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  1. #11
    Photo Engineer's Avatar
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    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

  2. #12
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    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.
    Gadget Gainer

  3. #13
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    Patrick;

    I know a lot about Rodinol from all three; experiment, theory and reading reports.

    I didn't mention contamination or pH effects from water due to the fact that pH and buffer capacity are two different things. A water source at pH 5 and another at 8 if 'pure' water do not have sufficient buffer capacity per se to have significant effect on the other ingredients. Also, a 1% or so impurity in any of the other ingredients is not likely to have much buffer capacity, but may affect pH which is quickly overcome during proper mixing.

    The discussion of buffer capacity vs pH has come up enough times in various posts to indicate to me that most non-chemists don't understand the difference.

    The use of PAP or PAP.HCl does affect final results for a number of reasons. Most tend to ignore this as well, but it is a not insignificant change. The presence of oxidized products also influences final solution strength to some small degree, but has a bigger effect during mixing of large batches. This is often ignored by non-chemists as well. On the small scale, oxygen entrainment in probably unimportant, but on the 100L or so size batch you can introduce problems. Having mixed developers at both scales, I can tell you that there is a significant difference as a function of batch size.

    Read the other thread on Rodinol mixing and formulations.

    PE

  4. #14
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    You didn't explain how precipitated anything can affect the pH of what is in solution. If pH is the principal concern, should we use a pH meter to decide when we have reached Rodinal, or will a development test suffice? pH meters are less reliable in the hands of amateurs than teaspoon measurements. What are the prime characteristics of Rodinal that would be affected by variation in pH? Most users of it are looking for a certain grain-gradation-sharpness character that they consider unique to Rodinal. If pH is a critical variable in determining those characteristics, then pH of the working solution should be critical as well. Is it?

    A saturated solution of potassium paraminophenolate should have a certain pH. The concentration of sulfite may have some effect on how concentrated a saturated solution can be, but I cannot see how precipitated p-aminophenol can affect pH. It is not ionized, is it?

    If using p-aminophenol or the hydrochloride makes a difference, why did not the recipe discovered during WWII use the hydrochloride? The amount of hydroxide is in fact almost but not quite enough to convert all the specified amount of aminophenol to the phenolate.

    If someone can show by experiment, and not by fanciful imaginings of what goes on in the mixing tank, that the order of mixing makes a noticeable difference I would appreciate it. You can refuse to listen to anyone who does not have a degree in chemistry. I can refuse to listen to anyone who does not have a degree in common sense, but I do not. In all my years at NASA, I engaged in many different research projects requiring many different fields of expertise. What I needed to know, I crammed into my brain both during working hours and at home. I found that having a degree in anything was a license to learn aout everything.

    Did I tell you I grew up in Missouri? Show me.
    Gadget Gainer

  5. #15
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    Please read the other thread. I don't intend to rehash what others have posted.

    PE

  6. #16
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    I read and reread the other thread long ago. Nowhere in it was there an indication that, let alone an explanation of how, precipitated p-aminophenol could affect the pH of a solution. Neither was there any prediction of exactly what would be the practical effects of mixing the ingredients in a different order. Furthermore, the fact that the Wolfen formula specifies p-aminophenol, not the hydrochloride, indicates that if the lacking potassium or sodium chloride is important, it did not seem so to AGFA. These are your theories that you could test in short order and perhaps explain in even shorter order.
    Gadget Gainer

  7. #17

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    The presence of a small amount of unreacted paraminophenol indicates that the solution is at the lowest pH where the phenolate can exist. There is nothing mystical about its presence.

  8. #18

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    Quote Originally Posted by gainer
    A saturated solution of potassium paraminophenolate should have a certain pH. The concentration of sulfite may have some effect on how concentrated a saturated solution can be, but I cannot see how precipitated p-aminophenol can affect pH. It is not ionized, is it?
    Patrick - if something is dropping out of solution, it will be removing ions from the solution. This can affect the pH of that solution (or the buffering capacity) depending on what it is that has dropped out.

    So it is not the unionized material sitting on the bottom of the vat that affects the solution pH, but it is the ions that were removed from solution that can affect the pH.

    Kirk - www.keyesphoto.com

  9. #19
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    That is not the point in this case. P-aminophenol does not drop out of solution. It is practically insoluble. It never gets into solution. Only that part that is combined with sodium from the hydroxide to form the phenolate gets into solution. If you start with a solution of p-aminophenol.HCl, the first hydroxide that hits the solution makes salt out of the HCl and precipitates the p-aminophenol. When that has been done, if there is more hydroxide available, the phenolate is formed. Whichever starting point you use, if there is not enough hydroxide to make phenolate out of all the p-aminophenol, some of it will remain undissolved. However, in the presence of sulfite, aerial oxidation will make monosulfonate and hydroxide which can act on precipitated p-aminophenol to make phenolate. But that is not the precipitated aminophenol changing the pH. That is a change in pH due to aerial oxidation unprecipitating some of the amnophenol.
    Gadget Gainer

  10. #20

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    I think that it is significant that the Wolfen recipe calls for the potassium sufite solution to be at 55C. This increases the solubility of the p-aminophenol and helps the formation of the phenolate when the hydroxide is added. As more phenolate is formed more pap will dissolve. I would assume that the reaction takes place in a closed vessel to prevent oxidation.

    BTW, there is a recipe in the Focal Encyclopedia of Photography that starts with p-aminophenol rather than the hydrochloride.

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