Dumb question about “pinch” of Sulfite mixing convention

Discussion in 'B&W: Film, Paper, Chemistry' started by Michael R 1974, Jan 31, 2013.

  1. Michael R 1974

    Michael R 1974 Subscriber

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    Metol doesn’t dissolve in Sodium Sulfite solutions, which is why the Sulfite goes in after the Metol when scratch-mixing. But a common convention is to dissolve a small amount (“pinch”) of the Sulfite before the Metol, which is supposed to scavenge dissolved Oxygen and therefore result in better preservation of the Metol while it is being dissolved.

    Haist says nothing about the pinch of Sulfite, using two examples, D-76 and D-72.

    So, why does the pinch of Sulfite (in say 750ml water) not cause any precipitation of Metol? The two possible explanations I can come up with are:

    1)The concentration of the Sulfite solution must be significantly higher to cause precipitation of Metol

    2)The entire pinch of Sodium Sulfite combines with dissolved Oxygen in the water to form Sodium Sulfate. My understanding (again from Haist) is this reaction is one of the ways in which Sodium Sulfite acts as a preservative in developers. And I’m assuming that Sulfate in solution doesn’t cause precipitation of Metol. That’s just a guess though.

    Answers?
     
  2. Alan Johnson

    Alan Johnson Subscriber

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    From "Photographic Theory and Practice-Part 4:Monochrome Processing" LP Clerc,DA Spencer,RE Jacobson 1971 p496:
    "Metol......is soluble in water 4.5 percent at 15 C........... (from a concentrated solution) the free base is precipitated when sodium sulphite is added."

    So,IMO,it could be that more concentrated sulfite solution forms the free base on the surface of any added metol particles that stops them from dissolving.

    IDK if that is the correct explanation, never come across this before.
     
  3. Bruce Osgood

    Bruce Osgood Membership Council Council

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    Snip
    End Snip

    This has been my understand as well.
     
  4. Rudeofus

    Rudeofus Subscriber

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    But we can assume that, once Metol is in solution, its free base ions float around freely, yet they don't precipitate when more Sulfite is added.

    Could it be that Metol dissolves better when its free base part can fully ionize? In other words it may not so much be an issue with Sulfite, but more with Sulfite's high pH. It may equally hard to dissolve Metol in Carbonate. The reason that the issue is usually associated with Sulfite may come from the fact that unlike other alkalis Sulfite is needed to preserve the Metol.

    When I get to it, I'll try to dissolve some Metol in 30 g/l Sodium Metabisulfite and in 30 g/l Sodium Carbonate. My guess is it will dissolve in the Metabisulfite but not in the Carbonate.
     
  5. Alan Johnson

    Alan Johnson Subscriber

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    More of the quote from LP Clerc:
    "On adding carbonate to a concentrated solution of metol,the free base is precipitated,as it is also when sodium sulphite is added.The free base melts at 85C. It is soluble in alcohol,ether and acetone and in strongly alkaline aqueous solutions."
    As you mention, that looks like ionization makes the free base soluble.
     
  6. Gerald C Koch

    Gerald C Koch Member

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    Metol is soluble in concentrated sulfite solutions. People do it every day when they mix up Kodak single package D-76. Remember in this case everything goes in at the same time. Metol just dissolves more slowly and requires more stirring. Kodak also recommends warm (125F) water. Without the small addition of sulfite the amount of metol that is oxidized is insignificant. When making up D-23, I have without thinking dissolved the sulfite first and can say that with heat and a magnetic stirrer to do the work that Metol will dissolve eventually. So the reason is mainly speed of dissolution.

    Developers which also contain hydroquinone in addition to Metol can precipitate the addition product of these two developing agents. I routinely mix up a concentrated developer which forms a slurry with the addition of the hydroquinone. The adduct dissulves when the alkali is then added.
     
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  7. Michael R 1974

    Michael R 1974 Subscriber

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    Gerald - interesting. Why does Haist say it doesn't work? In fact, why does everyone say it doesn't work? Could this be yet another myth busted?

    I assumed in the case of Kodak's single packages that all their secret additives, coatings etc somehow either made Metol soluble at the same time as Sulfite, or prevented the Sulfite from going into solution before the Metol.
     
  8. DREW WILEY

    DREW WILEY Member

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    About all I know is that once I forgot to add the pinch of sulfite first and the metol wouldn't dissolve at
    all! Packaged D76 contains a sequestering agent.
     
  9. Gerald C Koch

    Gerald C Koch Member

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    Hi Michael,

    Perhaps it is more a matter of patience than anything else.

    When in college one of the professors had a hobby to trace errors in text books back to the original source. In one case the original error had been "plagerized" from text book to text book for over 50 years! For over a hundred years the conventional wisdom was that the inert gases did not form compounds with other elements. Yet not too long ago a graduate student mixed xenon and fluorine gases together and produced a solid compound xenon tetrafluoride. This led to several other xenon compounds like xenic acid.
     
  10. Gerald C Koch

    Gerald C Koch Member

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    Metol will dissolve in plain water the solubility is 50g/l according to the MSDS.

    Some one noticed that oxygen in the water caused a discoloration. True, but the amount of oxidation is insignificant in the case of developers. But since then people have been adding a pinch of sulfite for none other than cosmetic purposes.

    The sequestering agent is to prevent calcium sulfite from precipitating when tap water is used.
     
  11. Michael R 1974

    Michael R 1974 Subscriber

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    Very interesting, Gerald. Thanks.

    Thanks, everyone.
     
  12. Gerald C Koch

    Gerald C Koch Member

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    Some developer recipes call for boiling the water first to remove any dissolved oxygen instead of adding a pinch of sulfite.
     
  13. Alan Johnson

    Alan Johnson Subscriber

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    If a solution of p-aminophenol in alkali is neutralized with acid, a precipitate ,almost certainly the free base, is formed.
    I have done this.
    Metol in alkali is like p-aminophenol with an extra CH2 group,it probably has similar chemistry.

    It may be that metol will dissolve eventually but not a myth that sulfite greatly slows the process down,and that this is due to the formation of metol base.It would be easy enough to test the rates of solution with and without sulfite.
     
  14. Gerald C Koch

    Gerald C Koch Member

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    Sodium sulfite is sufficiently alkaline to release the Metol free base. Remember that D-23 works as a developer because the solution is still alkaline even after 7.5 g of metol have been added.

    My objection was in the blanket claim that Metol will not dissolve in sulfite solutions. It WILL dissolve but at a slower rate. If it didn't then such developers as D-23 or D-76 could never be mixed no matter in what order the chemicals were dissolved for as soon as the sulfite was added the Metol base would precipitate out.
     
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  15. Gerald C Koch

    Gerald C Koch Member

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    Not all of it precipitates out since the solubility of p-aminophenol in water is 15 g/l.
     
  16. Rudeofus

    Rudeofus Subscriber

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    The problem with Metol and Sulfite (or probably any alkali) is not that the latter causes the former to precipitate out. It doesn't and there are plenty of working developer formulas loaded with Metol and operating at high pH. The issue seems to be that Metol crystals are slow to dissolve when the solution is already alkaline.

    And I really don't think Kodak's solution is as simple as writing in the manual to use hot water to mix D-76. Note Kodak holds patents for encapsulating compounds to make them go into solution later than they would do normally, and this gives them the option to dissolve different compounds in one powder in a preset sequence.
     
  17. Michael R 1974

    Michael R 1974 Subscriber

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    The "encapsulating" compounds are what I expected were involved in Kodak's single packaging. I assumed something like this kept the sulfite from dissolving before the Metol. It was just a guess though as I don't have anywhere near the chemistry knowledge required to make that claim with any kind of certainty.

    Also interesting is the wording used by Haist when he talks about mixing practices and why Metol must be dissolved before sulfite. He says the Metol must be dissolved first because it precipitates in a sulfite solution. This seems an odd choice of words since in my mind "precipitation" is when a substance already dissolved comes out of solution. If Metol actually precipitated in sulfite solutions than as Gerald says, it wouldn't matter if the Metol went in before or after the sulfite. Again though, I'm in over my head as far as the chemistry goes so what I'm saying might be nonsense.
     
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  18. Kirk Keyes

    Kirk Keyes Member

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    I always figured the pinch of sulfite was to neutralize the dissolved oxygen in the water. But since testing water for dissolved oxygen at work, I've learned that at 20C oxygen saturates in water at about 8 mg/L. That's not a lot of oxygen so it's really not that big of deal.