ECN-2 Sulfuric Acid Bath

Cid Cain · Jan 11, 2020

Have been reading here for a while on processing ECN-2 and it’s my understanding that a sulfuric acid bath is pretty much essential (as opposed to say, acetic acid).

My question is, I only own stainless steel tanks and reels. Will that pose a problem for the sulfuric acid stop bath, or not so much for the brief time frame of exposure? Wondering if I should get a plastic tank and reels.

Also, assuming I get the 37% sulfuric acid in battery acid, how much further should that be diluted for a stop bath? I think I read someone say 10ml battery acid per 1 L water.

Thanks much.

138S · Jan 11, 2020

Sulphuric stop is about 1% concentration...

AISI 316 (1.4401) allows 22% sulphuric concentration at room temperature, max concentration gradually falls with temperature up to 40 deg C and then falls more quickly to about 5% at 60 deg C.

AISI 304 (or 1.4301 type) allows 5% concentration at room temperature,

Not knowing what kind of s. steel is your tank, but I guess 1% should be safe, if you don't know the steel type then you put the 1% dilution in the tank for 24h to see if come corrosion appears.

Also use a magnet to see if it sticks to the tank, if it is low grade (420 or 430) ferritic type then it will stick a bit, like in kitchen spoons etc. If the magnet sticks like in spoons then your tank is low grade steel...

koraks · Jan 11, 2020

Cid Cain said:
I think I read someone say 10ml battery acid per 1 L water.

Something like that should be OK. It's not very critical. Since the stop bath is only in the tank for a minute or so, I wouldn't worry about corrosion too much.
If you rinse properly (eg two water changes) after the stop bath, you'll probably be OK if you use an acetic acid stop bath if the sulfuric acid worries you for some reason.
Use the stop bath one shot btw; don't reuse it.

Cid Cain · Jan 12, 2020

Thanks for the replies.

138S said:
Also use a magnet to see if it sticks to the tank, if it is low grade (420 or 430) ferritic type then it will stick a bit, like in kitchen spoons etc. If the magnet sticks like in spoons then your tank is low grade steel...

On two of my tanks, a magnet does stick a bit toward the rims (but not at the bottom). On another tank, it doesn’t at all.

I’ve just read that dilute (rather than concentrated) sulfuric acid is particularly corrosive to steel. But maybe at such a low concentration and short time frame (30 seconds for ECN), it may not be an issue?

138S · Jan 12, 2020

if the magnet does not stick then a higher Cr-Ni % is there, probably it won't rust at low sulfuric %

mshchem · Jan 12, 2020

Cid Cain said:
Thanks for the replies.

On two of my tanks, a magnet does stick a bit toward the rims (but not at the bottom). On another tank, it doesn’t at all.

I’ve just read that dilute (rather than concentrated) sulfuric acid is particularly corrosive to steel. But maybe at such a low concentration and short time frame (30 seconds for ECN), it may not be an issue?

Working stainless steel, ie drawing can convert the crystalline structure of the metal, thus some magnetic properties. I know just enough about stainless to know how to Google questions

https://www.greenwoodmagnetics.com/...fference-between-304-and-316-stainless-steel/

Cid Cain · Jan 12, 2020

koraks said:
Use the stop bath one shot btw; don't reuse it.

I realize the stop bath can lose its acidity with use, but is this something specific to color or ECN/sulfuric acid stop? I’m used to reusing stop at least a few times with BW development.

lantau · Jan 12, 2020

You can reuse the stop bath. Any stop bath will exhaust, eventually.

Metals can get corroded by acids, some alloys are more stable than others. 100% sulfuric acid will form a protective sulfate layer inside iron/steel vessels. Like many metals form a protective oxide layer on air. This will protect the metal from further attack. Dilute sulfuric acid will not do so. The sulfates will dissolve in the aqueous acid solution. Hence the acid will continuously corrode it. That is probably what you read. Your 37% acid is dilute acid, btw.

I may be wrong, but I wouldn't expect any problems with the ultra dilute ECN stop in your tanks. Acetic acid might corrode it just as well, if it was that sensitive. But store the stuff in glass or plastic.

koraks · Jan 13, 2020

Cid Cain said:
I realize the stop bath can lose its acidity with use, but is this something specific to color or ECN/sulfuric acid stop? I’m used to reusing stop at least a few times with BW development.

Normal stop bath (citric acid or acetic acid) forms a buffer upon use, as it's being diluted with hydroxide, forming a citric acid/citrate or acetic acid/acetate pair. Sulfuric acid, on the other hand, has virtually no buffering capacity. So it starts out very acidic, but then promptly "dies" once hydroxide is added.

lantau · Jan 13, 2020

koraks said:
Normal stop bath (citric acid or acetic acid) forms a buffer upon use, as it's being diluted with hydroxide, forming a citric acid/citrate or acetic acid/acetate pair. Sulfuric acid, on the other hand, has virtually no buffering capacity. So it starts out very acidic, but then promptly "dies" once hydroxide is added.

You are right that there isn't a buffer for constant pH. So when I'll do ECN again I will monitor the pH more closely to see how fast it will go up. But for a strong acid the curve should be quite flat until we get somewhat close to the point of equivalence. The original process is replenished, so that could be done without much fuss every couple of films.

Just a back of the envelope calculation: If you prepare 1L of stop and pour 500mL into your tank and return it to the bottle after use you'd need about 750mL of developer to completely exhaust the acid (acid and carbonate to bisulfate and bicarbonate). I think the stop bath can handle a handful of films before you need to check. Buffers are useful when you need a stable pH in a range were pH will otherwise be moved easily. Acetic acid is a weak acid and hence you'll need a buffer there. At the extreme ends of the pH scale the total capacity of the strong acid or base will take care of that. Buffers don't have endless capacity either.

EDIT: 750mL of developer (to neutralize all acid)

Rudeofus · Jan 13, 2020

Since it will be hard for you to determine the exact composition of your stainless steel tanks, it may be easier to just do a simple test: put a droplet of this stop bath onto some outer surface of this tank and observe what it does after several minutes. Do you see small bubbles? Does the stainless steel look markedly different in that location?

koraks · Jan 13, 2020

@lantau Yeah, you could just monitor pH. But the stop bath is so cheap and easy to make that I use it one shot and don't bother with replenishment. Since ecn2 often means 35mm (for me, it always does) this means that much of the home development will occur in jobo processors (again, at least for me this is the case). A 1510 requires about 150ml of solution in a rotation setup, so one shot processing (at least for the stop and in my case also developer) isn't too wasteful and you're guaranteed of consistent results. I do reuse bleach and fix as replenishment and reuse make perfect sense for those (again, at least to me).

lantau · Jan 13, 2020

@koraks Of course there is nothing wrong with one shot. I don't rotate, however. Also I need to bring up the conc acid from the basement, etc. I just wanted to point out that you can reuse it when it is more convenient.

But I will pay more attention when doing ECN again this year. Just to know how far we can go. Requirement for the pH is 0.8-1.5, hopefully that will last a good while.

Also I edited my post, I forgot the word developer after 750mL. It takes roughly that much developer to carry over for complete neutralization. So I hope it will turn out that 100mL will be tolerable, at least.

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