Potassium and Sodium Carbonate

[Harold33]

Some statements about carbonates in Steve Anchell, The Darkroom Cookbook, 3rd edition, Focal Press, 2008, are not clear at all for me:

First problem: "Potassium carbonate is far more soluble than sodium carbonate, hence its use in highly concentrated solutions. The two should not be interchanged" (p. 25), vs. "Potassium carbonate can be substituted for sodium carbonate as an alkali accelerator in developers". (p. 187).

Second problem: "Thirteen parts by weight [of potassium carbonate] are equal to 10 parts by weight of sodium carbonate. (...) To substitute potassium carbonate anhydrous, multiply the amount of sodium carbonate monohydrate by 0.90" (p. 187).

Not really easy to follow what is this about... Any comment ? clear informations ?

 

[Ian Grant]

You can't always substitute Sodium Carbonate for Potassium Carbonate because of the solubility issue, this is in concentrated formulae (stock solutions). The other way around is always possible.

In the US the more common form of Sodium Carbonate is the Monohydrate in Europe it's Anhydrous. the 13 parts forr 10 parts is if both are anhydrous.

Ian

 

[Ian Grant]

Harold Anchell's maths is right, to be more precise it's 0.89 times the weigh of Potassium Carbonate anhydrous when you substitute it with Sodium Carbonate monohydrate. I have it all in a spreadsheet makes life simple

Ian

 

[Gerald C Koch]

For one of the FX series of developers Crawley insisted that potassium carbonate sesquihydrate must be used. In this case no substitution can be made. So as Michael points our not an easy problem.

Be sure to check your copy of The Darkroom Cookbook. Early copies of First Edition contained a number of significant errors. There should be an Erratum.

 

[Ian Grant]

For one of the FX series of developers Crawley insisted that potassium carbonate sesquihydrate must be used. In this case no substitution can be made. So as Michael points our not an easy problem.

Be sure to check your copy of The Darkroom Cookbook. Early copies of First Edition contained a number of significant errors. There should be an Erratum.

All the formulae in the 3rd edition of The Darkroom Cookbook were cross-checked with original sources from the manufacturers etc and all the historical errors mainly perpetuated after mistakes in the Photo Lab Index where corrected. The OP did say he's using the 3rd edition. I know there were errors in the 2nd

Ian

 

[Rudeofus]

For some time Steve Anchell provided an errata page for the first and second edition of the Darkroom Cookbook. I never saw an errata list for the third edition, and despite some claims that it's "full of errors", nobody ever came up with anything credible.

To the original topic: the difference between Sodium and Potassium ion is their alkalinity (pKb 13 vs. 13.5), and this difference should be negligible in the pH range that most ordinary film developers work in. If a developer works at pH > 12.5, one wouldn't use Carbonate as alkali anyway. I would say that a mol for mol substitution should work in most cases. Differences could occur because of their different diffusibility as exemplified by the different fixing speed of Ammonium, Sodium and Potassium Thiosulfate.

 

[Ian Grant]

For some time Steve Anchell provided an errata page for the first and second edition of the Darkroom Cookbook. I never saw an errata list for the third edition, and despite some claims that it's "full of errors", nobody ever came up with anything credible.

To the original topic: the difference between Sodium and Potassium ion is their alkalinity (pKb 13 vs. 13.5), and this difference should be negligible in the pH range that most ordinary film developers work in. If a developer works at pH > 12.5, one wouldn't use Carbonate as alkali anyway. I would say that a mol for mol substitution should work in most cases. Differences could occur because of their different diffusibility as exemplified by the different fixing speed of Ammonium, Sodium and Potassium Thiosulfate.

Actually I've never seen anyone claim or say the 3rd edition is full of errors

Ian

 

[Photo Engineer]

If I were to use the quotes from the 3rd edition as being accurate, and which were posted here, I would say that there are a few errors in the 3rd edition. I would not care to comment though as these were posted here by second parties. So....

There are errors everywhere.

I would use the recommended base, as the actual size of K and Na differ and this can have all sorts of consequences in activity.

PE

 

[Gerald C Koch]

all the historical errors mainly perpetuated after mistakes in the Photo Lab Index

One should always check the Erratum of a technical book if one exists. When I was in college one professor's hobby was tracing textbook errors back to the original source. Amazingly one error lasted for over 50 years. In another case a mathematical proof depended on a superscript suddenly becoming a subscript. Proofreading technical books is a difficult and time consuming task.

 

[Alan Johnson]

Crawley BJP Jan 6 1961 commented on sodium vs potassium, including:
"Potassium Carbonate (dried) gives lower contrast than sodium,and therefore by the time the same contrast has built up will have produced better toe contrast,as they both give the same film speed this is probably the reason for our grandfathers' advice to use potassium carbonate for "detail development"."

 

[Photo Engineer]

And I would have to have this proven with a modern film and developer! But, it does seem possible due to the various differences between Potassium and Sodium.

 

[Harold33]

Anchell's maths is right, to be more precise it's 0.89 times the weigh of Potassium Carbonate anhydrous when you substitute it with Sodium Carbonate monohydrate.

I still don't understand. How would it be possible to reduce thirteen parts by weight (...) to 10 parts by weight using "0.90" ? (or 0,89) as multiplicator ?

If I were to use the quotes from the 3rd edition as being accurate, and which were posted here, I would say that there are a few errors in the 3rd edition. I would not care to comment though as these were posted here by second parties. So....
There are errors everywhere.

The first quotation (easy to check, you only have to open the book) is not an error but a contradiction.

(...) If you are serious about learning about the basics of photochemistry, The Darkroom Cookbook is not what you want to be reading.

I agree.

 

[Ian Grant]

I still don't understand. How would it be possible to reduce thirteen parts by weight (...) to 10 parts by weight using "0.90" ? (or 0,89) as multiplicator ?

100 g of Sodium Carbonate (Anhyd) is equivalent to 130 g Potassium Carbonate (anhyd), that's the fisrt page, then they are both equivalent to 120 g of Sodium Carbonate (monohyydrate) the ratio between the Sodium Carbonate (monohydrate) & the Potassium Carbonate (anhyd) is the 0.9 factor or (more accurately 0.89).

Agfa Ansco made the calculation between Anhydrous and Monohydrated the wrong way with one Agfa (Germany) formula they published, I don't know if that carried through to manufacture.

Getting the Carbonate weight right was a big problem with the Photo Lab Index as well.

Ian

 

[Ian Grant]

Crawley BJP Jan 6 1961 commented on sodium vs potassium, including:
"Potassium Carbonate (dried) gives lower contrast than sodium,and therefore by the time the same contrast has built up will have produced better toe contrast,as they both give the same film speed this is probably the reason for our grandfathers' advice to use potassium carbonate for "detail development"."

With warm tone papers using Potassium rather than Sodium salts in a developer gives slightly warmer tones, this is due to slightly finer grain, this must occur with film developers as well. Rodinal uses Potassium salts and give excellent fine grain.

Ian

 

[Harold33]

100 g of Sodium Carbonate (Anhyd) is equivalent to 130 g Potassium Carbonate (anhyd), that's the fisrt page, then they are both equivalent to 120 g of Sodium Carbonate (monohyydrate) the ratio between the Sodium Carbonate (monohydrate) & the Potassium Carbonate (anhyd) is the 0.9 factor or (more accurately 0.89).
Ian

Now it's clear. Thanks !