Sodium metabisulfite = sodium bisulfite

gainer · May 15, 2008

Bisulfite.

I wondered about that. It seemed like a high school chemistry problem, and I DID pass that. I got a 97 on the New York Regents Exam my junior year at Power Memorial Academy. I was pretty sure I counted the Na, H, O, and S atoms on both sides correctly. I got a 99 on the Regents Physics exam that same year.

gainer · May 15, 2008

I should also have consoled you a little. If you're going to make a mistake, that's the kind it should be-- one to laugh at.

dancqu · May 15, 2008

gainer said:
It appears that metabisulfite is anhydrous sulfite
as far as molecular weight is concerned.

Meta. Oxides of boron go through a series of dessicated
states; ortho, meta, tetra, and boron trioxide. Our interest
in boron's oxides is one of ph while with sulfur oxides our
interest is one of a redox nature. At least this thread.

I am still not convinced that the meta has any greater
potential as a reducing agent than the bisufite. The meta's
two sulfur atoms share 5 Oxygen atoms. The two bisulfite's
6th Oxygen is via hydration. Dan

gainer · May 16, 2008

dancqu said:
Meta. Oxides of boron go through a series of dessicated
states; ortho, meta, tetra, and boron trioxide. Our interest
in boron's oxides is one of ph while with sulfur oxides our
interest is one of a redox nature. At least this thread.

I am still not convinced that the meta has any greater
potential as a reducing agent than the bisufite. The meta's
two sulfur atoms share 5 Oxygen atoms. The two bisulfite's
6th Oxygen is via hydration. Dan

Exactly so. Neither has any reducing power without hydration. As soon as you add any measurable amount of water to 1 mole of sodium metabisulfite or 2 moles of sodium bisulfite, you can't tell the difference as long as you make the volumes, or better yet the weights of the solutions, the same. If you equalize the weights you need not worry about temperature.

Ian Grant · May 16, 2008

You're quite right in your chemistry, except there is a subtle difference between adding Bisulphite or Metabisulphite.

Metabisulphite forms an equilibrium with SO2 & H20 in solution, the SO2 can form Sulphurous acid (more probably SO2.7H2O) or Bisulphite. This is why Metabisulphite is used in water treatment as well as a food & wine stabiliser / preservative. Bisulphite can also be used but it usually needs acidifying.

Put another way, if you use a Camden tablet (which are Metabisulphite) to sterilise grape juice before fermentation it needs to be fresh to work. If it's been left unsealed and has partially decomposed to the Bisulphite form it is far less effective.

The bottom line is that while commercial Bisulphite (the Bisulphite/Metabisulphite mix) is still available in large volumes, and analytical "Pure" Bisulphite in small quantities usually all we can buy today is Metabisulphite for Photographic use.

Ian

gainer · May 16, 2008

Weakening of the Camden tablets due to atmospheric exposure comes eventually through combination with water vapor and CO2, which are always present, at least when and where grapes are grown. A molecule of metabisulfite ( also called pyrosulfite) is converted to 2 molecules of acid sulfite (bisulfite) by 1 molecule of water. 2 molecules of bisulfite are converted by 1 molecule of CO2 to one molecule of H2CO2, a gas, and 1 molecule of sodium sulfite, which removes the acidity of the tablets as time passes under exposure to the atmosphere. It has nothing to do with whether it is meta- or bisulfite. The same fate awaits either.

SO2 in water is a solution of sulfurous acid. It forms when either bisulfite or metabisulfite is dissolved in water.

Ian Grant · May 16, 2008

gainer said:
SO2 in water is a solution of sulfurous acid. It forms when either bisulfite or metabisulfite is dissolved in water.

The salts of Sulphurous acid H2SO3 are theoretically formed by the aqueous dissolution of gaseous SO2. However there is strong spectral evidence that very little of the free acid exists in solution, instead dissolved SO2 forms a clathrate in which the SO2 is trapped inside a water shell without chemical bond formation - in a manner analogous to other gas hydrates. (Cotton and Wilkinson,1966). This structure is usually represented as SO2.7H2O and although there is discussion about the exact form in which it exists.

When you start delving deeply into the chemistry you start to find that what appear at first to be simple equilibrium equations are quite complex. Certainly all references state quite clearly that both Sodium & Potassium Metabisulphite are the principle SO2 generating salts, and preferable to the Bisulphite and Sulphite, none say why.

BASF in their data-sheets for Photo-grade Sodium Metabisulphite give a figure of 66g/100g for potential SO2, while for BASF's 40% aqeous Sodium Bisulphite it's 23g/100g (57.5g/100g equivalent)

The basic equilibrium reaction between Metabibulphite - Bisulphite - SO2 & Water is what's most imortant. It's clear that in commercial applications that you start with Metabisulphite because of it immediate properties of liberating SO2 in solution.

It solution it dissolutes into Bisuplhite, in an alkaline solution the Bisulphite equilibrium is towards Sulphite and in an acidic solution towards SO2.

Certainly the food industry prefers adding Metabisuphite as a preservative in most cases, although Bisulphite is added to fruit juices which are naturally acidic.

Ian

2F/2F · May 16, 2008

Now you guys are just making me thirsty.

Out of curiosity, and to change the subject a bit, what would happen to one if he or she drank a glass of developer, and why? What about fixer?

I have tried a drop or two of developer and it was actually not half bad. Fixer seems much worse, however. I've never tried that. Guess it's the animal instincts saying: "Enough is enough with the tasting of the chemicals!"

Kirk Keyes · May 16, 2008

Thirsty - that's not a good response.

Anyway, you should start a new thread about the inappropriate tasting of chemical solutions instead of hijacking this thread...

2F/2F · May 16, 2008

Sorry. I'll be sure to leave my sense of humor behind when signing onto this Website next time.

Relax. Have a beer. Or would you prefer an HC-110 or D-76?

gainer · May 16, 2008

If you put the metabisulfite in water, it is not so easily attacked by atmospheric CO2. It still becomes the equivalent of a solution of bisulfite. Surely the equilibrium point can vary with time, temperature, but when it is possible for a gas to form, especially if the gas cannot be dissolved by the amount of water present, the equilibrium is shifted. The addition of one molecule of water to one molecule of metabisulfite will produce 2 molecules of bisulfite. The interaction of 1 molecule of CO2 with 2 molecules of sodium bisulfite will produce 1 molecule of sodium sulfite and 1 molecule of H2CO3 which is not very soluble in water, but there is no water anyway except that in the atmosphere, so the Na2SO3 is left on the surface of the Camden tablet. Whether it then protects the inner layers from being attacked, I do not know. That is a problem in physical chemistry I think, that could be solved by experiment.

If you keep the metabisulfite dry, you need not worry about CO2. OTOH, if you use the bisulfite, it can be attacked directly by CO2 in a perfectly dry atmosphere, leaving sodium sulfite. So it would seem that Camden tablets should be kept in a closed container with silica gel dessicatant. You might still lose SO2, leaving sodium sulfite, but you should be able to tell by the smell. I have a friend nearby who grows grapes and makes fine wines, who uses sulfur candles to sterilize bottles, but only because required by law IIRC.

These are IMHO possibilities. If there is a way to show that they do not occur, let us know.

Paul Verizzo · May 16, 2008

Haist says that metabisulphite and sulphite both have good preservative quantities. IIRC - and that's a matter of debate - it takes about 80% of the meta to equal the, um, naked sulphite for a given oxygen scavenging ability.

Of course, one results in an acidic solution, the other is alkaline. Setting aside cost differential, use what you want for pH. The reality, of course, is that meta costs many, many times the plain sulphite. The meta is often used to reduce the pH of an otherwise sulfite only mixture.

gainer · May 17, 2008

Paul Verizzo said:
Haist says that metabisulphite and sulphite both have good preservative quantities. IIRC - and that's a matter of debate - it takes about 80% of the meta to equal the, um, naked sulphite for a given oxygen scavenging ability.

Of course, one results in an acidic solution, the other is alkaline. Setting aside cost differential, use what you want for pH. The reality, of course, is that meta costs many, many times the plain sulphite. The meta is often used to reduce the pH of an otherwise sulfite only mixture.

The argument here was whether one could use sodium bisulfite (acid sulfite) when a recipe called for sodium metabisulfate (pyrosulfite). 1 molecular weight of sodium pyrosulfite + 1 moleculer weight of water is equivalent to 2 moleculer weights of sodium acid sulfite. If you want to make a solution in water equivalent to 10% bisulfite by weight, then 100 grams of sodium bisulfite or 91.35 grams of the metabisulfite in water to make 1 liter of solution will do the job. I contend there is no chemical test to distinguish one solution from the other if they are made with equally pure ingredients.

gainer · May 17, 2008

2F/2F said:
Sorry. I'll be sure to leave my sense of humor behind when signing onto this Website next time.

Relax. Have a beer. Or would you prefer an HC-110 or D-76?

Yes, you must leave your humor behind. We are discussing the serious business of things we can do with fart gas.

haryanto · May 17, 2008

very nice discussions indeed, thanks a bunch for u all

Kirk Keyes · May 17, 2008

gainer said:
I contend there is no chemical test to distinguish one solution from the other if they are made with equally pure ingredients.

Both Sodium Bisulfite and Metabisulfite in solution (at 50g/L) are about 3.5-5 pH. The difference between them is that there is more bisulfite (or sulfite if you wish, but at that pH it is as the single protonated or hydrosulfite form, HSO3-).

The difference is that you get more sulfite/bisulfite with the metabisulfite per gram of material than with the bisufite.

And as you like to point out Pat, you probably can't point out the difference in solution, unless you happened to have an atomic absorption (AA)spectrophotometer or an inductively coupled plasma spectrophotometer (IPC) or any of the other methods of measuring the amount of sodium that is in solution.

Metabisulfite also goes by the archaic name of "pyrosulfite", which was used when a compound could be made into another compound through the act of intense heating. The heating drives off only water from an "ortho" acid or it's salt, like sulfuric acid forms pyrosulfuric acid (2H2SO4 - H2O => H2S2O7), pryophosphoric acid from phosphoric acid (2·HPO3 => P2O5*H2O), and of course, our topic of converstation, sodium metabisulfite from bisulfite (2·HNaO3S - H2O => Na2O5S2).

This is not water of hydration, which would also give a compound of higher "concentration" in the dry form, say like making a decahydrate salt into the anhydrous salt, but it also resulted in a different molecular formula, hence the new name for the new compound.

So metabisulfite is a distinctly different chemical with distinctly different properties and formula and molecular structure from bisulfite. The "pyro" form has more of the ortho acid anion than the original ortho acid form does.

But as Pat says, once they are put into water and are allowed to rehydrate, they are back to the original starting ortho acids or their corresponding anions.

Kirk Keyes · May 17, 2008

By the way, an "ortho acid" is the compound that is a fully hydrated acid or it's corresponding salt. A good example of this is phosphoric acid H3PO4 and the various forms it takes, the simplest and "fully hydrated" for is orthophosphate, PO4---. See this page for a good comparison of the various forms it can take:
http://en.wikipedia.org/wiki/Phosphoric_acids_and_phosphates

Anscojohn · May 17, 2008

All photo Alchemists,
I sure wish I could remember whether I can use "sodium bisuphite" and "sodium metabisuphite" interchangeably in black and white photo chemistry. I ASS u ME I can, since, to me, the gist of your erudite discussions is that so bi is actually so met, except when it is both, or perhaps neither, except when.... or........oh, sheesh!! (vbg)

gainer · May 17, 2008

Anscojohn said:
All photo Alchemists,
I sure wish I could remember whether I can use "sodium bisuphite" and "sodium metabisuphite" interchangeably in black and white photo chemistry. I ASS u ME I can, since, to me, the gist of your erudite discussions is that so bi is actually so met, except when it is both, or perhaps neither, except when.... or........oh, sheesh!! (vbg)

In most cases where we need the acid sulfite, the amount is not very large and a small error in measurement would not be critical, especially considering that if it's a new formula to the user, there will usually be a test or two before serious usage. If you don't know which one you have, you could probably split the difference. If you have the meta but thought you had bi, you will be less than 5% in error on the high side, and less if the stuff is old and has been exposed to atmosphere. If it is so critical as to cause worries, you should know what pH you're looking for and have a good pH meter. You could try a practical test using the sulfite instead of either of the acid sulfites, or maybe no sulfite. Ascorbic acid sometimes works as well as sodium bisulfite.

Anscojohn · May 17, 2008

Gadge,
My needs are very simple: occasionally D25R and lots of EK F-24 fixer--so all this arcana is beyond my needs as well as my comprehension.

And i can always use the substitution chart in my old Ilford Encylopedia of Photography for rough equivalents on most anything I shall ever require. Thus, and meaning no disrespect, it's pretty much all fart gas to me. For my own needs it is always a case of KISS. About wine and beer, howemsoever......:rolleyes:

dancqu · May 17, 2008

Anscojohn said:
Gadge,
My needs are very simple: occasionally D25R and
lots of EK F-24 fixer--so all this arcana is beyond
my needs as well as my comprehension.

Most importantly is the bisulfite's reducing prowess
vs the metabisufite's. Ian has implied a greater prowess
for the meta. That greater prowess though is associated
with the amount of bisulfite vs the meta.

For example, with respect to the precipitation of gold
from a solution of gold chloride, either one will do.
I doubt the meta has any thing over the bisulfite
with regard to it's reducing potential. With both
that potential has to do with the oxidation of
sulfur or as often put the SO2 radical.

On evaporation the normal salt in solution leaves
a normal salt. With the bisulfite in solution the
metabisulfite is left. IMO pyro overstates and
meta is the more appropriate term. Dan

Kirk Keyes · May 19, 2008

dancqu said:
IMO pyro overstates and
meta is the more appropriate term. Dan

You may be right the use of "Pyro" overstates, but you needo to keep in mind the archaic name systems that were in use when these compounds were being discovered and what was in common use before IUPAC (International Union of Pure and Applied Chemistry) naming conventions were adopted in the 20th century.

Sodium metabisulfite = sodium bisulfite

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