Replacing sulfite with metabisulfite?

geirtbr · Apr 15, 2024

Where I live, potassium metabisulfite is readily available in brewing shops, while sodium sulfite is harder to get hand of. I wonder if one can replace sulfite in developer recipes. I know metabisulfite is more sour, so one also need to adjust the ph upwards. Is this doable and are there any rule of thumb here, or are there other effects that will occur and this better to be avoided. Sorry, I dont have advanced chemical knowledge, so I am happy if someone can enlighten me on this.

john_s · Apr 15, 2024

For converting sodium metabisulphite to sodium sulphite, see:

Will the real Mytol please stand up

I work with sodium hydroxide very often and I fail to see what the problem is. well, it's no problem if you handle it properly, but it can lead to very nasty damage if there's one moment of lack of attention, which is not something that is the case with 99% other other chemicals we usually use...

www.photrio.com

You will have to use sodium hydroxide, called caustic soda where I live and lye in the USA. It is dangerously caustic and you would need to read about using it. The maths I leave to you. Potassium salts are largely interchangeable with sodium salts for photographic purposes.

koraks · Apr 16, 2024

geirtbr said:
sodium sulfite is harder to get hand of

It's a fairly common food additive; that's how I get mine.
Instead of mucking about with bisulfite + lye (which will work OK, btw), I'd just look around a little more for some actual sulfite. Much more convenient, and it may even end up being cheaper to boot.

relistan · Apr 16, 2024

geirtbr said:
Where I live, potassium metabisulfite is readily available in brewing shops, while sodium sulfite is harder to get hand of. I wonder if one can replace sulfite in developer recipes. I know metabisulfite is more sour, so one also need to adjust the ph upwards. Is this doable and are there any rule of thumb here, or are there other effects that will occur and this better to be avoided. Sorry, I dont have advanced chemical knowledge, so I am happy if someone can enlighten me on this.

Food or industrial supply places sometimes have sodium sulfite in large amounts for cheap. I buy mine from a place that sells chemicals for cleaning. If I wanted, I could get 5kg for €30 or so.

Doremus Scudder · Apr 16, 2024

Vintners use sodium sulfite a lot. Maybe a little wine tasting is in order?

Doremus

geirtbr · Apr 16, 2024

Lye and metabisulfite it is then!
Im not so good with calculating PH, however, I can do it an empirical way.
For example, I can mix 0.5 liters of d19 with sulfite, and measure ph.
Then I can mix 0.5 liters of the same developer but substituting sulfite for metabisulfite. Then measure ph, and add a 10% solution of sodium hydroxide until the ph matches the first solution.
This should work shouldnt it?

lamerko · Apr 16, 2024

Potassium metabisulfite + Potassium hydroxide will give potassium sulfite. Sodium metabisulfite + sodium hydroxide will give sodium sulfite.
Potassium metabisulfite + sodium hydroxide = ? A potassium-sodium thing?

john_s · Apr 16, 2024

geirtbr said:
Lye and metabisulfite it is then!
Im not so good with calculating PH, however, I can do it an empirical way.
For example, I can mix 0.5 liters of d19 with sulfite, and measure ph.
Then I can mix 0.5 liters of the same developer but substituting sulfite for metabisulfite. Then measure ph, and add a 10% solution of sodium hydroxide until the ph matches the first solution.
This should work shouldnt it?

In my post above, I should have added:

The formula in the link in my post is theoretically correct. But measurement is critical because sulphite (whether sodium or potassium) is usually used in a rather large amount in a developer (up to 100g/L). The problem with measuring sodium hydroxide is that it is highly hygroscopic and is problematic to measure. And because it is so extremely alkaline (and sodium sulphite is not far from neutral) and the metabisulphite is rather acidic, if there is inaccuracy, and there probably will be, the pH could be rather different to that of sodium sulphite from a normal source. That would have a big effect on a developer.

Consequently, you would have to be very careful to get it right, and if sourcing the sulphite is possible, I would recommend doing so.

relistan · Apr 17, 2024

lamerko said:
Potassium metabisulfite + Potassium hydroxide will give potassium sulfite. Sodium metabisulfite + sodium hydroxide will give sodium sulfite.
Potassium metabisulfite + sodium hydroxide = ? A potassium-sodium thing?

Poatassium is more reactive than sodium so I am pretty sure you are going to get the same result. You will just have free sodium ions instead of free potassium ions.

koraks · Apr 17, 2024

relistan said:
You will just have free sodium ions instead of free potassium ions.

You'll have both potassium and sodium ions if you start with potassium metabisulfite and sodium hydroxide, or vice versa. Upon dissolution, the salts split apart into their respective ions and those basically float around in one big soup. As such, you can't really tell if the mishmash will contain sodium sulfite or potassium sulfite - there will be sulfite ions, and there will be potassium and sodium ions.

Now, what happens if you allow all the water to evaporate so things start to crystallize out is a different matter. I'm quite sure you'll first see the sodium species crystallize, and the potassium ones later on due to their higher solubility.

john_s said:
Consequently, you would have to be very careful to get it right, and if sourcing the sulphite is possible, I would recommend doing so.

Exactly. Just shop for sodium sulfite. Saves a lot of trouble.

lamerko · Apr 17, 2024

relistan said:
Poatassium is more reactive than sodium so I am pretty sure you are going to get the same result. You will just have free sodium ions instead of free potassium ions.

I'm no chemist, but I don't think it will work out that way. We will simply have the following reaction:

K2S2O5 + 2 x NaOH = K2SO3 + Na2SO3 + H2O = A potassium-sodium thing

koraks · Apr 18, 2024

lamerko said:
We will simply have the following reaction:

Not quite, because it doesn't occur in dry form. The K and Na ions aren't associated directly with the SO3 ions as the reaction occurs. It only happens upon crystallization. You've got a 'potassium sodium thing' as soon as you dissolve two salts with K and Na ions; at that point, which salt is which is indeterminate - the only thing you know is how much of every ion you've got in there.

relistan · Apr 18, 2024

koraks said:
Not quite, because it doesn't occur in dry form. The K and Na ions aren't associated directly with the SO3 ions as the reaction occurs. It only happens upon crystallization. You've got a 'potassium sodium thing' as soon as you dissolve two salts with K and Na ions; at that point, which salt is which is indeterminate - the only thing you know is how much of every ion you've got in there

Ah yeah, correct, until crystallization nothing is forming in there with the sodium and potassium ions anyway. But my main point was intended to be that it's not a problem, and that's correct.

koraks · Apr 18, 2024

relistan said:
But my main point was intended to be that it's not a problem, and that's correct.

Sorry to distract from this; I agree. It's not a concern in practice.

Replacing sulfite with metabisulfite?

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