[pure-silver] Re: RC to FB

----- Original Message ----- 
From: "Ole Tjugen" <oftjugen@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, January 31, 2004 4:11 PM
Subject: [pure-silver] Re: RC to FB


>
> Very simple: The main thing with my fixer is that it's 
> buffered, thus
> predictable.
>
> Since it's slightly alkaline, it washes out faster. So 
> there's no need for
> a wash aid - the fixer does that job well enough.
>
> Ole Tjugen
>
   This is somewhat of a misunderstanding. pH has two 
effects on the emulsion: one is that the wash rate is faster 
when the pH is above the isoeletric point. This will be the 
case if the emulsion is made neutral, it does not have to be 
alkaline. The second effect has to do with Aluminum 
hardeners. Most fixing baths use Potassium Aluminum Sulfate 
AKA white alum, as the hardening agent. Alum has a binding 
effect on thiosulfate and silver-thiosulfate complexes when 
below a certain pH. This "mordanting" effect is gone when 
the emulsion is made neutral plus at neutral pH the 
hardening is not undone as it is when the emulsion becomes 
more alkaline, as when treated in a Carbonate bath.
   Gelatin is amphoretic. Amphoretic materials have 
properties of both acids and bases. Because of this Gelatin 
will take on the pH of the last bath its in. So, a treatment 
in an alkaline bath before washing has the same effect as if 
it were fixed in an alkaline fixer. However, there are 
drawbacks to the alkaline bath such as sodium carbonate. For 
one thing, it removes the hardening where hardening is 
desired, because it makes the gelatin too alkaline. Also, 
its desirable that the swelling of the gelatin be minimised. 
Minimum swelling occurs at the isoelectric point. While 
gelatin has no definite pH it does have a sort of preferred 
pH, where the electric charges in it are in equalibrium, 
this is called the isoelecric point and is determined by the 
way the gelatin is made. At minimum swelling the diffusion 
path for ions that are to be washed out is minimum which is 
desirable. Also, there is an electrical binding of ions 
which depends on the condition of the gelatin. For rapid 
washing of thiosulfate and reaction products its desirable 
to have the gelatin at a pH above its isoelectric point. So, 
for maximizing wash rate the idea is to have the gelatin 
above its isoelectric point but not enough above it to 
create excessive swelling. For many photographic gelatins 
neutral pH is a good point. Given that neutral pH also 
eliminates the mordanting effect of alum hardeners without 
destroying their hardening it seems to be about an ideal 
point.
   While any alkaline can be used to ajust the pH of the 
gelatin there is another property which is important, namely 
an ion exchange process. This was discovered during research 
following WW-2 on the great effectiveness of sea water 
washing. The fact that sea water was much more efficient 
than fresh water for washing of photographic materials has 
been known for over a century, however, the reasons did not 
become the subject of concentrated research until the need 
for it was greatly expanded during WW-2 for washing of 
materials on board ship and other places where fresh water 
is at a premium. Sea water is very effective but must itself 
be washed out since it leaves halides behind which very 
rapidly attack the image.
   It was discovered at Kodak Laboratories that the effect 
of sea water was due in part to its ability to actively 
displace thiosulfate ions. A series of experiments was 
conducted to find what constituent of sea water was 
responsible. The result was the discovery that Sodium 
Sulfite, in the proper concentration, was even more 
effective than sea water. Several other salts were 
discovered to have accelerating effects on washing but 
Sodium Sulfite was significantly better than the others. 
Kodak developed Kodak Hypo Clearing Agent to take advantage 
of all the above accelerating effects on washing. It is 
essentially a 2% solution of Sodium Sulfite buffered to 
neutral with Sodium bisulfite. In addition, the commercial 
product has two sequestering agents in it, EDTA tetra sodium 
salt, and Sodium citrate. These serve to bind water borne 
carbonates and prevent their deposit on the film or paper.
   KHCA does the following:
1, It adjusts the pH of the emulsion to neutral.
2, It acts as an ion exchange agent for thiosulfate and 
fixer reaction products.

   Because it is at neutral pH it raises the pH to above the 
isoelectric point, where electical binding is eliminated, 
binding by Alum is eliminated, and the swelling is near 
minimum.
The sum total of all this is that wash rate is very much 
accellerated. Wash times are about 1/6th those of untreated 
materials which have been fixed in an acid hardening fixer 
containing alum. The was rate is less shortened for 
materials fixed in neutral, non-hardening fixing baths, but 
is significantly faster even for those.
  Since the effect of previous treatment is of no 
consequence film or paper treated in a sulfite wash aid like 
KHCA will wash out at about the same rate regardless of 
whether its been processed in an all alkaline line or a 
regular line with acid stop bath and acid fixer with 
hardener.
  There are other wash aids on the market. Some use Sulfite, 
some use other materials. AFAIK, Kodak's is the only one 
that is buffered.
  I've never seen any research published on the effect of 
detergents on washing other than some very early work which 
suggested they are not very effective. At least one brand of 
wash aid uses a detergent. It may in fact work well but I am 
unaware of any published research demonstrating that. OTOH 
there is plenty of research I havent read and don't even 
know about.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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