raisword - 1-7-2008 at 12:39
There is a bottle of KMnO4 in my lab and it was bought two years ago. The color is green now, but its aqueous solution is still purple....
I find that the correct color of solid KMnO4 should be black or deep purple. Does the green color mean that it actually decompose to some extent? What
makes it green?
Thanks.
12AX7 - 1-7-2008 at 13:58
Could be. Probably a layer of MnO2. The commercial form I have, in tiny prills, has a dull brown/gray color.
If you need it for analysis, it would be a fine idea to titrate it before use.
Tim
hashashan - 1-7-2008 at 15:03
it could be some trace amounts of potassium manganate
raisword - 1-7-2008 at 15:15
Thanks. I think that I need to buy a new one.
Jor - 1-7-2008 at 15:18
I think woelen has posted on this forum somewhere, that KMnO4 can be green. It was some guy who was asking why his KMnO4 was greeinish, while
'JT.Baker is supposed to be the best'.
JohnWW - 1-7-2008 at 15:27
The green color must be due to partial reduction to manganate(VI), MnO4--.
woelen - 1-7-2008 at 23:32
KMnO4 indeed can be somewhat green. It is a kind of reflection of light, which makes it appear green. I also have very pure KMnO4, large crystals
(some as big as 3 mm) and these crystals are very shiny and have a green lustre. Besides the large KMnO4 crystals, I also have a very fine powder of
general technical grade. This material is very dark grey.
So, there is no need at all for raisword to buy new KMnO4. This chemical stores indefinitely and is absolutely air-stable, as long as it is stored in
a dry place, preferrably dark as well.
DerAlte - 2-7-2008 at 08:11
IMHO Woelen has it right.
As a general rule, substances which absorb light on transmission reflect those wavelengths. The absorption band of KMnO4 (actually the permanganate
ion) is a broad band centered on 520-530 nm, in the green. Manganate has a similar broad band centered on the orange at 620 nm. By transmission MnO4-
thus appears magenta because it passes both blue and red, absorbing and reflecting green and yellow, producing magenta to the eye. On reflection ,
since it is transparent to red and blue, it is likely to reflect green.
MnO4--, on the other hand, absorbs in the orange-red region, which emphasizes green & blue on transmission. Manganate, in fact, should look red -
purple on reflection due to transmission of the green.
Permanganate does not convert to mangante in neutral conditions – it requires a heavily alkaline environment to achieve this. Your
green-on-reflection permanganate is fine.
The combination of both ions in a solution produces a very high absorption of all the central visual wavelengths, so such a solution is very dark,
tending to look dark blue/violet by transmission. I have been fooled by this several times. If you suspect manganate, use a drop of the solution an a
filter paper. Any manganate will give a green ring outside the pink permanganate center. A cheap sensitive chromatographic test.
Regards
Der Alte