ammonium vs sodium thiosulphate fixers and wash times

smieglitz · Aug 6, 2006

I was asked on another forum whether there would be any difference in washing the residual fixer from an emulsion (wetplate collodion specifically) if an ammonium vs. sodium thiosuphate fixing agent was used. This got me thinking that perhaps one form is more water-soluble than the other. If there is a difference, which might be easier to wash from the emulsion?

Joe

dancqu · Aug 6, 2006

Whatever compounds of silver are employed it's
the thiosulfate that loads. Although ammonia has
some affinity for silver and makes for a more rapid
fix it's the thiosulfate that loads up due it's much
greater affinity. The silver thiosulfate complex
is no match for the highly mobile sodium and
ammonium ions. Dan

smieglitz · Aug 6, 2006

So Dan are you saying it doesn't matter which salt is used as a fixer - that the wash times will be similar?

dancqu · Aug 7, 2006

smieglitz said:
So Dan are you saying it doesn't matter which salt is
used as a fixer - that the wash times will be similar?

Both sodium and ammonium thiosulfate are very soluble.
Sodium full strength fix is considerably less concentrated
than it could be. Any useful strength ammonium fix is also
way below maximum concentrations.

Very similar wash times. As we are reminded over and
over it's those silver-thiosulfate complexes with which the
wash must deal and take the time. Of course, all things
being equal. Ph is a factor. My unadulterated S. Thio.
has a ph of 6.8-7.0 and the A. Thio. 7.5-6.

HCA alone, has the most effect on washing times. Ilford
claims that two 5 minute washes with a HCA of 10 minutes
twixt will suffice. So, 10 minutes wash, 10 minutes HCA.
Silver levels in the fix determine the LE. Dan

don sigl · Aug 8, 2006

The recommended wash times are not the same. The reason (in the literature) is based on the fact that the shorter fixing times with ammonium helps limit the fixer from bonding to the paper. I suppose if you fixed a sheet in either type of fixer for the same amount of time, they would require the same wash. But the ammonium fixers are rapid, thus minimizing the need to leave a print in the soup.

I recognize that there are two schools of thought here, especially when you get into discussions on which process is more archival. But, an ammonium based fixer should require less wash time than a sodium fixer if the fixing times are followed.

raucousimages · Aug 8, 2006

Dito. Less time in fix = less time in wash. It is the fiber paper base that is the issue not the film, RC base or paper emulsion.

smieglitz · Aug 8, 2006

Well, the reason I'm curious about this is related to wetplate collodion photography on glass plates or metal sheet so the paper base issue isn't relevant. Also, the method will be used largely in the field with a portable darkroom which means hauling extra bottles of HCA around isn't desirable.

So my original question was about getting the fixer and byproducts out of the emulsion, not the base. Can anyone point to an authoritative published source that might address that aspect?

Thanks,

Joe

Photo Engineer · Aug 8, 2006

As you go up in fixer concentration (either sodium or ammonium), the activity increases up to a certain point at which it begins to decrease due to complex chemical reasons not important here. However, there is an optimum concentration for each of these at which fixation has a maximum rate (Mees, Mees and James and Haist).

Ammonium complexes with silver are more soluable and smaller in size (somewhat) and therefore diffuse more rapidly out of a coating. This is why ammonium fixes are more rapid. It is also the reason why ammonium fixes wash out faster. This is given the fact that the same amount is present when compared to a sodium fix.

So, Joe, look to the references above. They tell it all, rather well, I might add.

PE

dancqu · Aug 8, 2006

smieglitz said:
"...hauling extra bottles of HCA around isn't desirable."

Have you in the field good water? If so you could mix on
the spot. But:

"So my original question was about getting the fixer and
byproducts out of the emulsion, not the base."

You are talking about washing the fixer out of film or
RC paper; also emulsions on a water proof base. HCA
won't hurt but usually not used. Dan

dancqu · Aug 9, 2006

[QUOTES=dancqu]
"Both sodium and ammonium thiosulfate are very soluble.
Sodium full strength fix is considerably less concentrated
than it could be. Any useful strength ammonium fix is also
way below maximum concentrations."

"Very similar wash times. As we are reminded over and
over it's those silver-thiosulfate complexes with which the
wash must deal and take the time. Of course, all things
being equal. Ph is a factor. My unadulterated S. Thio.
has a ph of 6.8-7.0 and the A. Thio. 7.5-6."

I was OK up until the following paragraph. Not quite
indelibly imprinted upon my mind was the nature of the
media; an emulsion on a non-absorbing base. So I included
print paper as a subject. I see I'm not the only one.

"HCA alone has the most effect on washing times. Ilford
claims that two 5 minute washes with a HCA of 10 minutes
twixt will suffice. So, 10 minutes wash, 10 minutes HCA.
Silver levels in the fix determine the LE."

This is not the thread to discuss print paper short fixing
times. I think though that it's Ilford's work along those lines
that is the source of information so far mentioned. Dan

Gerald Koch · Aug 9, 2006

Traditionally collodian plates were fixed using potassium cyanide which is very rapid in its action. Of course, this substance is quit dangerous to work with. However, potassium thiocyanate is also rapid in its action and should wash out very rapidly in contrast to thiosulfates. The reason that cyanates are not used more commonly is that they causes softening of gelatin based emulsions. Since you are using collodian this is not a problem. They were once used in stabilization machines. Its use is something that you might want to investigate. Your plates could fixed but not washed in the field like stabilized prints. Asfterwards they could be then be washed like stabilized prints. Any of the thiocyanates; potassium, sodium or ammonium could be used. They are relatively non-toxic and readily available.

smieglitz · Aug 9, 2006

Gerald,

Your suggestion about thiocyanates is interesting and in fact, the reason I am using ammonium thiosulphate is to avoid the use of potassium cyanide. However, it is my understanding that once the collodion plate dries, the collodion vehicle (unlike gelatin) becomes impervious to further processing, including washing. This would necessitate washing the plates in the field or transporting them back while still wet to a conventional washing source which I can do anyways with the thiosulphate-fixed prints.

Thanks for the suggestion and I will give the thiocyanates a try. I have some stabilization chemicals around.

Joe

dancqu · Aug 9, 2006

smieglitz said:
Thanks for the suggestion and I will give the
thiocyanates a try. I have some stabilization
chemicals around. Joe

I wonder how ammonium sulfate would work?
If only the chloride of silver is involved it may.
Ammonium sulfate, lawn fertilizer.

Collodion, IIRC, nitrated cellulose. My
comments on ph may also not apply.
I'll need to read more closely. Dan

Photo Engineer · Aug 9, 2006

dancqu said:
I wonder how ammonium sulfate would work?
If only the chloride of silver is involved it may.
Ammonium sulfate, lawn fertilizer.

Collodion, IIRC, nitrated cellulose. My
comments on ph may also not apply.
I'll need to read more closely. Dan

Ammonium sulfate is not a good fix. Ammonium Sulfite is useful, but hard to use.

PE

ammonium vs sodium thiosulphate fixers and wash times

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