[pure-silver] Re: Alkali Fix - How to Mix
- From: "BOB KISS" <bobkiss@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 27 Aug 2007 17:14:58 -0400
DEAR RICHARD,
Thanks for the thorough reply. Yes, I know about the
instability of Ammonium Thiosulfate which, in addition to the shipping, duty
and vat, is reason I don't import it to my nice warm humid island home! I
have LOTZA hypo and can easily get ammonium chloride here which is why I
asked.
I recall from my photographic chemistry courses at RIT with Dr.
Ron Francis that ammonia was an effective silver halide complexing agent
(like hypo) but, for very obvious reasons, working with ammonia can be a
little less pleasant than hypo! ;-)) Perhaps the ammonium ion in solution
contributes to improved fixing and, who knows, there may be some interactive
effect between the thiosulfate and ammonium ion that accelerates the fixing.
That would be my guess (without digging out the old notebooks and texts).
CHEERS!
BOB
-----Original Message-----
From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of Richard Knoppow
Sent: Monday, August 27, 2007 4:54 PM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: Alkali Fix - How to Mix
----- Original Message -----
From: "BOB KISS" <bobkiss@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, August 27, 2007 8:06 AM
Subject: [pure-silver] Re: Alkali Fix - How to Mix
> DEAR RICHARD,
> Do you know if I might be able to make alkali fixer
> (ammonium
> thiosulfate) by mixing Ammonium Chloride or just Ammonia
> and Sodium
> Thiosulfate? If so, in what quantities do I mix to create
> either a stock
> solution or a working solution?
> CHEERS!
> BOB
>
First of all Ammonium thiosulfate fixer is not
necessarily alkaline. Ammonium thiosulfate is used to make
so called _rapid_ fixer. For some reason the Ammonium salt
fixes faster than the sodium salt. In addition, it is less
sensitive to Iodide ions leached out of fast film emulsions,
which tend to retard the fixing rate of Sodium fixer.
There are some older formulas for making up rapid fixer
using Sodium Thiosulfate and an Ammonium salt. They were
suggested because Ammonium thiosulfate, in powder form, is
not too stable.
The action of fixer using either salt is not dependant
on its pH, as is, for instance, the action of a developer.
Most fixing baths are acid for two reasons: one is that most
contain some hardener, usually White Alum (Potassium
Aluminum Sulfate). These hardeners require the bath to be
acid within some range in order to work. The other reason is
to prevent carried over developer from continuing to
develop. Such continued development can cause dichroic fog
because it can reduce some of the silver complexes dissolved
in the fixing bath into metallic silver which is deposited
on the film or paper. Of course, continued development in
the fixing bath can also make control of development more
difficult. Because an acid stop bath can not be used with a
neutral or alkaline fixing bath it is more difficult to
prevent carried over developer from being active. A water
stop can be used but must be quite thorough, really a short
but vigorous rinse. There are evidently alkaline agents
which will stop development but I've only seen them
mentioned by Ryuji Suzuki, but not specified. I have never
seen a formula for one or seen the theory of operation
described.
Neutral fixing baths are common in some types of color
processing because acid can convert the dyes into a
colorless form. Neutral baths are also recommended by some
for preserving the stain of imagewise staining or tanning
developers, typically those using Pyro or one of its
relatives. I have never experimented with these so I don't
know how much reduction of stain is caused by an acid
processing chain. Note that the imagewise stain is not a dye
but rather a pigment related to Humic acid. Tanning is cased
by other development reaction products produced by the same
developers which produce the pigment. It may well be that
normal acid stop and fixing bath will not damage the stain
image. It is important to realize that some of the "tricks"
for increasing the stain actually only produce an overall
stain, which is not desirable since it is the equivalent of
fog.
I suggest developing a negative in the staining
developer, cutting it in half and treating half in a normal
acid stop and fixer and the other in a water stop and
neutral or alkaline fixer.
Note that for color processing the fixing bath does not
have to be alkaline, only neutral. I see no advantage at all
in making the bath alkaline.
As far as washing goes, the use of a sulfite wash aid
after normal acid processing results in a wash rate faster
than that gotten by using either a plain alkali bath or an
alkaline fixer. I've posted a long treatice about this to
this list recently (unless it went to one of the Rolleiflex
lists). In short, acid stop and fixer has become the
standard method of processing for B&W for many decades
simply because it works better than the non-acid system.
Also note, Ammonium thiosulfate fixer, when acid, is a
mild bleach for metallic silver. This is more important for
prints than for film. It is the reason that a paper strength
(more diluted) version of rapid fixers is given. The
solution is simply not to over fix. When neutral the fixer
does not have the bleaching property so when fixing time can
not be closely regulated there is some justification for the
use of neutral fixer. Also note that the addition of Citric
acid to a rapid fixer makes a quite effective silver bleach,
in fact Kodak recommends it for removing dichroic fog.
Because of this the use of a Citric acid stop bath, like
Ilford stop, should be avoided when an Ammonium thiosulfate
fixer is used.
Kodak F-7 Rapid Fixing Bath
Water (at 125F or 52C) 600.0 ml
Sodium Thiosulfate (crystalline) 360.0 grams
Ammonium Chloride 50.0 grams
Sodium Sulfite (desiccated) 15.0 grams
Acetic acid, 28% 47.0 ml
Boric Acid (crystalline) 7.5 ml
Potassium Alum 15.0 grams
Cold water to make 1.0 liter
Dissolve the hypo first then add the Ammonium chloride. Do
not add the chloride to the completed fixer or a sludge may
form.
This may be made into a neutral bath by leaving out the
Acetic acid, Boric acid, and Alum. Where no acid is added
the amount of Sulfite can be reduced to about 5.0 grams per
liter but more sulfite will do no harm and will tend to
eliminate overall staining due to carried over developer.
Kodak had an alternative formula where this one caused
too much corrosion to stainless steel processing tanks. I
can't find it right now but will look.
Note that any standard Sodium Thiosufate fixer can be
converted to a rapid fixer by substuting Ammonium
thiosulfate for the Sodium Thiosulfate in the amount of
about half where the Sodium thiosulfate amount is given for
the crystalline form as is usual in older formulas. However,
again, powdered Ammonium thiosulfate is not particularly
stable in storage.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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