Sodium and potassium in Rodinal

[srs5694]

In furthering my general reading on photochemistry, I've recently stumbled across a puzzling contradiction between two sources concerning the composition of Rodinal, and I was wondering if anybody could cast any light on the matter. According to Jack's Photographic and Chemistry Site, Rodinal contains sodium metabisulfite and potassium hydroxide. According to Anchell's Darkroom Cookbook, though, Rodinal contains potassium metabisulfite and sodium hydroxide, and Anchell explicitly cautions against using sodium metabisulfite in place of the potassium metabisulfite. In doing further research, I've found other variants on these (mostly potassium metabisulfite and potassium hydroxide). Although I have no immediate need to know (I've got a bottle of Agfa Rodinal that'll likely last me a while), I'm curious about this discrepancy. Would both formulas work equivalently, and similarly to Agfa Rodinal, Calbe R09, etc., or is one seriously out of whack? Would these chemicals combine in solution to be equivalent?

 

[Ole]

"Rodinal" was originally the trade name for para-aminophenol hydrochloride, and Agfa's developer was thus called Rodinal developer - just like others were metol developer, hydrochinone developer, and so on. After a very short time the Rodinal name stuck to the particular mix made by Agfa.

There are many old published recipes for Rodinal developers. The only thing they have in common is the developing agent, "Rodinal". That may be the source of some of the confusion.

If the quantities of bisulfite and hydroxide had contained the same number of K/Na ions it would have made no difference at all. They don't, but I still think it makes no difference! The main reason for using potassium salts in concentrated developers is that these usually have higher solubility than the sodium equivalents. In this case it should all dissolve anyway, unless you use the sodium forms of both.

 

[Gerald Koch]

he MSDS for Rodinal does not mention any sodium salts.

According to Elie Shneour of Agfa, the original Rodinal (formula R9 in the Agfa Rezept book) was composed of p-aminophenol, potassium metabisulfite and sodium hydroxide.

Concerning potassium vs sodium, in many photographic applications potassium salts are more active than their sodium counterparts.