Affect dye stability? This is interesting. I though the blix and a generous final wash would remove all developer from the coating. After all, there is no stop in the machine process, if I'm right?
Anyway, I've never had any streaking with citric acid in trays used for the whole printing session and dumped after that.
Well based on what I have heard here from an RA4 specialist such as PE, it would seem sensible to use acetic acid even if citric will do the job albeit at much quicker exhaustion.
Steve, Fotospeed does a 1L pack which is quite strong, needing 1+29 dilution and at AgPhotographic's price is quite inexpensive at these dilutions. Actually cheaper than Champion which I always thought wrongly to be the cheapest but not so if you take dilutions into account.Acetic acid stop bath is what I always use for B&W and RA4 as I don't mind the smell. I think at 1+29 you might be hard pressed to beat this price-wise with white vinegar.
Couplers are dissolved in coupler solvents to disperse them in droplets in the emulsion prior to coating. During processing, the CD3 migrates into the droplets as either the oxidized product forming dye, or the unoxidized free base which can remain.
Originally Posted by hrst
This free base, if carried through the process with bad agitation or exhausted blix or bad wash will cause a pink stain and loss of dye. The stop bath is there as a safeguard as this problem is also related to the blue/cyan stain you get in rotary processes with no stop. The rotary (drum) processes such as the Jobo are prone to this problem.
We wanted to take no chance with bad dye stability and so if you see streaking on any prints or a cyan dmin it is indicative of poor removal of developing agent in the paper and needs a good stop. Acetic acid is the best choice for this IMHO having worked on the problem through vacation to make dye stability better for you. This is not something you would hear about generally because I feel bad about the problem. It cropped up after release and we had missed it due to our high quality processing. Medium to poor processes show this problem.
So, why take a chance.
Besides, a good stop gives better whites, longer life to the blix and improves the wash rate.
So, there you are! Remember that all processes, water supplies and peoples work flows are not created equal. The process is aimed at the worst possible conditions now, one that I personally missed way back then and am very very aware of!
BTW, one of the reasons it was missed was that EP2 was to have been EP3 with a final stabilzer. This pH 4.5 stabilzer with Citric Acid, was the final bath and did do the same job after the blix had first shot at the CD3. Most customers did not want the stabilizer so it was eliminated from the process thereby revealing the problem too late.
Interestingly, this pink stain appears and then vanishes so that was another reason why we missed it. But, once the process has begun, it does not stop even though the prints may appear normal.
If you wish to see the pink stain, use an acetic acid stop repeatedly without dumping it, until it is nearly exhausted, and it turns bright pink. Addition of sulfte (which is in the blix) eliminiates this pink color. If you add a pinch of sulfite to the stop, the pink color vanishes. Same reason as it vanishes in the paper. It is being sulfonated in this case by the Sulfite. Good blix was intended to prevent this, and so a well replenished fresh blix is also another reason why the photofinishers see little of this type of problem.
I hope now that you see the ramifications of color system development and the common problems you may experience if you make unwarranted substitutions. Good luck.
Acetic acid may be the way to go. I hope the smell isn't too offensive. My darkroom is a cupboard right next to the living room and smells emerging may cause some concern with my wife. Stop bath is my Achilles Heel with RA4: my heated Nova is two-slot necessitating a transfer into a tray of stop and then back into the processor. Works fine, vinegar stop wasn't too smelly, citric acid I hoped would be less so. Acetic acid stop from memory is stronger smelling.
PE, thank you again very much for this deep insight. The odor problem with acetic acid is not that bad. Good to know it's better for RA-4. As you say, why take a chance.
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Here comes the doubt: I knew citric acid is more acidic than acetic acid I guess, since it has a higher pKa than acetic acid (first dissociation).
Originally Posted by Photo Engineer
Am I wrong?
Actually, at equal Normality, both have about the same pH in solution and about the same concentration or about 5 g - 6 g/L for a 0.1 N solution. However, I have never heard of anyone getting citric acid burns from contact with concentrated citric acid solution, but I have gotten burns from concentrated acetic acid solutions.
Originally Posted by Alessandro Serrao
Yes, acetic acid can be more acidic, or be effectively more acidic in solution.
Remember that 1 N acetic = 1 M acetic, but only 1/3 M Citric.
Keep in mind there's a difference between corrosiveness and acidity.
Originally Posted by Photo Engineer
For up from the ashes, up from the ashes, grow the roses of success!
There is also solubility in fat and skin and diffusion rate. These are related to activity in swollen gelatin as well as in contacts with the human body. Citric acid is much more polar and a very large molecule compared to Acdtic Acid. All of these were factors in the design of this process.
Although corrosiveness and acidity are hard to separate out and measure easily.
Just discovered you can buy 500ml glacial acetic acid online for £8.96 in the UK from Abbey Chemicals:-
Not sure what the delivery charge is or if prices exclude VAT.
500ml should make about 25 litres of stop bath.
Works out less than 50p/litre when diluted.