Sodium Metabisulphite Solution

[Raghu Kuvempunagar]

Background:
In one of my experiments, I need to use very small amount (e.g. 0.25g per 500ml) of Sodium Metabisulphite to get the pH to my desired level. I would like to avoid the hassle of weighing such small amounts with my scale everytime. I'm instead thinking of preparing 1% solution in RO water and use appropriate volume of this solution in the experiments.

Question:
How long after preparing 1% solution of Sodium Metabisulphite does it remain stable? Assume storage in tightly capped thick HDPE bottle.

 

[Rudeofus]

There are two factors which work against you:

1. aerial Oxygen will attack the bisulfite ion. Since your solution is not very concentrated, this will happen quickly.
   
2. Sulfur Dioxide will escape from the solution. Since your solution is not very concentrated, small loss of Sulfur Dioxide will mean big difference in solution activity.

I would therefore use either a different acid to adjust pH (e.g. dilute Boric Acid would be a lot more stable), or prepare your solution fresh each time.

 

[Alan Johnson]

The darkroom cookbook 3rd edition p186 recommends using a 6 g/L bisulfite solution as a preservative
You could use 5 g/L bisulfite and take 50 ml of that to give 0.25 g .

 

[koraks]

In one of my experiments, I need to use very small amount (e.g. 0.25g per 500ml) of Sodium Metabisulphite to get the pH to my desired level. I

Perhaps this is a better starting point. What pH do you need and are there other ways to get that in a more stable and convenient manner? Whichever stock solution of metabisulfite you may use, the "working solution" of a fraction of a percent will likely not be pH stable and therefore of questionable reliability for your experiments. Wouldn't some kind of buffer be a better option? Does it really need to be just metabisulfite and nothing else?

 

[timmct]

Are you trying to "'slow down" a developer?

 

[Raghu Kuvempunagar]

I would therefore use either a different acid to adjust pH (e.g. dilute Boric Acid would be a lot more stable), or prepare your solution fresh each time.

Rudy, would dilute Boric Acid have stable pH for, say, a month if stored well?

You could use 5 g/L bisulfite and take 50 ml of that to give 0.25 g .

Alan, as a preservative, small fluctuation in pH over a period of time is probably acceptable. But in my experiments, I need pH to be stable for at least a few days. Would 5 g/L bisulphite ensure that?

Whichever stock solution of metabisulfite you may use, the "working solution" of a fraction of a percent will likely not be pH stable and therefore of questionable reliability for your experiments. Wouldn't some kind of buffer be a better option? Does it really need to be just metabisulfite and nothing else?

I've tried Boric Acid-Borax buffer at different pH by varying the proportion of the two components, but Borax seems to affect the result in a way I don't quite like. Adding small amounts of metabisulphite seems to work better. Will stick to it for now.

Are you trying to "'slow down" a developer?

Just doing some fun experiments for self-learning.

 

[john_s]

I use a solution of sodium metabisulphite for another purpose (stop bath) and I was interested in Ian Grant's comments about it needing to be fresh, even the dry powder. This is something I copied from somewhere (sorry I can't remember where but would have been a chemical supplier, possibly something connected to Dow Chemical). It looks as though a dilute solution wouldn't last very long at all.
<start of copied text>
Solid sodium metabisulfite has a typical shelf life of 4 to 6 months under cool, dry storage conditions. In aqueous solutions, however, sodium bisulfite can oxidize readily when exposed to air. A typical solution life can vary with concentration as follows:

Concentration (wt %) Solution life
10 1 week
20 1 month
30 6 months

 

[Rudeofus]

How about Sodium Bisulfate? It doesn't oxidize in air, it doesn't emit one of its components to air, it isn't eaten by bugs, and it has nothing which would buffer within pH 3 and 11. Any solution with this compound should be stable.

 

[Raghu Kuvempunagar]

In aqueous solutions, however, sodium bisulfite can oxidize readily when exposed to air. A typical solution life can vary with concentration as follows:

Concentration (wt %) Solution life
10 1 week
20 1 month
30 6 months

This is very useful! Thanks John.

Just wondering whether these are very conservative estimates. Some water based developers such as Pyrocat-HD and Obsidian Aqua use Metabisulphite as the preservative and are known to work fine for at least a few months. Pyrocat has 1% Metabisulphite and OA has 2%, much lower than the concentrations in the table above.

How about Sodium Bisulfate?

Seems like a good alternative to Metabisulphite for my experiments though I don't have this chemical with me at the moment.

 

[Rudeofus]

NaHSO4 the typical ingredient of "pH down" for swimming pools and should be trivially available everywhere. Even more commonly available is dilute Sulfuric Acid, which you could mix from battery acid.

 

[Alan Johnson]

Alan, as a preservative, small fluctuation in pH over a period of time is probably acceptable. But in my experiments, I need pH to be stable for at least a few days. Would 5 g/L bisulphite ensure that?

In absence of air, very likely yes. But it is improbable that that anyone has ever measured and published results for such a situation.

 

[jim10219]

In situations such as yours, I tend to avoid measuring by weight, and measure by volume. You can buy some tiny measuring spoons (I have some that go down to 1/64th of a teaspoon that I got of Amazon), and use those for mixing small quantities of stuff. I don't trust my spoons to be exactly what they say the are, but I do trust them to be the same every time I pull them out. So measure the weight of whatever powder you put into a spoon, and then write that down. Then figure out how much water you need to mix with that to get your solution. It's a bit of work the first time, but after that, it's really quick and easy to mix up a new solution of anything, and do it in small enough quantities that you don't have to worry about it going bad.

 

[John51]

Sodium metabisulfite is sold in home brew shops as a steriliser. It's cheap enough to mix fresh each time.

For belt and braces sterilisation of equipment, some add citric acid to the solution. This increases the amount of sulphur released by an order of magnitude. As confirmed by my nostrils, it's fierce. This may affect you if there is acid in your formula for the sodium metabisulfite to react with.

 

[john_s]

This is very useful! Thanks John.

Just wondering whether these are very conservative estimates. Some water based developers such as Pyrocat-HD and Obsidian Aqua use Metabisulphite as the preservative and are known to work fine for at least a few months. Pyrocat has 1% Metabisulphite and OA has 2%, much lower than the concentrations in the table above.

I suppose it's oxygen that does the damage. There's some dissolved in most water and if the bottle is opened with some frequency there'll be more. For a dilute solution I don't think it's going to work well for you, given that you're doing pH adjustments. Even powder might be iffy if it's not fresh (for pH adjustments).

There was a post from Ian Grant about sodium metabisulphite needing to be fresh, and suggesting using a bit more in Pyrocat-HD to be on the safe side.

As an aside: Incidentally, another post above says that acid accelerates the production of SO2 gas. I use sodium metabisulphite plus sodium sulphite as a stop bath. The sulphite raises the pH and makes the mixture somewhat more stable. How stable? I don't know, but for stop bath it's not critical. Obviously irrelevant for your discussion.

 

[Rudeofus]

Just wondering whether these are very conservative estimates. Some water based developers such as Pyrocat-HD and Obsidian Aqua use Metabisulphite as the preservative and are known to work fine for at least a few months. Pyrocat has 1% Metabisulphite and OA has 2%, much lower than the concentrations in the table above.

There are two facts which help Pyrocat HD or Obsidian Aqua:

1. There is other stuff in that stock concentrate, too. The more stuff is dissolved in water, the less Oxygen will be in there.
2. It really doesn't matter for Pyrocat HD or Obsidian Aqua, whether there is 1 %, 1/2 % or even 1/10 % of Sodium Metabsulfite in this stock solution, it will always work pretty much the same.

 

[john_s]

There are two facts which help Pyrocat HD or Obsidian Aqua:

1. There is other stuff in that stock concentrate, too. The more stuff is dissolved in water, the less Oxygen will be in there.
2. It really doesn't matter for Pyrocat HD or Obsidian Aqua, whether there is 1 %, 1/2 % or even 1/10 % of Sodium Metabsulfite in this stock solution, it will always work pretty much the same.

There have been some posts lately about "sudden death" of Pyrocat and stale or exhausted metabisulphite has been suggested as the cause. It has never died for me. When I mix my next batch I'll use fresh metabisulphite.

 

[Rudeofus]

Decomposition of sulite, bisulfite, development agents, thiosulfate etc. is catalyzed by trace metals, same story as with ascorbate, typically a lot slower, but just as unpredictable. You'd need to add a powerful sequestering agent to prevent this. This sequestering agent will still not prevent Sulfur Dioxide from escaping.

 

[Alan Johnson]

Do sequestering agents, eg NaDTPA , Na2EDTA dissolve in acidic solution of sodium bisulfite?

 

[Rudeofus]

They will not, and since they do not adequately protect Ascorbic Acid, they will likely not protect other compounds either from Fenton reaction. DTPA will also not stay in solution at low pH, but ATMP or HEDP should work. The latter is available from Suvatlar.

 

[john_s]

For the OP: Sodium metabisulphite is used to react with and remove excess chlorine in water treatment plants. The amount has to be calculated to remove the correct amount. So the effective strength needs to be known, either by knowledge of its freshness or by analysis at the time of use. This is from a water treatment web site (https://environex.net.au). I think this makes it quite unsuitable for the OP's use (adjusting pH by adding a known quantity of solution, unless it's mixed fresh). The suggestion made previously to use sodium bisulphAte seems like it would work a lot better.
<start copied text>
An SMBS solution is made by dissolving solid sodium metabisulfite into water and has a pH of 4.6 at 1.0 % (by weight) solution strength. The SMBS solution is not stableto air and reacts with oxygen as well as chlorine, therefore it is recommended that batches of less than 2 % by weight be used within 3 to 7 days and batch solutions less than 10 % be used within 7 to 14 days

 

[Raghu Kuvempunagar]

I think this makes it quite unsuitable for the OP's use (adjusting pH by adding a known quantity of solution, unless it's mixed fresh). The suggestion made previously to use sodium bisulphAte seems like it would work a lot better.
<start copied text>
An SMBS solution is made by dissolving solid sodium metabisulfite into water and has a pH of 4.6 at 1.0 % (by weight) solution strength. The SMBS solution is not stable to air and reacts with oxygen as well as chlorine, therefore it is recommended that batches of less than 2 % by weight be used within 3 to 7 days and batch solutions less than 10 % be used within 7 to 14 days

Very useful information, thanks John!

 

[Alan Johnson]

In solutions containing sulfite and a developing agent the two may be mutually protective vs oxidation, as discussed for hydroquinone here, p382.:
https://archive.org/details/TheTheoryOfThePhotographicProcess/page/n382
IDK if this also occurs in bisulfite solution.

 

[john_s]

In solutions containing sulfite and a developing agent the two may be mutually protective vs oxidation, as discussed for hydroquinone here, p382.:
https://archive.org/details/TheTheoryOfThePhotographicProcess/page/n382
IDK if this also occurs in bisulfite solution.

I seem to remember that hydroquinone is used as an oxygen scavenger also. I'll do a search when I can soon,

 

[john_s]

Off topic, sorry. There are lots of web articles citing the use of hydroquinone as an oxygen scavenger in boiler systems etc. So it seems that hydroquinone as well as sulphite are both preservatives in developers that have metol or phenidone in them. I never realised that, although maybe everyone else does, which is why the MQ and PQ developers have been the mainstay for so long, and why getting rid of hydroquinone hasn't always been easy.