[pure-silver] Re: Wash aid lifetime
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 17 May 2005 09:16:56 -0700
----- Original Message -----
From: "RH Designs" <richard@xxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, May 17, 2005 1:43 AM
Subject: [pure-silver] Wash aid lifetime
> Hi all
>
> I've been asked a question to which I don't know the
> answer but I'm certain
> there's someone here who does.
>
> What is the lifetime (i.e. time, not capacity) of Kodak
> wash aid working
> solution? Does it actually 'go off' ?
>
> Thanks
>
> Richard
>
Kodak gives the lifetime of Hypo Clearing Agent as 3
months in a closed container and 24 hours in a tray. This is
actually fairly long since Kodak gives the life of its
fixing baths as 2 months in a closed container.
The problem with this bath is that there is no easy way
to tell when its overage. The sulfite is slowly oxidized to
sulfate with time. Sulfate does no harm but does not have
nearly the wash aid properties of Sulfite.
In the paper describing this stuff the authors state that
some EDTA is added to prevent sludging when the solution is
reused. There is also some sodium citrate in the commercial
mixture. This is also a sequestering agent. I also saw
recently a suggestion that these two ingredients might help
preserve the sulfite but I am not sure that is correct.
Sulfite, but itself, works OK. Kodak uses Bisulfite (or
metabisulfite) to buffer it to neutral pH. Their reasons are
twofold: first that minimizes swelling and therefore the
diffusion path the hypo and reaction products must follow to
leave the emulsion; second, this pH falls in a window where
the binding effects of alum hardener are eliminated but the
hardening is preserved. The emulsion is also above its
isoelectric point at neutral so the electrical charges in
the emulsion will tend to repel rather than attract the
material one wants to wash out.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
=============================================================================================================
To unsubscribe from this list, go to www.freelists.org and logon to your
account (the same e-mail address and password you set-up when you subscribed,)
and unsubscribe from there.
Other related posts:
