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woelen
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[*] posted on 13-12-2011 at 23:40
Molybdenum hydrate?


I came across this compound on eBay:

http://www.ebay.nl/itm/MOLYBDENUM-HYDRATE-100g-Mo-95-63-High...

It is a compound of molybdenum and I ordered 100 grams of this for experimenting with this metal. I was attracted by the very special color of this compound (normally, molybdenum compounds are white in oxidation state +6 and dark blue in oxidation state +5).

This yellow compound does not dissolve appreciably in water. It is sparingly soluble in concentrated HCl, giving a pale yellow solution. It is easily soluble in a solution of NaOH, giving a colorless solution.

I also have some MoO3 (which is a very light grey powder, nearly white). This also dissolves easily in a solution of NaOH and this solution also is colorless.

There is a big difference, however, when both solutions are acidified. The solution made from the "molybdenum hydrate" becomes deep yellow when acidified with excess 2M H2SO4. The solution made from MoO3 remains colorless when acidified with excess 2M H2SO4.

Both acidic solutions become dark blue when powdered zinc is added (besides the change of color there also is formation of H2). So, it appears that the yellow stuff contains Mo in oxidation state +6 and this is reduced by the zinc.

My question now is, what is the real composition of this yellow compound? I think that it is of high purity, because its solution in NaOH is clear and colorless. If it were impure from other transition metals like iron, copper and so on, then it would give opaque/milky solutions and precipitates of hydroxides. And what makes the difference in the yellow solution and the colorless solution?

I myself could not find any conclusive info on "molybdenum hydrate" and my textbooks only mention formation of a yellow precipitate on standing of an acidified solution of ammonium paramolybdate, but they do not explain the real nature of this yellow material. I would expect it to be something like MoO3.xH2O, but I'm not sure that this is yellow. Normally, MoO3 is white and its solutions and compounds and acids, derived from this, also are white/colorless.

I think that this yellow compound brings up some nice and interesting molybdenum chemistry.




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[*] posted on 14-12-2011 at 00:21


Hi.

Indeed a attractive yellow color on that compound, almost looks like a nitrostyrene.

May it be phosphomolybdic acid or any of its salts? Only yellow Mo compounds I could find. Have you titrated to establish how much Mo it contains? If it really is phosphomolybdic acid it's really a cool compound, with the formula H3PMo12O40 and a mole mass of 1825.25g/mol.




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[*] posted on 14-12-2011 at 01:50


This is interesting. Your compound has 94.63% MoO3 by weight and 5.37% H3PO4. The eBay auction mentions 95.63% as the Mo-content. I am inclined to believe that this is a typo, there simply is too much coincidence.



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[*] posted on 14-12-2011 at 04:49


I think it is Molybdenum hydrate, maybe dihydrate or monohydrate

"The dihydrate loses water readily to give the monohydrate. Both are bright yellow in color."
--- from http://en.wikipedia.org/wiki/Molybdenum_trioxide

when I dissolved sodium molybdate (VI) in conc HCl or 50% H2SO4, I got these yellow precipitates as well
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[*] posted on 14-12-2011 at 06:37


A remark in Gmelin's Handbuch states that there exist two hydrates of molybdenum oxide. One is white/transparent, the other strongly yellow. I once prepared the yellow type, which has a tendency towards orange.
IIRC the yellow/orange one is the dihydrate, the white one the monohydrate. (This contradict the entry in wikipedia.)
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[*] posted on 15-12-2011 at 06:01


I must reject the option of the phosphomolybdic acid. Multiple sources describe this as freely soluble in water and this certainly is not the case with my yellow material. It hardly dissolves in water, not even in boiling water. It dissolves easily in a solution of NaOH and fairly easily (some heating is needed) in 5% ammonia. So, I think it must indeed be some compound of molybdenum in oxidation state +6 (because the solutions are colorless), but I cannot make anything of the number of 95.63% which is mentioned on the eBay auction. This seller uses this kind of numbers in many auctions and they always can be derived from the formula of the compound.

If the yellow is the dihydrate and the white material is the monohydrate (or anhydrous), then why does the solution in dilute NaOH or ammonia turn yellow on acidification when derived from the yellow compound and it remains colorless when derived from the white compound?




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[*] posted on 15-12-2011 at 23:41


You mean the white molybdenum trioxide dissolve in NaOH, but no yellow color is produced when re-acidified?

Perhaps in the crystal structure only when there's two water present, there would be a yellow color produced?? My sodium molybdate is the dihydrate, when i put the solid directly into acid, the crystals turns yellow immediately and dissolves with great difficulty compared with water only

However, when I got a solution of sodium molybdate in water, adding same volume of conc sulphuric acid does not produce any coloration

It's confusing ...
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[*] posted on 16-12-2011 at 00:57


Indeed, when I reacidify the solution of white MoO3 in dilute NaOH, then no yellow color is produced. If i do the same with a solution of the yellow material in dilute NaOH, then an intense yellow color is obtained.

So, in some way it is "remembered", even when dissolved in a solution of NaOH, from what compound the solution was made. Apparently the colorless solutions, made from white MoO3 and the yellow hydrate are not the same.

Your observations also look quite confusing. I have made some Na2MoO4 from NaOH and MoO3 myself and I will try your experiment this weekend.




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[*] posted on 16-12-2011 at 16:49


Maybe a related observation is the following (I admit this adds to the confusion):

1. add 1 mole of MoO3 to a solution of 2 moles of NaOH. A solution of Na2MoO4 is thus obtained. Addition of some 3% H2O2 yields a lemon yellow solution, that can be evaporated to dryness, yielding quite stable lemon-yellow crystals. (Now, 2.5 years after production, the colour has become a little less intense.)


2. a) put some MoO3 into a beaker with some water and add some strong H2O2 (37%). When the mixture is heated (not boiled), all MoO3 will dissolve, yielding a lemon yellow solution. Allowing the solution to dry, yields a stable orange compound:

b) take some of the orange compound, and a solution of NaOH in water. Addition of the hydroxide to the orange compound, while swirling the flask, yields a colourless solution!

Ergo, the orange compound is probably NOT a peroxide of molybdenum, but rather some hydrate.


I have attached a few pages from Gmelin's Handbuch dealing with the hydrates of MoO3.

Attachment: Molybdaenhydroxide (1MB)
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[*] posted on 16-12-2011 at 18:29


I'll try with stronger acids this weekend as well, I found that addition of conc sulphuric acid does not produce the yellow color from the colorless solution, but conc HCl did. I will try with nitric acid later...
it would be very interesting that the compound can "remember" where it came from!!! perhaps it's like chromium? the ligand displacement is not ready for Mo as well, so when it's re-acidified it reveals how's it's made? but I can't see any other unnecessary ligands except water...
another guess is that it forms clusters, just as vanadium do. Molybdenum can form clusters as well, but this simply can't explain why it can "remember" its original state...
it seems rare to encounter this yellow compound of molybdenum

@Bezaleel: should that be a peroxo complex of molybdenum? I also obtained the same results that a yellow solution is formed, but I didn't isolate the solid product. Before I have isolated the citrate-peroxo complex of molybdenum, and in alkaline solution, the peroxo (without citrate) complex is red, in acid solution, the complex is yellow; however, when both solutions are allowed to evaporate to dryness OR freeze for a few days OR stand in air for a few days, yellow crystals were obtained from both of them. However I use sodium molybdate dihydrate, not MoO3
So the orange compound could be a mixture of peroxo-complex and oxide? how long have you kept the orange compound for? because the peroxo complex of Mo have a very intense color when solid, but relatively less intense in solution
I will try your experiment as well. Nice pictures! :)
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[*] posted on 17-12-2011 at 15:50


I imagine your mysterious salt could be ammonium molybdophosphate. Ammonium molybdophosphate is formed as a yellow precipitate in a test for phosphate. The reaction only functions if 1) the solution is acidic enough (usually nitric acid is used) 2) there are enough ammonium ions. Ammonium molybdophosphate dissolves in a basic solution. I suppose, that in your case during acidification you didn´t obtained the right conditions for precipitation and so just a colour change was produced. Try by acidifying the reaction mixture with concentrated nitric acid and adding some drops of ammonia (wait some time). It is however only a desperate guess...;)


Quote:

I myself could not find any conclusive info on "molybdenum hydrate" and my textbooks only mention formation of a yellow precipitate on standing of an acidified solution of ammonium paramolybdate, but they do not explain the real nature of this yellow material. I would expect it to be something like MoO3.xH2O, but I'm not sure that this is yellow. Normally, MoO3 is white and its solutions and compounds and acids, derived from this, also are white/colorless.


This yellow precipitate is a monoclinic monohydrate of molybdenum trioxide which is made of layers of MoO6 octaeders, that are joined by their corners. Each Mo-atom in this structure is surroundet by 4 O-atoms that belong to two octaeders, one isolated O-atom and a water molecule which is trans to the isolated O-atom. (From the Holleman-Wiberg-I fear I did a bad job by trying to translate it from german to englisch)
Here a synthesis of this compound I performed: Synthesis of MoO3*H2O

Molydenum has really an intersting chemistry...:cool: Here for example another exotic compound:Tetramminezinc(II) Tetraperoxomolybdate(VI)
Another beautiful compound, which can be very easily made is ammonium tetrathiomolybdate (see Brauer). I have some pictures, but I can´t find them rigt now.
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[*] posted on 19-12-2011 at 01:27


I did some further experimenting with Mo.

In one experiment I took solid Na2MoO4.xH2O (home made stuff from NaOH and MoO3, hydration state is unknown, the solid is white and its solutions are colorless and remain colorless on acidification). To this solid I added some 50% or so H2SO4. When this is done, the solid remains white. On slight heating all of the solid dissolves and a colorless solution is obtained.

I took some of the yellow "molybdenum hydrate" from eBay and dissolved this in dilute NaOH. The solution is colorless. I allowed this solution to stand for two days. Still the solution is colorless. To this solution I added a few drops of 50% H2SO4 and swirled. The solution becomes intense yellow and remains clear. I boiled this solution for a while. While doing so, the yellow color slowly fades. On cooling down, the yellow color does not appear again. So, apparently the yellow color is unstable at higher temperatures and the hydration state changes on heating. More confusion is added to the puzzle ;).




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[*] posted on 21-12-2011 at 08:53


It is at least striking, that a yellow or orange compound of MoO3 is formed from strongly oxidizing solutions. For the preparation that Heuteufel (and also Gmelin) mention, HNO3 is needed, not H2SO4 or another strong acid.

I am inclined to believe that the coloured and white compound are stereo isomers.

Quote: Originally posted by LHcheM  
So the orange compound could be a mixture of peroxo-complex and oxide? how long have you kept the orange compound for? because the peroxo complex of Mo have a very intense color when solid, but relatively less intense in solution.
I will try your experiment as well. Nice pictures! :)

I think not, since, when dissolved in aqueous NaOH, no yellow solution is being formed.
The orange compound in the picture was prepared march 2009, and has since been kept at room temperature.
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[*] posted on 21-12-2011 at 17:20


This might be of interest: http://www.springerlink.com/content/t72h4t61r1j8n164/

As far as the influence of the acid used for acidification is concerned: google "sulphated molybdic acid" or "sulphate of molybdic acid".

Some more general information:Molybdenum compounds

(Sorry for not explaining much, I´m in a hurry:()
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[*] posted on 22-12-2011 at 02:12


Ok here's my update on the Molybdenum experiments I did... (sorry I was abit busy these days cause I still have tests to do :mad:)

1. With a fairly concentrated solution of sodium molybdate dihydrate (about 1g in 10ml water), upon fast addition of conc Sulphuric acid, a clear bright yellow solution is obtained, but the color quickily fades upon addition of more conc Sulphuric acid
2. Addition of conc Sulphuric acid under ice bath produced the same solution which the color lasts longer, but still the solution turned from clear yellow to colorless after standing in air (15oC) for 1 hour (so this is consistent with Woelen's results)
3. On addition of hydrogen peroxide to the sodium molybdate dihydrate solution, a dark red solution is obtained, with some oxygen evolved. Upon the addition of first drop of dilute sulphuric acid, the color of solution changes dramatically from dark red to yellow. The yellow color is somewhat different from that in 1) and 2), it's slightly more orange-yellow than that in 1) and 2)
* I tried to precipitate the yellow MoO3 but failed, since I don't have MoO3, I can't experiment with the compound.

I must apologize that my previous post saying that "addition of conc Sulphuric acid didn't give any coloration" is wrong, perhaps it's due to too dilute sodium molybdate solution I used last time... this time I double checked, I'm deeply sorry for the confusion...:(

So the orange compound, the yellow compound, the white compound are all moilybdates...:o oh my god...

By observation, at high pH, both molybdates and chromates tends to have lower degree of polymerization
when crystallized from NaOH, the pH is high, the salt formed is MoO4(2-) (we have never heard of Na6Mo7O24, this is probably due to the strong polarizing power of Na+ ions destroying the cluster?)
when crystallized from NH3, the pH is lower, the salt formed is Mo7O24(6-)
So, theoratically, when molybdate solution is strongly acidified, the polymerization goes to infinity, ie precipitation of MoO3.xH2O
But the ppt didn't come out...
from what we know phosphomolybdic acid has an intense yellow color, this is due to the Keggin ion formation, how about that when sulphuric acid is used, an analogue (sulphomolydate Keggin ion) is formed from sulphate? but then maybe for sulphates, the resulting Keggin ions is not as stable as the phosphomolydate ion, so upon standing/heating, it slowly decomposes by the high concentration of H+ ions into protonated HMoO4(-) ions, which may be colorless?

But this still can't explain what is the composition of the yellow solid that Woelen bought...and also the "memory" behaviour of the compounds...
The most confusing point is that, home made sodium molybdate from MoO3, either crystallized or prepared in situ, does not produce any coloration...
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[*] posted on 28-12-2011 at 12:18


Well, Wikipedia pointed me to a whole book on this:
Heynes, J. B. B.; Cruywagen, J. J. "Yellow Molybdenum(VI) Oxide Dihydrate" Inorganic Syntheses, 1986, volume 24, pp. 191

Hope that helps! I'm still waiting for my Ammonium Molybdate- COming from Bulgaria, I think it got caught up in customs :C
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[*] posted on 30-12-2011 at 13:31


I did another experiment with the yellow compound I have. I prepared a solution of this in dilute NaOH and allowed this to stand for two weeks. The colorless solution did not change noticeably in this period, only a very small amount of dirty brown stuff settled at the bottom, probably this is some impurity which settled in the two weeks.

When this colorless solution is acidified with 2M H2SO4, then NO yellow color appears. The solution remains perfectly colorless.

As a control experiment I also prepared a second solution in dilute NaOH of the yellow compound I have and to this fresh solution I also added 2M H2SO4. This solution becomes deep yellow.

So, when the alkaline solution of the yellow material in dilute NaOH is kept for a long period, then it looses the property that a yellow solution is made on acidification. This is a very interesting property. Apparently, the alkaline solution slowly changes over time and although this change is invisible, the structure of the compound in solution definitely changes. Maybe it depolymerizes?

@Wizzard: Your source is not a whole book on this, it is just a few pages on how to make MoO3.2H2O from Na2MoO4.2H2O and dilute HClO4. The book you mention is one of a series of 34 books. The method is very simple:
- Prepare a 0.6 M solution of Na2MoO4.2H2O in water.
- Prepare a 6M solution of HClO4 in water.
- SLOWLY mix equal volumes of the solutions while stirring.
- Put aside the clear liquid for a period of 4 weeks in a clean plastic beaker with a somewhat roughened/scratched surface. Yellow crystals of very pure MoO3.2H2O separate in this period.
- Thoroughy rinse the crystals with distilled water and let the crystal mass become dry at room temperature.

Yield is 80 to 84% relative to the amount of Na2MoO4 used.

The yellow monohydrate can be prepared from the dihydrate by heating in an oven at 100 C for 2 hours.

The dihydrate should not be stored above a dissicant, because it effloresces somewhat and then after some time its exact hydration state is not known anymore. So, simply put it in a tightly sealed container.




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[*] posted on 5-1-2012 at 15:13


I have my doubts whether the colour change is due to (de)polymerization, since none of the isopolymolybdates are yellow. I tried to set up a theory where the colour difference is due to stereo isomerism, but that is also difficult.

Point is, that the yellow colour seems to appear only when the complex is synthesized in an oxidative environment. Since the transition occurs only in solution and is SLOW (for normal temperature), you can have a complex with a "yellow" isomer that is colourless because of the "wrong" ligands/pH. Upon acidification, the colour immediately shows up.

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