about sodium and potassium

rore · Aug 27, 2006

Hi: i have a very simple question, are sodium carbonate and potassium carbonate interchangeable?

I used sodium carbonate instead of potassium carbonate in the agfa 120 formula, it resulted in cool tone, with almost no medium tones.

the original formula:
Water (52°C).............750ml
Sodium Sulfite............60g
Hydroquinone.............24g
Potassium Carbonate...80g
water to make...........1liter

thanks!

Tom Hoskinson · Aug 27, 2006

Potassium salts are generally more active than sodium salts and have a higher solubility.

Sodium carbonate has only 1/5th the solubility of potassium carbonate.

Bottom line - you would need more sodium carbonate (by weight) than potassium carbonate in this formula to reach the same solution pH and activity (ok as long as you don't hit the solubility limit). Or, you could always add some sodium or potassium hydroxide to increase the solution pH.

Photo Engineer · Aug 27, 2006

Tom Hoskinson said:
Potassium salts are generally more active than sodium salts and have a higher solubility.

Sodium carbonate has only 1/5th the solubility of potassium carbonate.

Bottom line - you would need more sodium carbonate (by weight) than potassium carbonate in this formula to reach the same solution pH and activity (ok as long as you don't hit the solubility limit). Or, you could always add some sodium or potassium hydroxide to increase the solution pH.

Tom, since the molecular weight of sodium carbonate is lower, you would need less of the sodium carbonate to match the molar equivalent of potassium carbonate. You would need more potassium carbonate to mach the molar equivalent of a given amount of sodium carbonate.

Equal molar solutions of both compounds, sodium carbonate and potassium carbonate, will yield about the same buffer capacity and pH value.

In round numbers, 106 grams of sodium carbonate is equal to 138 grams of potassium carbonate in reactivity based on molecular weight. The ratio of the two is the conversion factor between the salts.

PE

Tom Hoskinson · Aug 27, 2006

Thanks for the clarification, PE.

Thus, if I understand you correctly: In Agfa 120 example ~62 grams of sodium carbonate replaces 80 grams of potassium carbonate?

Photo Engineer · Aug 27, 2006

Yes. Everything else being equal.

PE

john_s · Aug 27, 2006

Apart from the higher solubility which makes potassium salts useful for making liquid concentrates, potassium salts are recommended for warm tones. However it depends on the paper. I used to use Agfa Neutol-WA (warm tone) developer, but the effect was quite subtle on most normal papers (i.e. not papers made to be warm tone).

You could try exposing more and shortening development at bit.

Tom Hoskinson · Aug 27, 2006

I forgot to mention in my previous posts that Potassium Carbonate (anhydrous) was called out in the original Agfa 120 formula. If monohydrated sodium carbonate (which is the most common form) is used to replace the anhydrous potassium carbonate, the amount used must be adjusted to compensate for the additional water molecule - you would need about 11% more Na2CO3.

However, I would advise using Ilford ID-78 instead (it's what I use). In my opinion ID-78 is superior to and more flexible than both Agfa 120 and Agfa Neutol WA.

Ilford ID-78 (there was a url link here which no longer exists)

dancqu · Aug 27, 2006

Tom Hoskinson said:
In my opinion ID-78 is superior to and more flexible
than both Agfa 120 and Agfa Neutol WA.

I'd think the comparison one twixt apples and oranges.
The OP's Agfa 120 had 24 grams of hydroquinone
and no other agent; similar to Wall's Normal
Hydroquinone Lith developer. Dan

nworth · Aug 27, 2006

My recipe book calls for 80 grams of sodium carbonate monohydrate in Agfa 120. Generally, sodium carbonate monohydate and anhydrous potassium carbonate are interchangeable, weight for weight, in developer formulations. The recipe also notes that tone is dependent on development and exposure, with normal exposure resulting in a brown-black tone. They were probably assuming a warm or warm-neutral tone paper as well. If you were using a cold tone or cold-neutral tone paper, your results do not surprise me. This looks like a pretty high-contrast formulation. Suggestions: 1- use a warm tone paper; 2- try diluting the developer and extending the exposure.

Tom Hoskinson · Aug 27, 2006

"My recipe book calls for 80 grams of sodium carbonate monohydrate in Agfa 120."

Which recipe book?

Tom Hoskinson · Aug 27, 2006

dancqu said:
I'd think the comparison one twixt apples and oranges.
The OP's Agfa 120 had 24 grams of hydroquinone
and no other agent; similar to Wall's Normal
Hydroquinone Lith developer. Dan

Dan, I was making a statement of personal preference based on my experience with these three warm tone developers.

about sodium and potassium

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