chemistic - 4-4-2011 at 10:15
A seemingly simple experiment starting with dissolving the shell of an old Sunbeam mixer in hydrochloric acid is giving strange results. It is a zinc
alloy casting, probably good old ZAMAK 5, and it does react nicely with the HCl, swimming pool acid for Home Depot. However, it is not making a
clear solution. It is producing a thin mud or slime that is composed of a fine grey particulate. This is still at a point where there is excess
metal present in the pot. The parts in the pot had all paint removed and were pretty well de-greased so I don't think this slime is organic
impurities. There seems to be too much of it.
Any thoughts on what the dark colored insoluble material might be?
I was hoping to end up with a solution of mixed chlorides of zinc, aluminum, magnesium, and copper, for a qualitative analysis experiment using
commonly available chemicals like NaOH, NH4OH, HCl, CH3COOH, etc.
m1tanker78 - 4-4-2011 at 11:01
It's probably the aluminum content of the alloy that's giving you the dark stuff. If you want to check, take some Al foil or aluminum scrap and put it
in a container with some of your pool acid. This is likely the product of hydrolysis of AlCl3. At least that's what I gathered back when I first
observed the black mud when reacting Al with HCl.
Tom
ScienceSquirrel - 4-4-2011 at 11:12
Personally I would use a mixture of hydrochloric and a small amount of nitric acid or I would try heating it to boiling or adding some hydrogen
peroxide.
Bulk metals seldom dissolve nicely in my experience. For example, steel wool is an excellent starting material for iron compounds, etc but it contains
a few percent of particulate carbon that has to be filtered off.
It also has a slight garlic, rotten egg smell when dissolving which I think is due to traces of hydrogen sulphide and phosphine being evolved.
blogfast25 - 4-4-2011 at 11:51
Aluminium really is an unlikely culprit here (although alloyed Al can also give problems of incomplete dissolution). AlCl3 doesn’t hydrolyse in
strong HCl and if it did, the product of hydrolysis (AlCl3-n(OH)n.mH2O) is white, not black.
But copper is a likely trouble maker: it dissolves poorly in HCl and has a tendency to form a mixed valence complex: Cu[I]-Cu[II] which is dark
brown/greenish (it's been reported on extensively on this board - use the search facility).
As SS suggested: use nitric acid for this dissolution or for the slimy residue. It will almost certainly dissolve if copper is involved.
chemistic - 4-4-2011 at 12:38
I did a few quick checks using a couple of suggestions from the replies.
1. I tried dissolving some strip of aluminum flashing in the HCl. It did not make a perfectly clear solution. There was some white ppt in it, but
not much, and it was not dark colored.
2. I have read of using HCl + H2O2 as a copper etchant, so I tried adding a little 3% H2O2 to the HCl with the aluminum strips in it and it turned
yellow and the reaction became much more vigorous.
I'm curious - why yellow?
[Note added later - the yellow color does not form if the HCl and H202 are mixed in a clean container. There must have been a bit of Kozy Shack
Tapioca left in there 
If starch binds chlorine does it become yellow? ]
As this mix became exhausted the remaining aluminum has become coated with a black layer. Fine oxide maybe? The yellow color also disappeared
eventually.
3. I don't have any HNO3 on hand at home and the peroxide-enhanced HCl seems promising. I took some of the sludge out of the ZAMAK pot and added
some of the peroxide to it. It has reduced the viscosity of the goo and lightened the color. It looks like I might get most of it into solution by
adding more HCl and peroxide.
If I have copper in this solution I would expect to be able to generate some blue color by adding ammonia water to it. I took some of the
peroxide/HCl treated material and added ammonia but it did not generate any color.
I wonder if what I have going on is at least partially that metallic copper is being precipitated from the solution by the remaining zinc metal going
in?
[Edited on 4-4-2011 by chemistic]
m1tanker78 - 4-4-2011 at 13:04
Going back over my notes, I see that I've gotten dark, cloudy precipitate from adding zinc (from galvanized steel) to conc. HCl. I think I was
confusing this with aluminum stuff I did before or after that.
Here's a link to that thread if you want to look over it...
http://www.sciencemadness.org/talk/viewthread.php?tid=15217
Tom
blogfast25 - 5-4-2011 at 06:22
chemistic:
Most metals turn black when exposed to strong acid. Al does too but that's usually due to finely divided metal particles (about to be dissolved) that
look black, not due to oxide (Al oxide for instance is snow white).
Adding H2O2 to HCl for the dissolution of Cu isn't as good as using HNO3 but it kind of works, albeit a bit slowly. You will obtain a solution that
contains a mixed valence ([I] + [II]) Cu complex of varying colour (brown, green, dark), see here:
http://www.sciencemadness.org/talk/viewthread.php?tid=5398#p...
Zinc does plate out copper but because it's more reactive Zn should go in solution first. In any case it doesn't matter: once all the Zn is dissolved,
the Cu must follow suit, at least in HCl/H2O2...
[Edited on 5-4-2011 by blogfast25]
chemistic - 7-4-2011 at 21:10
Blogfast, you make a logical point about zinc dissolving first. But when the metals are mixed in a solid alloy it seems like everything has to
dissolve at once. Then you have a solution with 4 metal cations and solid alloy bobbing around in it, mostly zinc.
A Related Observation -
While that ZAMAK in HCl reaction was going on I tried dissolving a penny in some of the HCl/H2O2 mix. First it produced a lovely light green
solution. After a few hours the copper layer became thin enough that the darker zinc underneath started to become visible. Once zinc was exposed it
bubbled vigorously, and shortly after that the green color disappeared and copper precipitate formed. There is still copper on the penny in patches,
but it does not make an appearance in solution any more.
Anyway, at this point I seem to have a gelatinous mud of, presumably,copper mixed valence complex, ZnCl2, AlCl3, MgCl2, and precipitated metals. I
definitely could use some nitric acid because this stuff won't filter for love or money. Probably if I keep adding HCl and heating it all will
eventually go into solution, but I have pretty much lost patience.