On 9/15/2011 11:56 PM, Mike Monett wrote:
Marshall, I would really like for you to do the thermal
decomposition experiment for silver hydroxide. It is one of the
easiest of all to perform, since it occurs at such a low temperature
- barely enough to cook hamburger, if you can wait that long:)
But there is something magic when you see the black deposit turn
gray right in front of your eyes. It reaches somewhere deep down
inside and you suddenly realize that what these people are saying
could actually harm you. So you begin to wonder what other mistakes
they have made, and what is the real truth about what is happening.
I am not questioning that you have seen the effect. I am however
questioning your interpretation of it. First I cannot find any
reference anywhere to silver hydroxide. It is not even in my CRC
handbook, and every reference I can find says that it can only exist in
a hydrated form. Thus I am unable to find anywhere that tells its color,
if it does exist. You are claiming that the black stuff is silver
hydroxide, I think it is silver oxide Ag2O, which my book says is black
or brown-black. The crystalline form shifts when it is heated. Silver
oxide is normal a cubic crystal, but when deposited on an electrode I
would expect most of it to be amorphous. If it is amorphous, it would
almost certainly be black. As is normal with amorphous materials that
are normally crystalline, they will form crystals when they are heated
sufficiently. So that goes right along with the idea that a black
amorphous compound crystallizes and changes color when heated.
Is there any test data to support either of these possibilities. In fact
there is. There is a paper that shows that when formed by an
electrolytic cell and deposited on an electrode, Ag2O is indeed laid
down as a mixture of amorphous and cubic forms. This can be viewed at
http://web.mit.edu/dsadoway/www/134.pdf
I am unable to find at what temperature you said this action takes
place. Another possibility is that the precipitate contains both AgO
and Ag2O. AgO decomposes at about 100 C lower temperature than Ag2O.
This is given in the paper at
http://www.iupac.org/publications/pac/2/1/0211/pdf/ which also shows
that various forms of Ag2O, specifically the the 220 and 331,
spontaneously form the cubic 111 form at temperatures no higher than 150
C, which is the temperature they did the annealing at.
Marshall
