Potassium or Sodium

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Can anyone tell me the practical differences when one substitutes things like sodium carbonate and potassium carbonate, and the various borates, bisulphates, and "Kodalk"? I often don't have the exact item called for, but have seen various substitution charts. What I want to know is will the developer charactoristics change significantly?

-- Conrad Hoffman (choffman@rpa.net), December 04, 2001

Answers

That's a very good question, and one for which there is not a simple answer. On the surface, one might assume if you use enough of, say, potassium carbonate in place of sodium carbonate, such that you equal the required pH value, the result would be equal. In fact, I believe, using those chemicals, that would be the case. I think I recall from one of Anchell's books that as long as you stayed within a given familial group (ie: carbonates), substitution was predictable.

However, simply matching pH while straying outside the familial circle invites interesting deviations from the expected. For example, sodium hydroxide is noted for producing high pH values. But what one seldom hears is that the high pH value falls rather quickly, and thus, is very valuable in compensating formulas. Carbonates, on the other hand, maintain a more consistent alkalinity and a steadier buffering factor during the cycle of development. The net effect is surprising. I've found developers using carbonate often exhibit more contrast and more grain than the same developing agents used with NaOH. This is quite the opposite of what I was led to believe from various published sources. Try substituting a 5% solution of NaOH in place of potassium carbonate as the "B" component in FX-2 to see a very pleasing case in point.

I know I haven't really answered your question, Conrad, but hopefully I will have opened lines of thought that others may contribute to and from which we may all learn.

-- Ted Kaufman (writercrmp@aol.com), December 04, 2001.


None, basically. Potassium salts are generally more active and have a higher solubility, and there may be a cost or stability difference between Na.X and K.X, but there appears to be no scientifically proven and documented difference between the results obtained.

-- Pete Andrews (p.l.andrews@bham.ac.uk), December 05, 2001.

I've just re-read the question, and now I'm not so sure what's being asked. I THOUGHT the question was simply about any difference between Sodium compounds and Potassium compounds, and my previous reply was with reference to that only. If I've mistaken the question, I apologise.

-- Pete Andrews (p.l.andrews@bham.ac.uk), December 05, 2001.

Thanks Conrad for the interesting question, and Ted also for that interesting reply. I had always assumed, based on what I had read, that the higher the alkalinity, the more coarse the grain structure (something about the high activity causing clumping). And therefore assumed that developers like Rodinal, would be improved if the hydroxide was replaced with a less base accelerator like either of the carbonates. Its not quite that simple after all. Wish the whole thing wasnt so complicated!

-- George Paltoglou (stellar@optusnet.com.au), December 05, 2001.

Conrad:

You might want to pick up Steve Anchell's books re photographic formulas. I don't have them handy to give exact titles but they can be gotten at amazon.com. You'll find that there are oftentimes simple substitution factors. A lot of times I have substituted potassium carbonate for sodium carbonate in warm-toned print developers to get warmer tonal renderings. I might even try substituting tri-phosphates some day to see what happens.

-- Ken Burns (kenburns@twave.net), December 05, 2001.



Thanks everyone for great answers and comments. I have the Anchell books and many others- that's where I've seen some of the subsitutions listed. I tend to have the very common chemicals on my shelf, and then see some developer I want to try, but it always needs one oddball thing I haven't got :-)

-- Conrad Hoffman (choffman@rpa.net), December 05, 2001.

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