No need for a pH meter. They can be fraught with problems, and the probes will need replacement from time to time.
I purchased pH strips from Edmund Scientific (100 strips for $1.00). They are certainly accurate for what is required here; ensuring that the developer is acidic, and checking the spectrum of clearing baths, etc.
Kerik says that he keeps his developer around pH 6. I like to drop mine down to about pH 5.5 because it will tend to drift upwards as papers are put into the developer (and the buffers leach out). So, starting at 5.5 will give a bit more time before the developer needs a bit more acid. It's one fewer thing to monitor closely. Oxalic acid is a clearing bath solution, so all that will happen is the print may clear a bit more in the developer.
There's no need to make sure about the absolute pH of the developer, just make sure it is decidedly on the acidic side.
As for EDTA, the tetra- version is basic, and does make a good final clearing bath for that reason. However, the di- version is acidic (around pH 4, if I remeber correctly), so it could safely be used in clearing baths without concern of ferrous conversion.
Thanks again to all. I appreciate your response.
This is something that does not have to be monitored all that frequently, so the difference between 5.5 and 6.0 is not that significant. For a little real world perspective, we've checked this during workshops. With a developer that started at 6.0 (measured with a meter) after ~100 8x10 prints have gone through 1.5 liters of developer, the pH only increased to about 6.4 or 6.5. Don't feel like you have to check every time you print (unless you're making 100++ prints during a session!)
Originally Posted by Michael Mutmansky
2014 Workshop Schedule Online
A lesson from organic chemistry: first EDTA itself is very weak acid. And secondly you don't have it as pure acid, but rather a salt (sodium or something). If you have a salt of strong base and weak acid, you'll get a buffer. And buffers can have various pH depending on acid, base and ratio of this two.
Originally Posted by eggshell
That leads me to another question - Is there a difference between sodium sulfite and HCA when used as clearing agent? My understanding is that HCA contains sodium metabisulfite and sodium sulfite. So what is the role of sodium metabisulfite? In my test sodium sulfite alone clears as well as HCA (after first clearing in citric acid for 5 minutes). And why the need for EDTA, since it is a weaker clearing agent than HCA?
Originally Posted by marko_trebusak
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I think the speed with which the PO will drift is dependant on the the temperature of the developer, the amount of time the paper is soaking in it, and the amount of buffering products in the paper. Of course, it also depends on how much developer you are using for each print.
I agree with you that the difference is inconsequential. I just make sure that I err on the acid side of 6 because I know the pH strips are not too accurate to begin with. So when I set my pH for the PO, I make sure it is on the 5 side of 6, which I equate to 5.5. Otherwise, it is nothing to be too concerned about, as you have indicated.
I will typically test it at the beginning of a session, and then not again the rest of the day unless I am moving a lot of paper through the developer.
Clearing Rives BFK
I'd like to revive this thread once again to ask about clearing Rives BFK. I've finally begun testing of this paper and am really liking what I see so far. Problem seems to be in clearing it though. I've gotten pretty aggressive with it to little success. I have not yet tried adding oxalic acid to my FO, but I have done various strengths of EDTA and HCA as well as a combo of the two, nothing seems to work. There is still some faint staining and it is not fogging. I'm hoping I can get away with not going with anything stronger. I will be mixing more FO later today and will add the OA at that time to give it a try, but I am hoping someone might have other suggestions. My water supply seems to be fine ph-wise, so does my developer and I am not looking to that as my problem. I have also done away with my usual post development water soak and gone directly to clearing bath. That has helped, but not solved the problem. Any suggestions greatly appreciated.
PS. It is my experience that HCA definitely does bleach the print over time.
HCA will bleach for sure, as will some other clearing bath solutions, like citric or oxalic acid, and even sodium sulfite/edta if left in long enough, so it is really hard to clear a difficult paper without getting some bleaching.
The OA in the FO should help, but you may have an issue of pH in the paper causing a problem. It will affect the spedd of the solution, so e prepaed to adjust your printing times. You will also have to be more careful to avoid fogging as the speed of the FO increases.
I presume you are pretreating the paper? You may have to use a stronger pretreat.
Using warm developer and warm clearing baths helps as well. Also, make sure you air-dry the paper before the coating solution soaks in too deeply. That can reduce clearing times as well.
I don't use the BFK, so I don't have a specific procedure to recommend, but I think some people are using pretty strong acids with it (phosphoric comes to mind).
Before trying the OA in the FO I will try to go a little stronger on my OA pre-treat. I have always used a 1% solution and perhaps a little stronger/longer treat will help as you say. I have also perhaps let the coating soak-in too much before drying. I will add that to my variable as well.
Thanks for the suggestions. Back to the coating room.
I've just started using BFK and I do not have a clearing problem. I treat the paper with 1.5% OA for 5 min. Develop in PO for 5 min. Soak in citric acid for 5 min, and 3 baths of Sodium Sulfite/Edta (a little stronger than needed for Platinotype paper) for 5 min. each bath. I'm using BFK Rives Velin 200 gms. Perhaps you are using heavier BFK paper or are double coating it. I see no difference in single or double coats, so I'm coating only once. I also let the coat rest for a min. or so before force drying with mild hairdryer setting. Hope this helps!