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blogfast25
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There's a thread here somewhere on chromous acetate, from potassium dichromate reduced by Zn in the presence of acid.
It seems strange then that Zn, by far the weaker of the three reducing agents (Mg > Al > Zn, acc. electrochemical series), can at least reduce
Cr(VI) to Cr(II).
Tomorrow I'll test my tubes for Cr(II) but I'm not hopeful: even the blue tube of Cr(NO3)2 + Al strip hasn't lost any colour intensity but it has
turned dark green! I suspect this may be a pH effect: the Al will have reacted with any acid reserve in there, thereby increasing the
OH<sup>-</sup> concentration, so that there may now be some OH ligands on the Cr<sup>3+</sup>.
I'll also try and reduce the Cr(NO3)2 with Zn powder and some HCl. The HCl should complex the Cr to the green complex, so if Zn does reduce the
Cr(III) chloro complex, I should see a colour change to blue. Then I can test with acetate...
[Edited on 17-3-2014 by blogfast25]
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woelen
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Do you have anything else than the nitrate? Nitrate ion is oxidizing and zinc will reduce the nitrate in acidic solution. If you have chromium
chloride or sulfate (or chrome alum), then I would try with that for a better chance of success.
I once made a solution of Cr(2+) (it's on my website) but this is amazingly air-sensitive. Even in strongly acidic solution, it quickly turns dark
green when exposed to air.
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blogfast25
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Quote: Originally posted by woelen  | Do you have anything else than the nitrate? Nitrate ion is oxidizing and zinc will reduce the nitrate in acidic solution. If you have chromium
chloride or sulfate (or chrome alum), then I would try with that for a better chance of success.
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Ooopsie. Hadn't thought of that. But I've got the alum. Will test Zn, Mg and Al in comparable circumstances.
Thanks!
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Brain&Force
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I'll be trying the reduction of chromium(III) sulfate with magnesium. (I had already tried it, but I left it unmonitored and all I got was a clear
solution with a bluish-gray powder on the bottom.)
On a side note, I have some chromium sulfate, but it's green, not purple, and dissolves to give a green solution (but not very easily). It was
purchased in 1984 if that helps (yeah, the chemicals are that old at my school).
At the end of the day, simulating atoms doesn't beat working with the real things...
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sasan
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Brain and force I have chromium sulfate that solve in water just a little and make the solution light green,if your sulfate is like mine(it is light
green not deep green such as chromium chloride ),then it would not reduce with Mg or either Al,but test it I'm not sure
I search this insoluble sulfate in handbook of chemicals,maybe it is chrome cake because it stated that it is insoluble form of chromium sulfate and
made by fusing sodium sulfate and chromium oxide(calcined form that is insoluble in acids and alkalies) followed by dissolving sodium hydroxide in
water and collecting insoluble chromium sulfate,again I'm not sure
chromium chemistry is really complicated,because of this chromium(3+)salts are more expensive comparing to cooper and nickel and just sold by big
famous chemical suppliers like sigma aldrich
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blogfast25
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I prepared a 100 ml of 0.35 M chromium alum stock solution and tried to reduce it in various ways.
20 ml was mixed with 10 ml HCl 37 w% and chips of Al and Mg added. In both cases the blue solution turned green and stayed green. No reduction was
observed.
I then tried zinc. I tried clean battery zinc, pellets of good purity zinc and even very pure fine powder. I never obtained proof positive of any
reduction. Adding some of the solution to some glacial acetic acid or the other way around never precipitated any red Cr (II) acetate dimer.
I also reduced some potassium dichromate with a pellet of zinc and some HCl 37 w% and it reduced to a green solution very quickly but I never observed
any blue.
I’m somewhat baffled that Al and Mg can’t reduce the complexed Cr(III): the complexation constants must be very large or something else is in
play.
B&F:
My (now spent) Cr(III) sulphate was also green and of a very strange texture: like green wool.
[Edited on 19-3-2014 by blogfast25]
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S.C. Wack
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Or perhaps hydrogen peroxide.
Perhaps contamination with a little mercury would be helpful.
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blogfast25
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There's a published procedure (by Science Squirrel, IIRW) somewhere on this board for preparing Cr(II) acetate with mossy zinc and HCl. It doesn't
call for any mercury. But I'd try it if I had any.
Here:
http://www.sciencemadness.org/talk/viewthread.php?tid=18216#...
Re. reduction of dichromate with H2O2: works very well but it does have the disadvantage that any excess, remaining H2O2 will start oxidising the
Cr(III) back to chromate, in alkaline conditions (for instance to precipitate Cr(OH)3).
[Edited on 19-3-2014 by blogfast25]
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S.C. Wack
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There is chromous literature using zinc both with and without mercury or acid. An interesting quote which the authors go on to demonstrate in Anal
Chem 8, 188 (1936) (thx Russians. here, have Crimea.): "A chrome alum solution that has been freshly prepared, that is made up in dilute sulfuric acid
solution, and that is held at room temperature or below, is violet in color and rapidly reduced by the zinc amalgam in a Jones reductor. If the
solution is allowed to stand, if acid is not added, or if the solution is heated, there is a shift to the green color and the rate of reduction is
less."
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blogfast25
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| Quote: Originally posted by S.C. Wack | | There is chromous literature using zinc both with and without mercury or acid. An interesting quote which the authors go on to demonstrate in Anal
Chem 8, 188 (1936) (thx Russians. here, have Crimea.): "A chrome alum solution that has been freshly prepared, that is made up in dilute sulfuric acid
solution, and that is held at room temperature or below, is violet in color and rapidly reduced by the zinc amalgam in a Jones reductor. If the
solution is allowed to stand, if acid is not added, or if the solution is heated, there is a shift to the green color and the rate of reduction is
less." |
S.C.:
That confirms the role of complexation. Strange how the electrochemical series (values for CrIII to Cr0 or CrIII to CrII) doesn't seem to reflect
that... CrIII seems to be complexed almost always, in some way or another.
Crimea to the Crimeans, as far as I'm concerned.
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