Plain Sodium Sulphite as a paper printing stop bath?

Lately I had a soar throat when printing.

When I finally consulted a GP about it, after a lot of questioning and reflecting, we came to the conclusion that the fumes coming from the buffered stop bath (vinegar + sodium hydroxide) could be the cause.
Although my darkroom is well ventilated with a system evacuating most of the fumes coming from the developing bath, but clearly not good enough...

So, I started to use a citric acid stop bath, the throat troubles went away (vinegar is a part of the buffered stop bath).
But, as always there is a but, in Belgium citric acid may mot be sold freely as it can be used in hallucinating drugs (heroine?), what makes it rather hard to find (and somewhat expensive)...

The trouble is that I have a nose problem after a medical intervention that didn't work out entirely as intended, I can't identify a smell anymore, if I can smell at all...
So I thought to use Sodium Sulphite as a stop bath, and just want to be sure of the dilution, I use 2,5%, that's 25g on 1l, but as I read about BISULPHITE and not just SULPHITE, I have doubts.
I happen to have a lot of Sodium Sulphite and no Sodium Bisulphite, and neither have I Potassium Bisulphite...

Any suggestions?

Philippe-Georges said:

fumes coming from the buffered stop bath (vinegar + sodium hydroxide)

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Does it smell of vinegar? If it doesn't, this isn't your problem.

What is oossible, is that you're having issue with the tiny amount of sulfur dioxide formed as prints go from the developer into the stop bath, and the sulfite from the developer decomposes in the stop bath. Using a more dilute developer or a formula with much less sulfite will help. The developer will have a shorter tray life, though.

Philippe-Georges said:

in Belgium citric acid may mot be sold freely

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You can buy it by the kilo from many stores, I including online. It's not regulated. You generally get good prices at stores that retail cleaning agents or brewery/winemaking supplies.
If vinegar/acetic acid bothers you, a citric acid stop bath is indeed a good alternative. Here's a source I've used several times in the past: https://www.naturalspices.be/citroenzuur-e330 But there's a plethora of source including chains like "Dille en Kamille" etc. (with generally higher retail prices, however).

In my mind, buffering acetic acid and citric acid stop baths is a bit silly. Carryover from the developer (carbonate) will buffer it all by itself anyway. Starting out with a buffer is basically "pre-aging" the stop bath and somewhat reducing its capacity. Both paper and film these days withstand the unbuffered pH of a weak solution of citric or acetic acid just fine.

Buffering does make sense if you want to prevent the formation of SO2 (which may be the real cause of your throat issue); in this case, you'll have to maintain the pH of the stop bath above 4.5. But obviously also below 6 or so to prevent prolonged activity of the developer (e.g. metol is still active at a neutral pH). The practical range of an acetate buffer is 3.5 - 5.5 or so, which makes it suitable, as long as you keep it at the higher end of its working range. This means a shortened lifespan of the buffered stop, or you need to mix it fairly strongly ('high molarity') to keep it alive as long as possible.

Sodium sulfite by itself won't work as a stop bath. Carryover from the developer will just make a weak developer since the pH of the sulfite solution will be well above neutral. See e.g. D23.

Your main issue at this point is to figure out what's actually causing the problem. Sounds like at this point it's still guesswork.

I assume you've tackled the issue of ventilation and that it's already excellent in your darkroom. If not, focusing on the chemistry is a futile exercise until you address it.

PS: doctors are great people and of course very useful, but I consider them a poor source for figuring out darkroom-related problems with. For instance, I bet (s)he overlooked the SO2 issue entirely. In my experience, the chemical background of GP's is generally fairly weak - which is not a disqualification to the profession. As a matter of course, their strengths lie in different areas.

PPS: as to regulation of citric acid; I don't think there's any country worldwide that limits its sale whatsoever. Citric acid is a common chemical present in many fruits (lemons and limes, notably) and it's widely used as a preservative, buffering agent in foods and industry and as a chemical feedstock for a variety of processes. What has been controlled, at least to an extent, is the sodium hydroxide you mentioned. To the best of my knowledge it was never controlled legally, but certain stores chose (perhaps still do) to not retail it anymore in dry/pure form because it was apparently used in producing GHB. That fad seems to have waned a bit and NaOH pellets remain easily obtainable from drugstores as well as chemicals retailers. Pure acetic acid ('glacial') may be difficult to obtain for private persons in Europe (along with things like pure/highly concentrated sulfuric acid, nitric acid etc., and of course also hydrogen peroxide) - and as far as I'm concerned, that's actually a good thing given the safety implications of handling these acids (and peroxides). Fortunately, for darkroom use, the weaker dilutions commonly available are nearly always a perfectly viable substitute. I've personally never run into legislation being a problem in photography, with the exception of applications of dichromate.

Which begs the question of squeezing a lemon or lime into a tray of water, as a stop, pulp and seeds be damned?

Have you ever measured the pH of your stop bath? Perhaps it is too low. It should be in the range of 3-5. You can change it increasing the ammount of hidroxide or lowering the ammount vinegar. Could be also that it is too concentrated.

I use straight acetic acid stop bath at 1.5-2% concentration. I don't find this annoying, I find more disturbing the fixer (also acid) on open trays.

eli griggs said:

Which begs the question of squeezing a lemon or lime into a tray of water, as a stop, pulp and seeds be damned?

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I'm sure that would work just fine in a pinch. Citric acid has gone up in price rather drastically in retail here, so this might become economically viable!

I use sodium metabisulphite but I don't recommend it if you have a nose sensitivity (SO2). Citric acid seems to be the obvious choice.

Is there another acid that would do, and not smell? Maybe sulphuric or hydrochloric VERY DILUTE?

Is tartaric acid suitable (maybe availability similar to citric).

grain elevator said:

Citric acid has gone up in price rather drastically in retail here, so this might become economically viable!

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Using limes/lemons etc. will always be more costly than buying citric acid in somewhat reasonable amounts (a few pounds and upwards). Just run the numbers on the lemons you can buy from your average Turkish supermarket (they tend to be relatively cheap there, especially when in season) and their citric acid content vs. a kilogram bag of citric acid you obtain from your favorite brewery supplies store. You'll see.

Technically, it'll work. The additional sugars and potentially oils etc. from the rind/peel might be a problem. I wouldn't recommend it and see no practical reason for it, either. It's one of those "hey look, we can make a bulb light up by putting some wires into a lemon" kind of things. A fun demonstration without much practical use.

As I mentioned before, citric acid is not regulated in Belgium or anywhere else AFAIK, it's cheap and an obvious choice, and even if it's difficult to obtain (although I couldn't figure out how on earth it might be), there's always products like odorless stop bath from a variety of brands that are basically citric acid solutions with an indicator added (if you're lucky). Try any Ilford, Adox etc. odorless stop bath.

halfaman said:

You can change it increasing the ammount of hidroxide or lowering the ammount vinegar. Could be also that it is too concentrated.

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Yes, well, sort of. It's a buffer, so pH will drift only slowly once the buffering action starts. This means that if you start with just vinegar/acetic acid in solution and add hydroxide, pH will rise fairly rapidly until it reaches ca. 3.5 and from there on it will rise only slowly as you keep adding hydroxide. When it reaches 5.6, it will rise steeply again. Working out to get a decent starting point is a matter of just pulling out one of the many online buffer calculator tools and keying in the numbers. Here's a nice one: https://www.aatbio.com/resources/buffer-preparations-and-recipes/citrate-buffer-ph-3-to-6-2 Just use the sliders to get to where you want and it'll give the amounts you need to mix to get the target pH and molarity. No difficult maths or chemistry involved.

'Too concentrated' is not an issue; quite the opposite. A high molarity stop bath with have a longer service life. However, due to carry-over of developer, it's wise to discard stop bath regularly, so there's no point in making it very strong. It doesn't hurt either.

halfaman said:

I use straight acetic acid stop bath at 1.5-2% concentration.

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That will have a rather low pH; much lower than the limits you indicate, at least at the start of the session. It'll drift up and start buffering as developer carries over into it. Which goes to show that the 'too low pH' is not all that much of a concern in practice as it's a situation that remedies itself rather quickly. But like I said earlier, if a person is highly sensitive to SO2, it makes sense to start out with a buffer instead of using an unbuffered stop bath until it starts buffering itself.

Use water in the stop tray. Put the print in, then dump the water and rinse the print. Stop bath is obviously better, but if you cannot use it, rinsing off all the developer under running water does work.

But not with Ilford Art paper. That needs stop bath.

koraks said:

Using limes/lemons etc. will always be more costly than buying citric acid in somewhat reasonable amounts (a few pounds and upwards). Just run the numbers on the lemons you can buy from your average Turkish supermarket (they tend to be relatively cheap there, especially when in season) and their citric acid content vs. a kilogram bag of citric acid you obtain from your favorite brewery supplies store. You'll see.

Technically, it'll work. The additional sugars and potentially oils etc. from the rind/peel might be a problem. I wouldn't recommend it and see no practical reason for it, either. It's one of those "hey look, we can make a bulb light up by putting some wires into a lemon" kind of things. A fun demonstration without much practical use.

As I mentioned before, citric acid is not regulated in Belgium or anywhere else AFAIK, it's cheap and an obvious choice, and even if it's difficult to obtain (although I couldn't figure out how on earth it might be), there's always products like odorless stop bath from a variety of brands that are basically citric acid solutions with an indicator added (if you're lucky). Try any Ilford, Adox etc. odorless stop bath.


Yes, well, sort of. It's a buffer, so pH will drift only slowly once the buffering action starts. This means that if you start with just vinegar/acetic acid in solution and add hydroxide, pH will rise fairly rapidly until it reaches ca. 3.5 and from there on it will rise only slowly as you keep adding hydroxide. When it reaches 5.6, it will rise steeply again. Working out to get a decent starting point is a matter of just pulling out one of the many online buffer calculator tools and keying in the numbers. Here's a nice one: https://www.aatbio.com/resources/buffer-preparations-and-recipes/citrate-buffer-ph-3-to-6-2 Just use the sliders to get to where you want and it'll give the amounts you need to mix to get the target pH and molarity. No difficult maths or chemistry involved.

'Too concentrated' is not an issue; quite the opposite. A high molarity stop bath with have a longer service life. However, due to carry-over of developer, it's wise to discard stop bath regularly, so there's no point in making it very strong. It doesn't hurt either.



That will have a rather low pH; much lower than the limits you indicate, at least at the start of the session. It'll drift up and start buffering as developer carries over into it. Which goes to show that the 'too low pH' is not all that much of a concern in practice as it's a situation that remedies itself rather quickly. But like I said earlier, if a person is highly sensitive to SO2, it makes sense to start out with a buffer instead of using an unbuffered stop bath until it starts buffering itself.

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If Citric acid is not regulated how coms that for instance Halland&Barret (a large Dutch store) doesn't sell it over here, only in (dissolving-) pastilles and pills with orange flavor, and when asked the drug reason is given...

I had some left and contacted the supplier mentioned on the label, a bakery wholesale that also sell to private buyers: the same answer...

BTW: the buffered stop bath I was using had a pH of about 6.

Have you asked them to refer you to the specific legislation? There's nothing I can find about it and frankly, I think there's some kind of confusiog going on here that results in your belief that you cannot buy something that's evidently being sold all over the country.

Philippe-Georges said:

only in (dissolving-) pastilles and pills with orange flavor

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That's a common way to retail ascorbic acid, but not (or much less so) citric acid, which is barely used as a supplement anyway. While I also don't see any good reason why a country would regulate the sale of ascorbic acid either, it's a little less far-fetched than citric acid. Are you sure you're not confusing the two? Citric acid = "citroenzuur". Ascorbic acid = "ascorbinezuur", but mostly sold as "vitamine C".

Anyway, if buying it in Belgium is difficult, just buy a couple of kilos in The Netherlands and pay the additional shipping fees. You'll be set for years with that amount and the total cost will be very manageable. If you want me to help out, just send me a message and I'll pack a box of citric acid and send it over to you. I'm quite confident I won't be breaking any laws, so I'm fine with it.

Philippe-Georges said:

BTW: the buffered stop bath I was using had a pH of about 6.

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An acetate buffer has a useful range up to about 5.6. pH 6 would be a poor choice for setting an acetate buffer because its buffering capacity is very minimal at that pH. Is this a commercial product you're using, or something someone DIY-ed for you?

koraks said:

That will have a rather low pH; much lower than the limits you indicate, at least at the start of the session.

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Yeah, I discover those limits here in Photrio after years printing. It is hard to change habits...

Philippe-Georges said:

BTW: the buffered stop bath I was using had a pH of about 6.

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That is a very soft acid stop bath, practical concentration of acetic acid is well below 1%.

Philippe-Georges said:

If Citric acid is not regulated how coms that for instance Halland&Barret (a large Dutch store) doesn't sell it over here, only in (dissolving-) pastilles and pills with orange flavor, and when asked the drug reason is given...

I had some left and contacted the supplier mentioned on the label, a bakery wholesale that also sell to private buyers: the same answer...

BTW: the buffered stop bath I was using had a pH of about 6.

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Many health stores used to sell food grade citroenzuur (E330) in NL.
This acid must be >99.5% pure (EU regulation) and it's usually made by fermentation.
A quick search showed that ietsgezond, deonlinedrogist, bol, and labshop sell it for about 5Euro/kg

I don't kow if fotohuisrovo.nl still sells chemistry but they had an odorless stop bath which was citric acid base

titrisol said:

in NL.

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Yes, but @Philippe-Georges is based in Belgium. He argues the situation is different there. Which again I find very odd given the many Belgian sources that retail citric acid in powder form to the general public.

titrisol said:

I don't kow if fotohuisrovo.nl still sells chemistry but they had an odorless stop bath which was citric acid base

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I think he's still in business, yes. Alternatives would be fotofilmfabriek (e.g. Adox chemistry) or, in Belgium, retrocamera.be. Their selection of citric-acid based stop bath products includes:

Plenty to choose from!

I imagine none of these are buffered, and if the desire exists to make a buffered stop bath out of them, it's a matter of diluting any of the above products to working strength, and then incrementally add sodium hydroxide to the stop bath, using indicator pH strips or a pH meter to observe the pH. Adjust until the desired pH is reached; I'd recommend 4.5-5.0 and replacing the stop bath frequently (at the start of each brief session, and multiple times throughout extended sessions).

koraks said:

Using limes/lemons etc. will always be more costly than buying citric acid in somewhat reasonable amounts (a few pounds and upwards). Just run the numbers on the lemons you can buy from your average Turkish supermarket (they tend to be relatively cheap there, especially when in season) and their citric acid content vs. a kilogram bag of citric acid you obtain from your favorite brewery supplies store. You'll see.

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I wasn't entirely serious.

Water.

koraks said:

Have you asked them to refer you to the specific legislation? There's nothing I can find about it and frankly, I think there's some kind of confusiog going on here that results in your belief that you cannot buy something that's evidently being sold all over the country.



That's a common way to retail ascorbic acid, but not (or much less so) citric acid, which is barely used as a supplement anyway. While I also don't see any good reason why a country would regulate the sale of ascorbic acid either, it's a little less far-fetched than citric acid. Are you sure you're not confusing the two? Citric acid = "citroenzuur". Ascorbic acid = "ascorbinezuur", but mostly sold as "vitamine C".

Anyway, if buying it in Belgium is difficult, just buy a couple of kilos in The Netherlands and pay the additional shipping fees. You'll be set for years with that amount and the total cost will be very manageable. If you want me to help out, just send me a message and I'll pack a box of citric acid and send it over to you. I'm quite confident I won't be breaking any laws, so I'm fine with it.

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I once tried to buy two pots of 1kg Vit-C powder at the webshop of Holland&Barrett (NL webshop) then at others too, and it was prohibited to send to Belgium, idem ditto for citric acid.
But I found the Vit-C at Billini, but apparently missed the citric acid, thanks for the tip...

koraks said:

An acetate buffer has a useful range up to about 5.6. pH 6 would be a poor choice for setting an acetate buffer because its buffering capacity is very minimal at that pH. Is this a commercial product you're using, or something someone DIY-ed for you?

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It's the buffered stop bath I use with Pyrocat-HD (Sandy King) as formulated by R.Suzuki, and for 'simplicity' (I made a stock), easy to wash out the gelatine and long lasting (and in combination with Fuji-Hunt's C-41 fixer), I use it for paper too...

What is strange is that I am using this stop bath for nearly 15 years now, for paper aswel as for film, and all the sudden this problem occurred a few months ago.

And for the developer (E-72) and the fixer the same, +/-15 years, somewhat around the demise of AGFA...

Philippe-Georges said:

What is strange is that I am using this stop bath for nearly 15 years now, for paper aswel as for film, and all the sudden this problem occurred a few months ago.

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Yes, these kinds of problems happen as we get older.

titrisol said:

Yes, these kinds of problems happen as we get older.

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Oh no!

Water stop works for me on both paper and film.

john_s said:

Is tartaric acid suitable (maybe availability similar to citric).

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I wondered the same thing.

I think I can confirm it: after two sessions in the darkroom and not using that buffered stop bath but citric acid instead, I had no soar throat.
It might be something like autosuggestion (what my wife stated), but I don't care the trouble went away!

And I have a pod of Sodium Metabisulfite I use for mixing Pyrocat-HD, next session I will mix 2,5% for a stop bath.
I now have to find out how long that one will last, perhaps by checking the pH but my (old-) pH meter isn't working anymore. But I read somewhere that there is an easy to find product used for swimming pools: something with '******red' in the name but can't recall exactly...
I will have a look at a DIY shop.

Anyway, thank you all for your answers and concern!

Philippe-Georges said:

I have a pod of Sodium Metabisulfite I use for mixing Pyrocat-HD, next session I will mix 2,5% for a stop bath.

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It will work, but it won't have any buffer capacity. I.e. it's a relatively wasteful way to make a stop bath. What happens is that as developer carries over into your bisulfite stop bath, the pH will just go up and the bisulfite will convert to regular sulfite. This will get you back into "D23-territory" again with the stop bath acting as a weak developer instead of a stop bath.

If the citric acid works well for you, I'd strongly suggest sticking to it because that's a very effective stop bath. There are very good reasons why virtually all odorless stop bath on the market is based on citric acid. It just works very, very well. Part of this is because citric acid is triple-protonated, which simply put (in laymen's terms) means that each citric acid molecule can react with three basic/alkaline molecules before it stops being an acid. It actually works better than acetic acid in this sense, which is only a singular acid. The consequence is that "a little goes a long way". It's very economical, it's odorless, doesn't emit any vapors, inherently safe, etc. It's the superior choice.

john_s said:

Is tartaric acid suitable (maybe availability similar to citric).

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Yes, tartaric acid will work just as well. It's generally significantly more expensive than either citric or acetic acid and offers no benefits especially in comparison with citric acid. But it'll work. Tartrate will buffer around pH 3.5 (buffer range ca. 2.0-5.4) which is on the low side (see the argument on SO2 production), but you could set it on the high side (with limited capacity). So basically, it'll be a bit less efficient than citric acid and far more expensive than either citric or acetic acid.

Being solid at room temperature is another thing in favor of citric acid, much better to handle than acetic acid which is a liquid. I really not like to handle glacial acetic acid but I have three bottles of 1 liter...

Guess what, I just came back from the DIY store (HUBO) and there I found Citric Acid in 400g pods, but it was described as a "rustic method for cleaning" but it is 100% Citric Acid, speaking of circumventing regulations...

Philippe-Georges said:

speaking of circumventing regulations

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They're not circumventing regulations. There's no regulation in effect that prohibits sale of citric acid to individuals in Belgium. You were either misinformed by the shops you contacted, or there was some kind of misunderstanding in the communication between you and the stores you contacted. Hence my question which specific legislation they referred to - a question which remained unanswered, and probably will, because there is none.

Either way, nice that you've found some. There are of course many more options. For instance, di.be (I understand Di is a big drugstore chain in BE) has this: https://www.di.be/nl/the-fabulous-citroenzuur-2191053
Or via Bilsen: https://www.dbilsen.be/nl/citroenzuur-1kg.html
The stuff is literally everywhere, which is sensible since it's such a common and relatively benign/harmless material, not subject to any legislation (apart from proper labeling)...