This subject is to offer data from "the photographic Lab Handbook"[D[D[D 
by John S. Carroll.  If anyone is a B&W home processor and wants film data
or chemistry, let me know & I will try to look it up for you.
This is good for times/temperatures for various developers on B&W films.
My handbook copy is old, so I do not think it covers T-max.

9 responses total.

Does anyone have a batch of thoroughly exhausted hypo they would be
willing to give me? I want to try some silver plating (for radio
purposes), which I've read can be done from exhausted hypo. 

re#1:  how much do you need?  i might be able to give you some out of my
        x-ray darkroom.

A pint would be fine (not exactly offering to dispose of all your used
hypo....). The more "spent" the better, of course - more silver. Do you
recycle or recover silver values? I know they do from the film itself,
due to some horror stories a few years back of police finding big trucks
filled with old film that had been soaked in cyanide to recover the
silver, and then abandoned (the film was soaked, not the trucks, though
the trucks were kinda soaked too....). 

Well, I use it at about a gallon at a time...and I usually try to squeeze out
as much use of the fixer as I can.  Sometimes, I go through 3 changes of
developer for one change of fixer.  I can hold you a quart, or gallon, next
time it's changed if you have a bottle for me to put it in.  I'll have to
check with my office manager to see when it is due for changing.

Let me know. I have lots of gallon milk bottles. 

Rick did provide me with spent 'hypo' (thanks!) and it did sit around for
a while, but this week I finally tried to plate silver on copper with it. 
I set up my copper disk as the cathode and used carbon rods for the anode,
and somewhat arbitrarily chose a current of 2 ma/cm^2. I added the 'hypo'
and turned on the current - and copious flakes of silver swirled off the
copper disk. I also tried it without current, and only a very loose plate,
easily rubbed or even washed off was formed. The problem is that the
thiosulfate complex of silver is not 'strong' enough to reduce the free
silver ion concentration to a point where it does not spontaneously react
with copper (copper being more electropositive than silver). 

I am now considering adding cyanide to the used hypo, to complex the
silver as the cyanide, which is what is usually used for silver plating. A
problem is to know the concentration of silver (and unused 'hypo') in the
used hypo, for estimating the amount of cyanide required (and dilutions
necessary to attain the silver cyanide complex concentration recommended
for plating).  Therefore this is the right item - what does the handbook
say in answer to: 

1. What is the usual concentration of sodium thiosulfate in 'hypo',

2. What fraction of this is likely to have been consumed in the used
   'hypo', conventionally?

3. About how much rinse is likely to have been carried over into the
   'hypo' during film processing, until the 'hypo' is exhausted?

I would be glad for information or speculation on any of these matters,
although I would only need rough estimates, as I think the silver will be
titratable with cyanide - that is, solid silver cyanide will first
precipitate as cyanide is added, and then redissolve as the complex with
further addition of cyanide. However I don't work with cyanide unless I
have planned ahead to the extent possible.

You might actually just look into silver recovery systems which are
commercially available.  I think some "cheap" ones are in the range of a few
hundred dollars.  Will pay for themselves if you recover enough used hypo.

I intend to get a stainless plate (for the cathode) and try just plating
out the silver. (If that works OK, I may be looking for more used hypo. 
From silver I can make silver nitrate, and prepare a proper silver plating
bath from that.) 

After looking further into plating directly from hypo, it appears I would
have to complex all the silver anyway with cyanide after neturalizing the
used hypo's acidity. Another problem is that ammonium thiosulfate was used
in the hypo, so it will form free ammonia when made alkaline, and free
ammonia attacks copper. It looks like it would take a lot more
experimentation to get a coherent silver plate directly from used hypo. My
inquires on usenet if anyone had done it also met with no success.

I did this (#8), but no silver. Instead, a black suspension formed. I ran
at 124 ma for a little over two hours. I suspect that the suspension is
silver sulfide (Ag2S). The activity of silver w.r.t. silver sulfide is
less than w.r.t. the thiosulfate complex, so either thiosulfate ion was
reduced to sulfide ion, precipitating Ag2S, or silver was released but
was immediately oxidized by thiosulfate by the reaction 2Ag + S2O3(-2) =
Ag2S + SO3(-2). [Since no one else is using this item to seek data from
the lab handbook, I presume no one minds this digression in photographic
chemistry. But maybe someone will come along who knows how to do what
I am trying to do. In the meantime I will do a search on silver recovery.]

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