Fixer: What does the dissolving?

[Ulophot]

In days of yore when I taught photography, developers were alkaline, stop acidic, and fixer acidic, so keep your hands and print tongs clean. With only glimmers of pre-high school chemistry remaining in memory even then (starting on college), I assumed that the acidity dissolved the undeveloped silver salts.

Now that I use TF-5 fixer, which is reportedly close to neutral PH, and darkrooms are seemingly populated with everything from lemonade developers to fixers made from extract of Antarctic clover, I think I need a little better grasp of my basic chemistry if I am going to teach again, as may come to pass.

I said, a little better grasp. I don't intend to delve into chemical atomic equations any more than to begin studying optical formulas to explain lenses and DOF. If someone has a good layman's reference book or site to answer my question of what it is in fixer that does the work, or can simply supply an answer here, I will be grateful.

[Michael R]

Thiosulfate does the work. Either sodium thiosulfate in a "standard" speed fixer, or ammonium thiosulfate in rapid fixers. There are some additional compounds added to a few formulas to make them super-rapid.

You are correct regarding TF-5 pH which is very near neutral. Note pH isn't really relevant to the process of fixation per se, and the whole pH thing with fixers is generally a much overblown topic, particularly in the case of rapid fixers.

[Drew Wiley]

Well, the alkalinity of TF4 certainly helps with respect to more efficient removal of any residual dyes, though there are post-fixing ways of doing that too. I like the speed and convenience of a single step prior to washing. That was especially important when I was working my butt off all week long, and needed to squeeze in darkroom work into my tight schedule as efficiently as possible. But if I seem to have more time after retirement, the gradual drain off of surplus energy with advancing age amounts to the same thing - wanna get it done efficiently enough that I have plenty of time and energy left over for more important aspects of the overall darkroom experience.

Plus, needing to shake those jugs of TF4 before measuring out the concentrated solution helps to wake one up if the morning coffee didn't sufficiently do that already.

[Ulophot]

Thanks. I subsequently discovered a useful discussion here as well:

https://web.tech.uh.edu/digitalmedia/materials/3351/PHOTCHEM.pdf

[Michael R]

Just don’t accidentally drink the TF-4 instead of the coffee.


Well, the alkalinity of TF4 certainly helps with respect to more efficient removal of any residual dyes, though there are post-fixing ways of doing that too. I like the speed and convenience of a single step prior to washing. That was especially important when I was working my butt off all week long, and needed to squeeze in darkroom work into my tight schedule as efficiently as possible. But if I seem to have more time after retirement, the gradual drain off of surplus energy with advancing age amounts to the same thing - wanna get it done efficiently enough that I have plenty of time and energy left over for more important aspects of the overall darkroom experience.

Plus, needing to shake those jugs of TF4 before measuring out the concentrated solution helps to wake one up if the morning coffee didn't sufficiently do that already.

[Drew Wiley]

Well, one more reason for the alkalinity of the fixer - to buffer my stomach in case the coffee is too acidic. I do best on milder blends of dark roast like those based on Kona or Sumatran, not on the corrosive rocket fuel varieties.

[Duolab123]

Thanks. I subsequently discovered a useful discussion here as well:

https://web.tech.uh.edu/digitalmedia/materials/3351/PHOTCHEM.pdf

This is fun to read. Compares silver halide in film emulsion to bananas in Jello. Pretty good analogy :rolleyes:

[LabRat]

The reason for pH adjustment with fixers is that it helps them operate iin their most efficient environment... Different plain solutions will operate somewhat, but due to solution carryover etc, they loose their efficiencies fairly quickly upon re-use...

Steve K

[interneg]

The reason for pH adjustment with fixers is that it helps them operate iin their most efficient environment

Acid fix was only necessary for getting hardeners to work. With modern vinyl ether hardening etc in manufacturing, you don't need that - near neutral pH rapid fix is the best bet in terms of efficiency & smell - and is essentially what C-41 fixes are (and there are potentially even more efficient fixers, but which run the risk of blistering less well hardened emulsions).

[Tin Can]

Very good to know

also may explain why very old plate emulsion is so fragile


Acid fix was only necessary for getting hardeners to work. With modern vinyl ether hardening etc in manufacturing, you don't need that - near neutral pH rapid fix is the best bet in terms of efficiency & smell - and is essentially what C-41 fixes are (and there are potentially even more efficient fixers, but which run the risk of blistering less well hardened emulsions).

[Doremus Scudder]

Actually, the thiosulfates in the fixer react with the silver halides left in the emulsion and, through a number of reactions (which become less effective with dissolved silver building up in the fixer) render those former-silver-halides water soluble. In the end, it's the water that does the dissolving.

Best,

Doremus

[nmp]

Water is the person pushing the cart made up of sodium thiosulfate and the cargo is silver halides.

[Ulophot]

Thanks, all. I think I had assumed that sodium or ammonium thiosulfate was the acid that was doing the work (did I mention may masterful command of chemical knowledge?), and therefore that something had been substituted for it in the low-acidity fixers such as TF-5.