>I asked my question about chelating agents for good practical reasons. I
>noticed that prints cleared with EDTA tend after a time to be subject to iron
>staining. I also noticed, that in my experience, that this only happened where
>ferric oxalate had been used in the sensitiser.
I cannot answer chemically for your experiences with ferric oxalate, Terry,
because I never, ever use the stuff. It is an ill-defined mess, which is
capable of almost infinite variation in composition and state of
hydrolysis, so no two people are usually talking about the same thing.
Terry continues -
>I am quite capable of reading books and other literature but when I do not have
>a particular specialisation I find it more helpful to listen to a discussion
>between the specialists and intelligent lay persons.The reading of archives
>often does not answer one's own questions.
You could try the List Archive for 10 January, 23 August and 12 October '95.
To encourage you to retrieve it, here is a sample:
>I have tested Arches Platine with the platinum/palladium print-out process
>based on ammonium salts devised by Pradip Malde and myself. It appears to
>clear perfectly in the following wet sequence:
>
>Disodium EDTA 5% for 5 minutes
>Kodak 'Hypoclear' working strength for 15 minutes
>Tetrasodium EDTA 5% for 15 minutes
>Water wash for at least 30 minutes
>
>This may not apply exactly to a 'development' Pt process (using ferric
>oxalate), where a more energetic first bath may be needed. The rationale
>behind this treatment sequence was outlined in my post to this List of
>23/8/95. Luis Nadeau also explains it in the new edition of his book.
In case Terry has not yet bought Luis's new edition (8-> here is the
rationale in a nutshell:
Iron(III) must first be complexed in an acid (pH 3-4) environment by
disodium EDTA (the trisodium EDTA which Terry uses has too high a pH, and
probably causes hydrolysis to iron hydroxide - hence his yellow stains).
Following this, the sulphite in KHCA probably helps reduce any residual
iron(III) to iron(II), it is an alkaline bath (pH 9).
Iron(II) is most strongly complexed at alkaline pH, without hydrolysis, by
the final bath of tetrasodium EDTA (pH 9-10).
The paper is thus left in a nice healthy alkaline condition and iron-free.
Mike