FX2 and Potassium Carbonate Question

[waynecrider]

I'm buying some chemicals to mix up FX2 and the formula calls for crystalline vs Anhydrous. Apparently the crystalline form can contain a small amount of buffer which I think FX2 needs if I remember right. Since Photographers' Formulary doesn't carry it what's the work around; FX2K with the sodium metaborate substitute?

 

[Ian Grant]

The Crystalline form Crawley used back around 1961 had more impurities including bicarbonate. It would depend on the grade is used. Photographic grade was a commercial grade and not the laboratory grades we tend to buy now.

Steve Anchell has some variations of FX-2 in the 3rd Edition of The Darkroom Cookbook.

Ian

 

[Ole]

FX-2 works just fine with "modern" laboratory grade Potassium carbonate. Both anhydrous and crystal.

 

[Ian Grant]

Be that as it may Ole, Crawley states quite vigorously that it doesn't. So how do we know that you achieved the absolute optimum. Personally I'd try it and see, possibly with modern films it's not as critical

Ian

 

[waynecrider]

From what I read, the nature of the carbonate is as a buffer.

 

[Alan Johnson]

Regarding carbonate-bicarbonate buffers, I did not find any data for potassium but there is this table for sodium:
www.biochemj.org/bj/039/0245/0390245.pdf
It does appear that the going from 10% bicarb to 20% the pH only changes by 0.3. But this does not cast any light on potassium carbonate cryst bp 1960 version.

 

[waynecrider]

Regarding carbonate-bicarbonate buffers, I did not find any data for potassium but there is this table for sodium:
www.biochemj.org/bj/039/0245/0390245.pdf
It does appear that the going from 10% bicarb to 20% the pH only changes by 0.3. But this does not cast any light on potassium carbonate cryst bp 1960 version.

I did some online research and read about how potassium carbonate is made and where in the industry it is used. Being that it is used as a buffering agent at times leads me to believe that Ole is correct; There may be no difference. I had thought about getting some bisulfite and doing a film test but I think I'd rather compare the FX2 formula with the FX2K formula and get some sodium metaborate instead to replace the carbonate.

 

[nworth]

FX-2 is generally used as a one-shot. Considering how little effect 10 percent bicarbonate has on pH, I can't see why straight potassium carbonate wouldn't do just fine. (Add to this that I've found that anything Ole says is usually right.) There is also a great deal of photographic folklore that potassium carbonate produces results that are superior to sodium carbonate. I'm not sure there is any basis in fact to this. The purpose of the carbonate is just to adjust the pH, and the cation shouldn't have much, if any effect.

 

[Ole]

Potassium carbonate is more soluble than sodium carbonate, so you can make more concentrated stock solutions. That's the main reason for choosing one over the other.

The carbonate is not a buffer but the main alkali. For that reason the FX2k is a completely different developer from FX2, the alkali (and thus pH) is significantly different.

Also: The miniscule amounts of bicarbonate found in anything sold as "carbonate" will not have useful buffer properties - at least not in photographic chemistry. So one made with 1960 crystalline potassium carbonate and one with modern anhydrous carbonate and a third with sodium carbonate (assuming the amounts are adjusted to give similar carbonate concentration) will be identical for all practical purposes.

 

[Ian Grant]

Crawley, would still disagree with you Ole. I assume you've read all he's written on the subject

Personally I agree with you but even 47 years after first publishing the formulae Crawley insists it needs the Crystalline form of the Carbonate.

This is a bit like a previous post about Metabisulphite & Bisulphite, they are different but often sold as the same in the US. These differences are subtle.

Ian