Re: VDB test results: related concerns

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From: Richard Urmonas (rurmonas@senet.com.au)
Date: 12/15/02-02:11:58 PM Z


On 2002.12.09 10:42 Sandy King wrote:

> I was thinking on the assumption that a 3% citric acid clearing bath
> could remove the soluble iron compounds at any point in the wet
> processing procedure. What you are saying is that the soluble iron
> compounds are actually changed in some way with any alkaline wet
> processing that takes place before the compounds are removed, right?
> If so, this would also have implications for regular kallitype
> processing as well as VDB and n theory I should eliminate the water
> rinse that I have been using between development and the citric acid
> clearing bath. This assumes of course that ferric oxalate and ferric
> ammonium citrate compounds function alike at this particular stage.
>
>
> In any event your processing proposal makes sense. However, what
> percent solution of citric acid do you propose in step #2?
>

I did some investigation and ran some tests this weekend. The
iron is forming iron hydroxide. The formation
of the hydroxide is a strong function of pH. It takes
tens of seconds at pH = 8, minutes at pH = 7, and hours at
pH = 6. Once formed, the hydroxide appears to be difficult
to remove.

For my tests I coated some paper with straight ferric ammonium
citrate. I then exposed this to full sun for a few minutes.
This should ensure a good mix of ferric and ferrous states.
I then tried clearing the paper is plain water, 1% citric,
2% citric and 3% citric.

My conclusions were:
1) The FAC reacts with some papers, resulting in iron hydroxide
formation even in a 3% citric wash. This is the yellow-brown
stain which some papers produce when using cyano, VDB etc.

2) A plain water wash (tap water filtered through mechanical and carbon
filters) does allow a small amount of iron hydroxide to form. With
a citric acid wash, there is no difference between 1%, 2% and 3%
acid concentration, on both staining and non-staining papers.

Thus only a small amount of acidity is required to ensure that
no iron hydroxide is formed. A non-staining paper should be used.

Richard

---
Richard Urmonas
rurmonas@senet.com.au

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