12AX7 - 23-6-2007 at 18:03
On a larf I picked up a gallon of 10% ammonia ("janitorial strength", I forget if they mean 10%wt NH4OH or NH3).
One thing I've tried with it is complexes. Obviously, a copper solution turns intensely blue, for instance. I have a sample of mixed nickel and
chromium chlorides, probably with a lot of iron as well (think "stainless steel" chloride, with much of the iron crystallized out). I added a sample
to some ammonia and poked it around a bit. Later, it turned deep reddish purple, while insolubles settled. I poured off the clear layer and added
more ammonia. It looks like I'm pulling nickel out of the whatever that remains.
But what of chromium? I recall iron doesn't form an ammine complex, but does chromium? So, I took a gram or two of crude (i.e., once crystallized)
potassium dichromate, dissolved it in ammonia giving a yellow solution, then added sodium metabisulfite to reduce it. Reason being, chromium is
supposedly troublesome to change ligands on, so I want to reduce it right into the ammonia. The odd thing is, the reduction prefers acid conditions.
While decontaminating my chromate-stained glassware with sulfite takes only seconds, this didn't appear to be moving at all. So I put it in a pyrex
flask and boiled it. The color darkened, seemingly in response to the loss of ammonia (*cough, choke*). It ultimately turned a turbid reddish-brown
sort of color. What kind of compound is this supposed to be? I later acidified it (with HCl, producing thick "London fog" within the flask!), giving
the usual dark green chromium solution I always get.
Which reminds me, "chromium" isn't. I always get green from the stuff! Isn't there supposed to be a purple or blue chloro- or hydrato-species
somewhere? Chloride and sulfate solutions always give me green.
Tim
not_important - 23-6-2007 at 19:12
The violet Cr(III) salts tend to go green on heating, and it can take a long time to return toe the violet state. Reduction of dichromate with
sulfuric acid and alcohol or sugar, warmed but not brought near boiling, then set aside for some time, will become blue or violet, allowing it to then
evaporate will give purple crystals.
I remember the ammonia complex being made by oxidising Cr(II) in the presence of ammonia.