Fixer Failure - Any Ideas Why??

Rudeofus · Nov 25, 2012

Pat Erson said:
"That's why clip tests are recommended"
Allright but do you have to prewet your clip in plain water before you dip it into the fixer or can you use a dry clip?
I've heard that to get an accurate result you have to use a "wet" clip...

Clip test is not an accurate test anyway since all it tells you is whether and in which time frame fixer dissolves enough silver ions to make the test clip look transparent. Theoretically it tells you nothing about retained silver or how long it takes from clearing to archival fixing.

If you start with a dry test clip, the fixer will take longer to clear the test clip, so chances are you will overestimate the time you need for fixing. On the other side, the time difference between dry clip and prewet clip should be less than half a minute. IMHO it really doesn't matter. Give the fixer an extra minute if you want to be sure. Or use one of these commercial tests for retained silver to verify that your fixer procedure, whatever it is, works reliably.

Doremus Scudder · Nov 25, 2012

Rudeofus said:
This is exactly the technique I warned against and I will give you the reason why:

Fixing is a competition between AgBr/AgI and the complex of Silver with the fixer, usually Thiosulfate.

The balance between these two is governed by the solubility product of AgBr/AgI and the complex stability constants of the various Silver Thiosulfate complexes.

Note that all soluble Silver complexes contain several molecules of the fixer, which means the concentration of fixer goes in with an exponent of 2-5.

A low concentration of fixer will therefore shift the balance strongly towards AgBr/AgI and those Silver complexes, which contain only one or two molecules of Thiosulfate. Note, that these are insoluble, i.e. they will remain in your film. And they are not archival!

This means you will end up with retained silver ions, regardless of how long you fix and what your clip test tells you.

And this is the reason why I advise against one-shot fixing: it becomes expensive, people start diluting their fixer, and archival stability goes out the door. Using two bath fixing with full strength fixer as described by Haist (and many others) is cost effective and avoids exactly these problems.

Rudeofus,

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.

Certainly, if there is not enough of the thiosulfate ions in solution to achieve complete fixing, there will be residual silver in the film. However, that is just a question of making sure one has enough capacity. I would think that making sure the capacity is not exceeded and extending the fixing time would be enough to ensure that no insoluble silver compounds remained in the film. Much more risky, in my view, is using fixer that is near exhaustion, where lots of insoluble compounds have built up.

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.

I do agree that two-bath fixing is more economical when processing larger amounts of film, but for just a few sheets, I find a one-shot approach the best solution.

Best,

Doremus

Rudeofus · Nov 25, 2012

Doremus Scudder said:
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.

Doremus Scudder said:
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!

Doremus Scudder said:
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.

Doremus Scudder said:
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.

Doremus Scudder · Nov 27, 2012

Rudeofus said:
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

Fixer Failure - Any Ideas Why??

Rudeofus

Doremus Scudder

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