D-19, Dense or Light Soda Ash?

[Mani_Reshad]

I was wondering whether dense sodium carbonate is used in d-19 or the light version?

 

[koraks]

The terminology "dense" or "light" aren't well-defined chemically, neither is "soda ash". Photographic formulas often call for sodium carbonate as an activator, and it's usually safe to assume the monohydrate unless specified otherwise. In the case of D19, there's no need to guess since it's specified explicitly: https://www.digitaltruth.com/data/formula.php?FormulaID=117

Sodium Carbonate (monohydrate*)

52.5

g

*multiply amount by 0.833 if using Sodium Carbonate in anhydrous form.

Sodium carbonate monohydrate = CAS 5968-11-6
Sodium carbonate anhydrous = CAS 497-19-8

The monohydrate is usually the most convenient to use, because it is stable and not particularly hygroscopic. Anhydrous sodium carbonate, on the other hand, is hygroscopic and absorbs moisture from the air, turning itself into the monohydrate. There's also the possibility to use the decahydrate, which is sold in many/most European supermarkets as 'crystalline' cleaning soda. This is also stable, but due to the high water content, more voluminous, so it takes up more space. It also tends to dissolve slower than the monohydrate, which tends to come in small pellets/grains instead of larger chunks.