Home-made PO and pH

[Shinnya]

Hi,

I have been mixing my PO for Pt/Pd developer for some time now. I mix 2lb of Potassium Carbonate and 1.75lb of Oxalic Acid. Whenever I make the solution, pH seems to be higher like 7 to over. I usually end up adding more Oxalic Acid when I use it to lower pH.

Chemistry is not my background, but I was wondering if I could add more Oxalic Acid when I mix it in the first place. If I use the same amount, could I make a solution with lower pH?

Also,by mixing this, what % solution do it get? I would like to know how to calculate that. If you could tell me the basics, I will find out from textbook...

Thanks as always.

Warmly,
Tsuyoshi

 

[Gerald Koch]

Your PO solution has very little buffer capacity so any small excess of carbonate shows up as a large deviance from pH 7. If your stochiometric calculations are correct you need not worry about the pH difference.

 

[clay]

Actually, you do need to worry about the pH. The problem is that if your developer is slightly basic, you will lock in ferric oxalate to the paper fibers to the point you will never get it to adequately clear. A pH of 6 for your developer will make your life infinitely easier when you get to the clearing steps. The pH worry also is relevant for the commonly used water-only clearing step a lot of workers use right after development and before chemically assisted clearing. If your tap water is slightly basic (even pH 7.5) you will get permanent ferric locked into your paper. I speak from experience here.

 

[Shinnya]

Hi,

My reason for lowering pH is to be able to clear irons. So, can I add more OA to begin with if I am adding later anyway?

My concern, without understanding chemistry, is that if I change anything important if I do add more OA in the first place... It does not sound like it, but I want to be sure and I want to understand the mechanism of it.

Thanks again.

Warmly,
Tsuyoshi

 

[bill schwab]

A pH of 6 for your developer will make your life infinitely easier when you get to the clearing steps.

How would one go about lowering the ph to 6? I know I know this, but my head is swimming with all of the info I have read. Thanks in advance...

Bill

 

[Michael Mutmansky]

Clay is absolutely correct, avoid neutral or basic pH balance at all costs.

When I mix, I add the oxalic acid and then get it all into solution. I then add a little extra until the pH is down below 6. A little goes a long way on this. Check the pH with test strips from Edmunds Scientific (about $1.00 for 100 strips).

I think that the problem is that oxalic acid can be hydrated, or it could be somewhat dehydrated (it will hold 2 water molecules per oxalic molecule). If the oxalic is not fully hydrated, it will be more potent per given mass, and vice versa.

There's absolutely no concern about ensuring proper pH by adding more oxalic initially, and I recommend you do that as a matter of course. It also makes sense to check the pH of your developer solutions every once and a while to ensure that they haven't drifted too much. If they have, add oxalic and test again.

---Michael

 

[Shinnya]

Bill,

Just add Oxalic Acid to PO. If you are using Sodium Citrate, use Citric Acid. I would assume the same for Ammonium Citrate from the name.

Happy printing,
tsuyoshi

How would one go about lowering the ph to 6? I know I know this, but my head is swimming with all of the info I have read. Thanks in advance...

Bill

 

[Shinnya]

Micheal,

I think this is the problem. My OA does seem to be a little more "wet." I bought a lot from Chemistrystore. So, it just needed more than what I weighed, which makes a sense.

So how about %? How can I figure this out?

Thanks again.

Warmly,
Tsuyoshi

I think that the problem is that oxalic acid can be hydrated, or it could be somewhat dehydrated (it will hold 2 water molecules per oxalic molecule). If the oxalic is not fully hydrated, it will be more potent per given mass, and vice versa.

 

[Michael Mutmansky]

Tsuyoshi,

There is a lot of flexability in the formulas. I would base it on the Carbonate amount, and then just balance the solution to pH of 6+-. That will work well without any difficulty.

Since the oxalic acid is always in some form of hydration, it's hard to know how much you are really putting in, but it is a simple matter to put in an amount and then test for pH and add more to obtain the correct pH. Ultimately, that is all that is important anyway, assuming you are mixing a high enough concentration of PO in the first place, which the amounts you provided indicate you are.

I think when I got my container from The Chemistry Store it wasn't as wet as it is now, so I suspect that the hydration has changed with time. Pennsylvania humidity will do that to you...


---Michael