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  1. #31
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    Quote Originally Posted by Doremus Scudder View Post
    I'm interested in learning more about why dilute fixer does not fix archivally. I'm no chemist, but I've hashed this out with others on this forum who are and corresponded with the Ilford tech rep about using rapid fixers 1+9 for film. The consensus seemed to be that it worked fine, but that fixing times needed to be extended.
    Quite to the contrary, I've seen a Delta film remain unfixed in Ilford Rapid Fixer 1+9. Refixing in 1+4 solved the problem.
    Quote Originally Posted by Doremus Scudder View Post
    Certainly, if there is not enough of the thiosulfate ions in solution to achieve complete fixing, there will be residual silver in the film.
    There are two things at work: first, you need to have enough Thiosulfate available to convert all Silver ions into a soluble Thiosulfate complex. Second, you have two or more Anions competing for Silver ions: Br- and S2O32-, and in case of film I-. If you look at the solubility products of AgBr and especially AgI, and compare these against the complex stability constant of soluble Silver Thiosulfate complex, you see immediately that there is quite some competition for these Silver ions:

    [Ag+][Br-] = 5.3 10-13

    [Ag+][I-] = 8.7 10-17

    [Ag+] [(S2O3)2-]3 / [Ag(S2O3)35-] = 2.9 10-14

    The balance between AgBr and Ag(S2O3)35-, and particularly between AgI and Ag(S2O3)35- depends on the concentration of (S2O3)2-, and not linearly but with an exponent of 3. Plot x3 for x between 0 and 10 to see what that means. An 1+9 dilution is not twice as good as 1+19, it's eight times as good!
    Quote Originally Posted by Doremus Scudder View Post
    Much more risky, in my view, is using fixer that is near exhaustion, where lots of insoluble compounds have built up.
    The biggest issue will be buildup of Br- and I- which shift the equilibrium towards AgBr and AgI. But note that the Br- and I- concentrations enter the balance only linearly, not with an exponent of 3. You can estimate that reusing fixer eight times is as bad as diluting it twice as much, i.e. reusing 1+4 eight times is about equal to using 1+9 single shot. And using two bath fixing always fixes the film with fairly fresh fixer in the second fix, which makes two bath fixing not only more economical but also more archival.
    Quote Originally Posted by Doremus Scudder View Post
    When I first started processing negatives with weaker dilutions, I tested them for residual silver. They passed the test with flying colors. I used the Formulary residual silver test kit. I would think that that would show the residual silver you mention.
    If the test says it's fixed we might as well assume it is fixed. Case closed. But do note that what works for one emulsion does not necessarily work for other emulsions.
    Trying to be the best of whatever I am, even if what I am is no good.

  2. #32

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    Quote Originally Posted by Rudeofus View Post
    Quite to the contrary, I've seen a Delta film remain unfixed in Ilford Rapid Fixer 1+9. Refixing in 1+4 solved the problem.

    There are two things at work: first, you need to have enough Thiosulfate available to convert all Silver ions into a soluble Thiosulfate complex. Second, you have two or more Anions competing for Silver ions: Br- and S2O32-, and in case of film I-. If you look at the solubility products of AgBr and especially AgI, and compare these against the complex stability constant of soluble Silver Thiosulfate complex, you see immediately that there is quite some competition for these Silver ions:

    [Ag+][Br-] = 5.3 10-13

    [Ag+][I-] = 8.7 10-17

    [Ag+] [(S2O3)2-]3 / [Ag(S2O3)35-] = 2.9 10-14

    The balance between AgBr and Ag(S2O3)35-, and particularly between AgI and Ag(S2O3)35- depends on the concentration of (S2O3)2-, and not linearly but with an exponent of 3. Plot x3 for x between 0 and 10 to see what that means. An 1+9 dilution is not twice as good as 1+19, it's eight times as good!

    The biggest issue will be buildup of Br- and I- which shift the equilibrium towards AgBr and AgI. But note that the Br- and I- concentrations enter the balance only linearly, not with an exponent of 3. You can estimate that reusing fixer eight times is as bad as diluting it twice as much, i.e. reusing 1+4 eight times is about equal to using 1+9 single shot. And using two bath fixing always fixes the film with fairly fresh fixer in the second fix, which makes two bath fixing not only more economical but also more archival.

    If the test says it's fixed we might as well assume it is fixed. Case closed. But do note that what works for one emulsion does not necessarily work for other emulsions.

    Rudeofus,

    Thanks for the detailed information. You make a very convincing case, and in light of your superior knowledge of the subject, I will refrain from recommending using diluted fix from now on, or at least until I make some more tests.

    I rarely use more modern emulsions like Delta or T-Max, so perhaps my 1+9 dilution is indeed working well as my tests indicated. Nevertheless, I can see good reason to save up negatives until I have a batch that I can use the 1+4 dilution for. I'm wondering what the effect of two-bath fixing with the 1+9 dilution provides in the way of better fixing as opposed to just fixing in a single 1+9 solution.

    As I mentioned, the great majority of my film is fixed using the two bath method, and most of that at the 1+4 dilution. However, for less than 15 sheets or so, I have been using 1+9 two-bath. I'll be repeating my tests and trying to use 1+4 more.

    Thanks,

    Doremus

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