[pure-silver] Re: PMK and MCS

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <pure-silver@xxxxxxxxxxxxx>
  • Date: Fri, 28 Apr 2006 10:13:04 -0700


----- Original Message ----- From: "David Foy" <dfoy@xxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, April 28, 2006 8:35 AM
Subject: [pure-silver] Re: PMK and MCS


I read and re-read and re-read Anchell and Troop, who I believe popularized the idea of avoiding acid fixation, to try and find out why they advocate against it. The only thing that I could find were suggestions tucked away here and there that modern films have higher iodine content than earlier films, and there is apparently something about this, having to do with how long things take, perhaps too long wash times, that lies behind the preference for alkaline processes straight through from start to finish. Perhaps I missed something. I know of no professional photochemist who sees it as an issue.

David Foy

I think Ryuji can comment better than I on the effect of Iodine on fixer but from what I understand iodides from film can slow down fixer. The iodide has greater effect on Sodium thiosulfate than ammonium thiosufate but if a two bath fixer is used the effect is minimal. High speed films, perhaps all films today, have a high percentage of Silver Iodide in them plus iodide can be added to the emulsion in other ways. The pH of the fixer should have no effect at all on this. Wash rates are affected by completness of fixing. Incompletely complexed silver is not completely soluble and also binds to the emulsion so is not washed out. That means that film or paper fixed in an exhausted fixing bath can never be washed down to archival levels no matter how long washing is continued. Sulfite wash aid does remove some of these complexes so it aliviates the problems caused by partially exhausted fixer. Nonetheless, especially if sodium fixer is used, a two bath system will go a long way in insuring complete fixation and rapid washing.
Again, there is confusion about the effect of an alkali on washing. Gelatin is amphoretic, that is, it is not either acid or alkaline, although it has what might be called a preferred pH, called the isoelectric point. Gelatin takes on the pH of whatever solution it is in. So, if an emulsion processed with acid fixer is treated in an alkaline bath before washing it becomes alkaline. If treated in a neutral bath, like KHCA, it becomes neutral. The pH of the fixer makes absolutely no difference.
The isoelectric point is important because the electrical charges in the gelatin can either attract or repel ions. The isoelectic point of traditional photographic gelatin (perhaps not the stuff used now) is very slightly on the acid side of neutral. If it is on the alkaline side of the isoelectric point it will tend to repel the ions of thiosulfate. This is why an alkaline bath works to increase wash rate (aside from the mordanting effect of alum hardener). Kodak Hypo Clearing Agent is neutral pH, this is sufficiently on the alkaline side to have the desirable charge polarity. The mordanting effect of alum hardener disappears at neutral pH but the hardening action remains. When the emulsion is made more alkaline the hardening disappears. I don't know the value of pH where this happens. Also, the swelling of gelatin is dependant on its pH compared to the isoelectic point, swelling is minimal at the isoelectric point. In the Kodak research paper on KHCA it is pointed out that minimal swelling is desirable becaue it also minimizes the diffusion path ions must take in being washed out.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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