

Ian

gainer · May 15, 2008

Maybe so, but I have a difficulty. If one puts a certain number of moles of the metabisulfite in one pot and twice that many moles of bisulfite in another and adds water to the same level in each pot, is there any measurement you can do that will identify which pot had the metabisulfite added? Will it be SO2 content, pH, color, what? If the chemical reaction you are about to start depends on SO2 content, it seems to me that either it will get the needed SO2 from either or will get it from neither.

Kirk Keyes · May 15, 2008

gainer said:
Will it be SO2 content, pH, color, what?

pH - that's the one...

Bisulfite is the acid salt form of sulfite. You can see this in the formulas - sodium sulfite is the di-sodium salt, and bisulfite is the sodium hydrogen salt.

Since you have the acid salt form of sulfite, the pH is lower, and that's why you are driving sulfite (as SO2 gas) from the solution.

Think about trisodium, disodium, and monosodium phosphate and what that does to pH - and the names associated with each - tribasic, dibasic and monobasic. And then think about sodium carbonate and bicarbonate - sodium carbonate is the disodium salt (i.e. dibasic and monobasic). And then think about sodium sulfite and bisulfite - one's dibasic and the other is monobasic.

Kirk Keyes · May 15, 2008

Sorry - misread question. Please disregard the previous post....

Sodium metabisulfite = sodium bisulfite

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