[pure-silver] Re: Pyro in a Jobo Processor
- From: "John Black" <jblack@xxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 18 Mar 2005 12:43:48 -0500
> this problem. I assume this is why we now see DTPA
> (diethylenetriaminepentaacetic acid) and similar compounds being used
> instead of EDTA in photo products.
Very possibly. All I have here is EDTA because I use it for other (non
photographic) purposes. The presence or absence of the EDTA seems to have
no noticable effect on the rate of oxidation of sulfite in my hands. My
buffer happens to contain EDTA but I think it would perform as well without
it (see below).
>
> I am curious about your statement "my developer at 5gm/L", what
> developer are you using that has such a low concentration of sulfite.
> This is usually the concentration that one sees in the working solution
> of a one shot developer.
Actually that is the final concentration of my developer sulfite and I
really don't store it that way. I like the negatives that I get using PC
based developers with a pH not exceeding 8.3. But I find that, at that pH,
the activity of the PC at the concentration I like to use it, is too low in
the complete absence of sulfite. So I add 5-10 gm/L back to the working
developer to activate the PC superadditivity (I suppose). Larger amounts of
sulfite result in a grain structure and loss of acutance that I find
objectionable. The working strength of my developer is 50-60gm/L phenidone
(not dimezone), 2-3gm/L ascorbic acid and 5-10gm/L sulfite at a pH of 8.3. I
do use it one-shot and used to keep the buffer and sulfite solution at a 5X
concentrate (25gm/L sulfite) but found the sulfite to loose potency slowly
over 2-3 months. The buffer I use is Tris-Borate at a working molarity of
50-100mM. I use Tris/borate because it has a pKa of 8.3 and is a powerful
buffer at this pH. As Ryuji has pointed out, Tris has a binding property
for silver but this has not proven to be a problem.
The same developer with Na Carbonate replacing Tris makes a pretty good
paper developer also.
JB
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