Quote Originally Posted by Photo Engineer
The fundamental problem with Rodinal is that the pH is critical no matter what you do to mix it or how you mix it.

At the correct pH (which was also rather hard to do back then) there should be a small amount of undissolved PAP remaining in the bottle. If it dissolves, then the solution is too acid and if too much precipitates (more than 1 or 2 crystals) then it is too alkaline.

This will be critical if the concentration is wrong as the pH will then be off as well.

This information in complete form was published elsewhere in another thread with complete instructions.

PE
And you know this by experiment or by theory or by some inside knowledge of how it was done most recently by AGFA? There are three sources of contamination in Rodinal: the sulfite or metabisulfite, the p-aminophenol, whether the hydrochloride or not, and the hydroxide. I'm assuming that the purest possible water is used. Will leaving a few crystals in a vat of, say, 1000 liters, guarantee always the same pH, or only within some tolerance? How far off would the pH be if there were 1 gram of precipitate per liter instead of, say, 0.1 or 0.01? How does precipitate affect the pH of what is in solution?

The implication in the quoted recipe is that all the amounts are dumped in the vat, probably stirred or otherwise agitated, and left for 2 weeks to age. A minimum of stirring is guaranteed when the components are in solution to begin with, but it is not likely that there is such a thing as a 34% solution of p-aminophenol in water unless it is extremely hot water. That is why the aminophenol is dry. There was no indication of any sort of titration to produce only a few crystals. In point of fact, I have made Rodinal Expedients all the ways I could think of and some that others thought of, and the only difference I could find in operation was due to concentration.

There is something to be said for adding the p-aminophenol last. The solution is less colored, indicating less initial oxidation. However, no one has to my knowledge successfully correlated the color of Rodinal from the factory with its activity in the developing tank. If you add it last, the solution may be at room temperature.