> I'm interested - and not a little surprised - to learn of the use of an
> oxalate developer for platinum/palladium which contains dichromate to
> 'control' contrast.
>
> The reaction between these two ions is well-known to inorganic chemistry,
> and is indeed one of the standard methods for preparing a whole range of
> oxalato- complexes of Chromium(III). Admittedly the lab preps entail
> heating, but this is just to speed up the process. I doubt that any
> solution containing the two will remain unaltered for very long, even at
> room temperature.
>
> Keith has just described one consequence:
>
> >KOx does not become
> >exhausted. I don't know about the dichromate. I have found some green
> >needle-like crystals in one of the bottles.
>
> This is, very probably, potassium tris-oxalatochromate(III): K3Cr(C2O4)3.3H2O
>
> The other consequence is that when the Cr(VI) of dichromate is thus reduced
> to Cr(III) it will cease to have a 'contrast enhancing' effect.
>
> Mike
>
I guess the only thing to do to determine if Mike is right on this
(and I have little doubt that he is) will be to repeat the test
sequence at intervals of a few weeks and compare the results to those
obtained with fresh KOx/NaDichromate developer. Its been several
weeks since the first round already so I'll try to get to it this
weekend.
Anyone else (David) who has been using this method for a while have
any observations on this count?
Keith