woelen
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manganese complex of ClO2?
Some years ago, I did an experiment in which I tried to see whether Cl2 is capable of oxidizing Mn(2+) in conc. HCl, to get manganese(III) or
manganese(IV). I found that a solution of MnCl2 in conc. HCl, when treated with Cl2 does not change color, it only become weakly green. However, when
I added some solid Ca(OCl)2 from swimming pool chlorinator, then the liquid turned dark brown at once. When first Ca(ClO)2 was added to conc. HCl and
then MnCl2 was added, then I also obtained the dark brown liquid, but if this experiment was repeated in sunlight and some minutes were allowed to
pass between adding Ca(ClO)2 and adding MnCl2, then no deep brown color was obtained. Some investigation revealed that the brown color is caused by
presence of ClO2 besides Cl2 when swimming pool chlorinator is added to conc. HCl.
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Now I have reagent grade NaClO2. When this is added to conc. HCl, then nearly pure ClO2 is formed. I now have done some further tests with ClO2 and
manganese(II) and now I found a new riddle. The ClO2 seems to form a complex with manganese(II), but only in the presence of a lot of chloride ion.
I did the following experiments:
EXPERIMENT 1:
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Dissolve some NaClO2 in a little water and add a single large drop of this concentrated solution to 2 ml of conc. HCl (appr. 35%): A deep yellow
solution is obtained, some solid NaCl precipitates from the liquid.
Allow the NaCl to settle and then add a small amount of a solution of MnCl2 to the yellow solution: At once, the liquid becomes dark brown, but it
remains clear. When the liquid is shaken, then all of the liquid becomes dark brown. After several hours of storage, the liquid still is dark brown.
EXPERIMENT 2:
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Add a single large drop of a concentrated solution of NaClO2 to 2 ml of conc. HNO3 (appr. 65%): A yellow solution is obtained, not as intensely
colored at the solution obtained with conc. HCl.
Dissolve some manganese(II) acetate in water and add this solution to the yellow solution in HNO3: Along the glass of the test tube and on top of the
more dense HNO3, a very dark brown liquid is formed. This liquid nearly is black. On shaking, however, the dark color disappears again and the liquid
becomes yellow again. This is quite remarkable. So, apparently, when there is excess Mn(2+), then the liquid becomes very dark brown, but in the
larger amount of HNO3, the brown color disappears again. The dark brown material is a solid, which stains glass. Most likely it is hydrous MnO2.
Pour appr. 0.5 ml of the yellow liquid (which is mainly conc. HNO3 with some NaClO2 and some Mn(II) acetate) into 2 ml of conc. HCl: The liquid
becomes brown at once. On shaking, the brown color remains.
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What the above experiments show is that with chloride, apparently a complex is formed, which is stable. With excess HNO3, at seems as if ClO2 is
capable of reducing MnO2. With excess HCl, a dark complex seems to be formed.
I am wondering what this complex could be. Literature mentions the existence of a brown complex MnCl6(2-), but this seems to be formed from MnO2 and
conc. HCl at low temperature. ClO2 is capable of reducing Mn(IV) in nitric acid, but apparently, ClO2 causes fomation of Mn(IV) in conc. HCl, while
Cl2 does not. Anyone has experience with this?
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Metacelsus
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Couldn't the brown liquid be an extremely fine suspension of manganese dioxide formed by oxidation of the manganese(ii) salt? Can you try passing
light through it to see if it scatters?
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Brain&Force
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Could it be manganese(III) chloride?
No Tears Only Dreams Now had a pic of it but I couldn't remember if it was purple or brown.
At the end of the day, simulating atoms doesn't beat working with the real things...
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Amos
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Quote: Originally posted by Brain&Force  | Could it be manganese(III) chloride?
No Tears Only Dreams Now had a pic of it but I couldn't remember if it was purple or brown. |
Here's the picture in question. I obtained that solution, which has been filtered no less than 4 times, by treating pottery grade manganese dioxide
with boiling hydrochloric acid under reflux for a couple of hours. Searching for "manganese(III)" gives pictures of amber or brown colored solutions
just like mine, though the coloration could also be due to impurities of iron(III) chloride. I'm not entirely sure that MnCl3 exists under normal
conditions. Treating my brown solution(which contained remaining hydrochloric acid) with sodium metabisulfite made it give way to a pale yellow
solution, which might be manganese(II) chloride with some residual iron.
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Boffis
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Mn3+ and Mn4+ chlorides are not stable in the presence of water. However, in the presence of a large amount of hydrochloric acid they form
chloro-complexes which are significantly more stable but are still not completely stable, loosing chlorine over time, rapidly if boiled and on
dilution of the hydrochloric acid the corresponding hydrated oxide is precipitated. These complexes are generally brown, red brown or purplish. Nitric
and sulphuric acids do not form such complexes hence even the presence of a strong oxidising agent does not generate a brown colour. Bleaching powder
by contrast is both an oxidizing agent and a chloride ion source hence the brown colour formed when it is added even to nitric acid solution. Sodium
chlorite contains a little sodium chloride impurity but I doubt that ClO2 is as powerful an oxidizing agent to generate Mn3+ in the presence of small
amounts of chloride ions except when the solutions are not mixed fully then locally you get a higher chloride ion concentration and a local brown
colour. This doesn't preclude the formation of a dark colour Mn3+ chlorito complex but I suspect if such did form it would follow the same rules.
Phosphoric acid forms a stable purplish brown complex with Mn3+.
This raises an interesting question; can chlorite ions form complexes? This is not easy to test because sodium chlorite always contains 15-20% sodium
chloride and chlorate and you would need to be able to distinguish complexes of these ions from those of chlorite ions. I have never come across a
reference to chlorite or chlorate complexes but chloride complexes are many and varied.
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bismuthate
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Well, I doubt it is a chlorite complex because chlorite would react with HCl wouldn't it?
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woelen
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@Boffis: I need to mention that bubbling Cl2 through a solution of MnCl2 in conc. HCl does NOT lead to a deep brown color, while adding ClO2, e.g.
pouring the deep yellow gas over a solution of MnCl2 in conc. HCl, leads to formation of a very dark brown solution. The latter actually is quite nice
to watch, you see formation of a nearly black liquid at the surface and when the liquid is shaken, it becomes dark brown all over.
With Cl2 the liquid only becomes pale yellow/green. Some Cl2 does dissolve and this gives the pale yellow/green color to the solution.
How would you match these observations with what you explained above?
The liquid (in HCl) really is a solution, it does not consist of fine solid particles.
Chlorite ions can form complexes, I have investigated these as well some time ago. If you lead ClO2 gas over a solution of NaClO2, then a very dark
brown complex is formed. This is discussed in this thread: http://www.sciencemadness.org/talk/viewthread.php?tid=24208
This, however, is not a transition metal complex.
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