From: Judy Seigel (jseigel@panix.com)
Date: 01/22/03-12:07:00 AM Z
On Tue, 21 Jan 2003, Neil Miller wrote:
> One thing I queried with my chemical supplier is why the ferric ammonium
> citrate changed colour from brown to green. I was assured that both were
> the "green" variety, but I use the one that is faintly lime-green in colour
> to make the
> solution up with. A and B mix fine, and no deposit is formed with up to
> 20ml of the silver nitrate solution mixed into A+B, although you can see a
> milkiness when
> each drop hits the mix right from the start. It just mixes in and
> doesn't precipitate out until around the 20cc mark. If I don't go really
> slow from then on, the "milkiness" gets really bad. I'm talking about 5
> mins or more to drop the C solution in - a real pain.
Neil, where are you? Possibly in the UK? Because Spirits of Salt comes
from there of course -- and that's the first I've heard about precipitate,
milky, whatever, with VDB -- I probably have entries for the process in 15
books, and have led, pushed, or followed 100s of space cadet undergrads
through it, and never seen what you describe -- except the one time we got
fake distilled water at the local hardware store, and that simply
precipitated out the silver, as silver chloride, bam, right to the bottom
of the cup, dead. And as I said, I've found that if I get impatient and
pour too fast,it still goes right back into solution.
What comes to mind now, though, reading your description, is Mike Ware's
term for ferric ammonium citrate, something about it being (if I remember
the phrase correctly) "ill defined," or maybe it was "ill characterized,"
meaning it's not one precise identical substance but has a lot of wiggle
room and still "F A C." (Somewhere I have that in the file, but probably
someone else knows the exact term.) I have seen brown FAC (it performs
differently BTW, is slower for one thing), but never "brown" that was
supposedly "green." Etc.
In any event, sounds to me like your FAC is EXCEPTIONALLY ill defined.
For curiosity, if nothing else, you might want to get a small quantity
from another supplier to compare.
The other thing is, have you tested your tartaric acid? Maybe it's
something else... Or dead. ?
Judy
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