[rollei_list] Re: OT: development
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 20 May 2005 17:16:28 -0700
----- Original Message -----
From: "Jerry Lehrer" <jerryleh@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, May 20, 2005 4:11 PM
Subject: [rollei_list] Re: OT: development
> Richard,
>
> I seem to recall that sodium chloride was one of the
> components of a hypo eliminator.
>
> BTW, I would be loath to use a non hardening fixer
> ever for film. Of course I used a non hardening fixer
> for PAPER that was going to be toned.
>
> Jerry
>
Maybe. I think sodium chloride was tested along with a
number of other salts in the Kodak tests. I have to dig out
the paper. Its not an effective wash aid. It is the anion
that does the work and the most effective one is sulfite.
Kodak wash aid has three features:
Neutral pH. At this pH the gelatin is above its isoelectric
point so does not bind ions due to internal charges. This is
also beyond the window of pH where white alum hardener acts
as a mordant binding thiosulfate ions. However, neutral is
still within the window for preserving hardening.
Also neural pH, near the isoelectric point so it minimises
swelling and the length of the diffusion path.
Ion exchange. Sulfite acts to displace thiosulfate and
reaction product ions.
The third is the main accelerator of washing. A mild
alkaline bath will raise the pH of the gelatin above the
isoelectric poing eliminating binding of ions by charge and
by the mordanting effect of alum in hardening fixers.
However, other alkalies do not have the ion exchange
property of Sulfite, or have it to a lesser extent.
Other wash aids have Ammonium sulfite. There is probably
no advantage to this but it probably works as well as sodium
sulfite.
Kodak has curves of treatment time vs: wash time for
complete washing. The curve is asymptotic, after about 4
minutes there is no advantage to longer treatment. I am not
sure without looking if this was for emulsion on film base
or for printing paper.
The elimination of white alum hardener in fixing baths
eliminates its mordanting effect. Also, the main reason for
making fixing baths acid is the presence of the hardener,
which works only over a narrow range of pH. Even with a
neutral fixing bath the ion exchange effect of the sulfite
bath will accelerate washing.
Gelatin is amphoretic, that is, it has characteristics of
both an acid and an alkaline. It takes on the pH of the last
bath its in. Where that is an acid fixing bath it leaves
behaving like an acid. Where the last bath is a alkali it
leaves behaving like an alkali. The sulfite leaves it
neutral.
BTW, since the wash rate at neutral pH is about as fast
as it gets there is no reason for using an alkaline fixing
bath, a neutral bath will do.
However, an acid fixing bath, even if it does not have to
support a hardener, has the advantage of preventing carried
over developer from remaining active or causing stains.
Balanced against this is the lack of odor of a neutral bath
and the lack of bleaching effect of Ammonium thiosulfate
when not acid.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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