From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 12/02/00-06:42:14 AM Z
Hans Nohlberg wrote:
> ... Every time when I have mixed
> Potassium Oxalate and Ammonium Citrate, i. e. 50/50, there is a heavy
> precipitation. ...
> I prepare the Pot.Oxalate in the following way: After having dissolved 210g
> Potassium Carbonate in 1000ml dest. water I continue with 180g Oxalic Acid
> and some more water until everything is completely dissolved.
> I prepare the Am.Citrate in the following way: After having dissolved 300g
> Citric Acid in 1000ml dest. water I continue with 300 ml Ammonium Hydroxide
> (27%).
> Using the developers unmixed works fine. The question is, can the mixed
> developer be done in a different way without any precipitation?
I am not a chemist, but have first hand experience.
I mix the Potassium Oxalate (PO)(crystal from Bostic & Sullivan) as
follows.
280 grams PO and 900 ml H2O (warm) and no Oxalic Acid (none)
This may be over saturated as some PO will not dissolve. However, a bit
more H2O can be added until the PO remains dissolved. In any event, the
solution I use is not weaker than yours. And Oxalic Acid is NOT added.
The Ammonium Citrate (AC) I use is that from Bostic & Sullivan in liquid
form. I do not use it frequently so I have a couple quarts still on
hand. I do not know the strength of the solution or if it has
additives. As a developer, it provides a more neutral color but looses
some of the depth compared with PO.
I discovered (about 1987) that a 50-50 mixture of PO and AC would
provide the more neutral color while maintaining all the substance and
depth of the PO. I also found that a separation would occur
necessitating the mixing or shaking of the developer each time prior to
poring it over the coating to be developed. I suspected that the AC and
PO did not want to mix. But the solution always has provided consistant
results as far as the print is concerned.
Using this developer for several years and having some in the same
bottle for at least a decade, there has been no precipitation as you
have described. However I do note that after many prints, the developer
(PO or 50PO-50AC) will contain a greenish-gray crystaline precipitate.
I suspect that this may be PO with some of the metalic salts. This
crystal precipitate can be removed from the developer and has not seemed
to affect results.
Note that I have noticed that any developer, after working on many
prints, may tend to fog the print. I suspect that this may be due to
exposed coating material being collected in the developer. At a certain
concentration the exposed coating material might travel onto a
subsequent print and be noticed as fog. This is my reason for periodic
replacement of the developer after a significant amount of coating
processing with replenishment in the interim to replace that carried of
by the substrate.
I would first suggest to use Potassium Oxalate instead of the components
and see if this solves the problem.
Second check the purity of your ingredients.
Third check the concentration and components of the AC, however if too
strong, a precipitate should form before the 50-50 mix.
Forth make sure the PO and AC are completely dissolved before mixing.
Perhaps a chemist will respond.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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