Jor - 10-2-2008 at 05:33
Today I did some experimenting with dichromate. I tried different reductors and different acids.
I did the following:
-1mL moderately concentrated sodium dichromate
-3mL 2M nitric acid
-3mL ethanol
The reaction proceeded slowly, somewhat slower than the reaction with sulphuric. I saw some , not much, very small bubbles coming out the reaction
mixture. I concluded this couldnt be carbon dioxide: dichromate isnt able of oxidising ethanol to the 3rd step (1 being acetaldehyde and 2 being
acetic acid).
So the only 2 gasssen that came into mind were acetaldehyde and nitrogen dioxide. Nitrogen dioxide is not detected at such low levels, acetaldehyde is
said to smell fruity.
The smell I smelled was unpleasant, reminds me of when I was sick in the hot tropics of Dominicana Republic. Am I missing something or? What could
this be? The smell really became noticable when I diluted the solution ....
woelen - 10-2-2008 at 08:08
Acetaldehyde has a bad smell. It reminds me of people being drunk with a smell of stale alcoholic beverages. Acetaldehyde to my opinion definitely
does not smell 'fruity', unless you have a very peculiar notion of 'fruity' things 
.
It also is understandable why dilution makes the smell even more noticeable. Any other oxidation product (acetic acid) and the original smell of the
ethanol almost disappear with dilution, while the smell of acetaldehyde remains (the boiling point of the latter is 20 C or so, and even from dilute
solutions it escapes quite easily, while acetic acid and ethanol are bound to their solution much more when diluted).
Of course, if you used denatured alcolhol or spiritus for your experiment, then one cannot tell really much about this. The other ingredients may also
become oxidized, but with unknown stuff one can get any smell.
The bubbles you observed also can be explained. With a concentrated solution of sodium dichromate you probably also noticed heatup of the solution.
The reaction is fairly exothermic and if you have a concentrated solution of a dichromate and you reduce it at a fairly high speed (within tens of
seconds), then it certainly heats up. Given the low boiling point of acetaldehyde, one can easily imagine formation of bubbles of gas.
[Edited on 10-2-08 by woelen]
Jor - 10-2-2008 at 08:38
The way you describe the smell, thats exactly how I ment it!
And my ethanol is no spiritus, its Fisher's '99% methylated spirit industrial'. The one you saw in the picture of my lab on the work bench.
YT2095 - 10-2-2008 at 08:53
Methylated.... I wonder if there may also be Formaldehyde in there then, that stuff really DOES stink! 
[Edited on 10-2-2008 by YT2095]
12AX7 - 10-2-2008 at 08:59
I once made some, using crappy ethanol (I distilled myself, described in the "solar still" thread, if you can find it), potassium dichromate and ~50%
sulfuric acid. As the reaction was heated and turned from orange to green, I noted a smell that I assume was ethanal, smelling only remotely like
apples. I think because of the bad source, I also smelled what I might describe as burnt matter (I forget now if it was more like seared protein or
carbohydrate), which is probably due to yeasty residues (i.e. containing nitrogen) in the spirits.
Tim