Re: precipitation of dichromates

On Thu, 26 Feb 2004, Christina Z. Anderson wrote:
> In the old BJPs it was talking about the two at a time when pot di was the
> norm, and how pot di, because it is so low in saturation compared to am di
> (10 vs 30%), with a drop in room temp you tend to get crystallization around
> the bottle stopper all the time. That's always plagued me, too, in a
> inconsequential but messy way with using saturated solutions, so I thought
> that'd be another goood reason to use am di at lower than saturation point.
> And then I came across this alcohol statement and was wondering.

Chris, I mix ammonium dichromate to 26%, it doesn't precipitate out in the
bottle with change of temperature & is always the same concentration. I
still dilute it for use, but prefer keeping the stock solution
stronger.... theoretically it keeps better, and it definitely takes up
less space.

However, IME crystallization around the bottle neck is going to happen,
also with k di... Because (I theorize) unless the bottle is EXTRENELY
tightly closed, and maybe in any event, the water in solution there will
evaporate, so the concentration will get greater, hence precipitate out.
If you use a dropper bottle (Ginsberg has them in 8 oz size, or did) it's
easier to keep drip off the neck, but I usually wipe with paper towel
anyway.

Judy

>
> Can't believe it--it is SNOWING here in SC. Looks like MT outside....I'm
> waiting to see if school is cancelled. In MT it'd have to be 2 feet of snow
> and 30 below zero before they'd consider closing anything.
> Chris
> ----- Original Message -----
> From: <mmatusz@pdq.net>
> To: <alt-photo-process-l@sask.usask.ca>
> Sent: Tuesday, February 24, 2004 1:36 PM
> Subject: Re: precipitation of dichromates
>
>
> > Chris,
> > Potassium dichromate is insoluble in alcohol, ammonium dichromate is
> > soluble, although I do not have the exact solubility numbers. You gum
> > solution is mostly water anyway. If you like to print with concentrated
> > dicromate solutions (around 15% in the final solution or even higher)then
> > it is likely that you can not add a lot of alcohol, or else potassium
> > dichromate will precipiate. Again the alcohol addition is also small,
> > maybe 5%. If you are printing with a dilute dichromte solution you might
> > not have a problem with potasssium dichromate and alcohol. This is one of
> > those things that it is so much easier to test in practice anyway.
> > Happy printing
> > Marek Matusz, Houston
> >
> > > Do any of you chemists out there know why potassium dichromate would
> > > precipitate out of solution when alcohol is added vs. ammonium
> dichromate
> > > which doesn't? Could it just be potassium's lower saturation point
> that
> > > makes it seem that way or is there a property of alcohol that makes a
> > > potassium form do this whereas an ammonium form doesn't? Or is this even
> > > true in your observations, and from a practical standpoint, at what
> point?
> > > Any conjecture would be appreciated.
> > >
> > > My reason for asking this is just thinking about the addition of alcohol
> > > in
> > > a coating solution in gum printing that has been suggested in the past,
> to
> > > make it thinner and easier to coat, whether this is viable with
> potassium
> > > dichromate.
> > > Chris
> > >
> > >
> > >
> >
>
>
Received on Thu Feb 26 16:53:58 2004