Urnes
Hello,
Can anyone tell me what ratio I should use when preparing the stop bath for film and paper if I’m using 60% acetic acid? How long does the solution last? Is there a different ratio if I’m only using the stop bath once? I’ve always used indicator stop bath before, but I couldn’t get hold of any at short notice; however, I reckon I could manage with acetic acid, which is cheaper.
Regards, Sven.
Gast
Hi Sven,
1+20 = 3%.
Without an indicator, you can’t determine the exhaustion level precisely. It depends on the paper and developer, and on how well you let it drain.
There is a method, but it’s harmful to your health and therefore NOT my recommendation. (Dip your finger into the developer and rub your thumb and index finger together in the stop bath. If the ‘soapy/slimy’ feel of the developer stops within a few seconds and your fingers ‘squeak’, the stop bath is still OK).
Best regards,
Mirko
Gast
Hi lads,
I usually throw out the stop bath at the same time as the fixer. In the worst-case scenario, the development process might continue slightly in the stop bath, but the fixer stops it reliably. If it’s no longer effective, you’ll need to make a fresh batch anyway. Do you know how to test the fixer?
Regards
Martin
Gast
Hello
Citric acid is cheaper and better (as it’s almost odourless). You can find it at Schlecker or DM, made by Heitmann (sold as a limescale remover), at a bargain price. 15 g per litre is fine (a kitchen scales will do the job).
You can also use sodium carbonate (SODA); I’m not sure how pure it is, but it’s certainly effective as a rinsing aid.
Regards, Rolf
Gast
Citric acid is odourless, but that stinking vinegar somehow lasts longer.
I don’t know why. You’d think 3% acid is just 3% acid, but somehow 3% stinking vinegar lasts almost 50% longer than citric acid....
(Comparison made not with Schlecker powder but with odourless stop bath – citric acid-based – vs. acetic acid).
Regards
Mirko
Urnes
So, first of all, a big thank you to you all.
I reckon the most practical thing for me will be to dispose of the stop bath together with the fixer. I usually leave it for no more than four weeks, and given the amount I make, it doesn’t get used up anyway – it tends to oxidise instead.
Regards, Sven.
Gast
Sven,
The fixer doesn’t oxidise! You need to calculate the capacity (square metres of paper processed).
Only the developer oxidises, and its ineffectiveness tells you that it’s gone off.
The fixer doesn’t show you this, and after six months your prints will start to ‘bloom’.
This is important!
Mirko
Urnes
Hi Mirko,
I need you to explain that bit about the fixer to me again.
Ilford states that the diluted concentrate has a shelf life of about a week. Andreas Weidner mentions something about four weeks in his workshop, regardless of whether you’ve used up the fixer or not. I’ve also had fixer standing around for a while that had barely been used, and at some point it stopped fixing properly. Something must be happening to the fixer when it’s left standing for a long time. It doesn’t evaporate, I’d have noticed that. :D
On the other hand, I’m often quite lazy and would rather make a fresh batch than go to the trouble of heating up 2.5 litres from €14 to €20.
Regards, Sven.
Gast
Hello!
Even Fix goes off over time – even the undiluted, unused concentrate – which is evident from the cloudy, milky yellow-white streaks in the concentrate. I’ve no idea what that is – sulphur deposits, perhaps? Maybe Franz will weigh in on this as well...
Gast
Yes, over time it can build up, but it doesn’t oxidise like the developer does.
Are you sure your fixer stopped working because it had been standing for too long?
Couldn’t it just have run out?
We use A324 from 1987 here and it fixes perfectly :D
Mirko
Urnes
Yeah, I don’t think it had gone off. I’d developed four rolls of film using that batch (0.5 l) and then left it standing for about six months; I had to re-fix the next roll because there wasn’t enough left – it had yellow streaks on it. Yeah! I did stick to the fixing time. :D Since then, I’ve only left the fixer standing for a maximum of four weeks.
Regards, Sven.
cfb_de
Well, I’ll be...
Roman, do you actually realise what you’ve done to me? I was just about to go to bed.
First off: there are a few ingredients in the fixer. Only one of them is the infamous ‘sodium thiosulphate’ (which has already sparked religious wars), or alternatively the equally infamous ‘ammonium thiosulphate’ (which has also sparked such religious wars).
A mixture of sodium thiosulphate and ammonium chloride (as found in some powdered quick fixers) produces nothing other than ammonium thiosulphate when water is added. And a little table salt, but that doesn’t matter.
What else is in it? As in almost every truly essential photochemical process: some sort of sulphite. Usually sodium sulphite. This stuff acts as an ‘antioxidant’. It is capable of attacking elemental silver, which is why leaving film or paper in the fixer for too long is a bad idea. On the plus side, though, the solution doesn’t go off so quickly.
Then there are other bits and bobs in the fixer: water softeners, pH buffers, usually some sort of germ killer (this is where expensive fixers differ from cheap ones).
So. How does it all work?
- Thiosulphate ions (whether bound to ammonium or sodium): These fix the image by dissolving undeveloped silver halides from the emulsion in the form of a ‘complex’. Thiosulphates are, without exception, chemically unstable and can be both reduced and oxidised. Sulphur chemistry is pretty much the chapter of classical inorganic chemistry where the most action takes place. What’s more, there are nasty little bacteria and fungi for whom thiosulphate ions taste almost better than a decent steak does to humans.
- Sulphite ions: These are supposed to prevent the thiosulphate from being oxidised away. Unfortunately, however, they redissolve elemental silver, which limits the fixing time. Furthermore, everything mentioned above from ‘sulphur chemistry’ to ‘steak’ applies here as well.
- Water softeners (‘Calgon’, ‘Photoplex’). These are rarely added nowadays, as there is no longer any truly hard drinking water. What is termed “hard” today is, at best, hardness level 4 and requires no special treatment when used as a fixer. The situation is different before and after the fixer.
- Buffer substances: “Normal” fixer is acidic. Since the pesky photo lab technician is constantly running all sorts of chemical-contaminated papers through the system, care must be taken to ensure that the fixer is maintained within a pH range where it performs its function reliably. So the scenario of “paper straight from the developer” must be covered just as much as the scenario of “40% acetic acid is the best stop bath”. And that’s what buffer substances are for. These are usually not sensitive to oxidation.
- Germ killer: Well. Here we have a difference between Great-Grandad’s homemade concoction and modern chemistry from the branded bottle. Great-Grandad simply dumped his trusty thiosulphate and sulphite together into water. Nowadays, we want to buy liquid concentrates that still do the job even after three years. And because we all have to do something about the “nasty little things” mentioned in the section on thiosulphate, a “preservative” is added, for example, which is “antibacterial”. Ordinary old benzoic acid is perfect for this and doesn’t even need to be listed on the label. The best way to tackle mould is with properly acidic concentrates containing plenty of sulphite.
Conclusion: Of course, fixer breaks down in the air or due to tiny organisms without any “contamination” from photographic material. All without any outside intervention. Even worse: over time, sulphite and thiosulphate actually react with each other. And this can produce elemental sulphur. This then accumulates as a yellow mist in the solution. Sometimes it also smells of rotten eggs; this means hydrogen sulphide has formed. As I said, the paragraph on thiosulphate – here, everything from “sulphur chemistry” to “works”.
Has that cleared things up?
Best regards,
Franz
(In the lab, we used to secure our thiosulphate measurements against bacterial contamination – mostly against the rules – using lanthanum or mercury compounds.)
Gast
Hey Franz,
thanks for the detailed explanation. :D
But what’s still not entirely clear (and this is probably what interests us most): how long does the decomposition process usually take in a laboratory setting without any specific bacterial contamination?
I mean, with some developers the oxidation only takes 2 hours (highly diluted Lith) and others (N113) sit in an open tray for months.
As far as I’m concerned, with the fixer, it’s always been the case that no special precautions are needed, because it sits for longer than the residue anyway without causing any problems.
So it would be good to know whether the (in my opinion, overly cautious) recommendation of one week for a 1+9 solution actually makes sense.
Regards,
Mirko
cfb_de
Hello Mirko,
An unpreserved thiosulphate solution in the lab goes off within a week. With a fixer (Tetenal Superfix / Amaloco), I haven’t had any problems so far, even after three months, provided the bottles are clean (dilution 1+9, or 1+7 for film).
Homemade fixer (thiosulphate and sulphite, which I take with me when travelling) barely lasted two weeks in a motel before sulphur started to precipitate.
Best regards,
Franz
Gast
Hi lads,
Here’s another tip on fixing and storage:
I always keep some film (35mm film on a cassette is the easiest to handle) in the darkroom. Before any serious session, I test the fixer: I clip a piece of film in the forceps and pop it into the fixer. If nothing clears up within 15 seconds (and it’s completely clear after a minute at the latest), the fixer is ready for the bin. It costs next to nothing, and I find it more reliable than coffee grounds that have been sitting around for four weeks or however long – which I’ve probably forgotten about anyway by the time they’re past their best. And when it fixes, it fixes.
Cheers
Martin
