Sciencemadness Discussion Board

Dissolution of Group 6 metals

Kitsune - 13-10-2014 at 04:37

Hi all, this is my first Post on this forum. Recently I have decided to look at the compounds of certain Transition metals, namely those of Group 6 (Tungsten and Molybdenum).

From my research I see that both Tungsten and Molybdenum, at least for the home chemist are very difficult to put into solution. I happened across a few different scientific papers (sadly most abstract only) which eluded to the use of Hydrogen peroxide to dissolve Molybdenum; unable to find any more research I set about testing the theory that these metals though invulnerable to the Hydrochloric, Sulphuric and Nitric acids nor the bases, could be put into solution by Hydrogen Peroxide.



My Method was as follows:

Taking a small piece of clean Molybdenum wire (salvaged from old Incandescent light bulbs that were no longer needed) I placed this into a solution of 5cm3 6% Hydrogen Peroxide (due to where I live, this is the most concentrate Solution available) and 5cm3 of 1M Acetic acid.

Right away the very fine ends of the wire where it had been cut started to form few bubbles, heating this solution weakly the few bubbles increased to a continuous stream, after one day, the wire had disappeared into solution, leaving a rather fetching lemon yellow hue.

I have been allowing this solution to sit and evaporate, it is currently at about 5cm3



I have also done this with Tungsten also recovered from an old incandescent bulb (filament) which also dissolved, slightly faster. I am soon going to be testing this also on the group 5 metal, Tantalum due to it's similar chemistry.

The idea of using Acetic Acid along with the Hydrogen peroxide came from the standard route for making Transition Metal Acetates from the metal such as Copper Acetate.

My Question is, what are the products likely to be from these reactions, which oxidation state am I looking at, what intermediate compounds were there (product of Hydrogen Peroxide on the metal) and whether they are likely to decompose when out of solution (upon crystallisation).

My original thoughts were Dimolybdenum (ii) Tetraacetate (the colour in solution would support this) and Ditungsten (ii) Tetraacetate though with these metals, but I'm not sure. Anyway, all suggestions welcome.

j_sum1 - 13-10-2014 at 14:43

Cool work. Nothing much to add except to say I intend to try this too.
Following this thread for more info.

AsocialSurvival - 13-10-2014 at 14:46

They form soluble molybdate and tungstate with bases like Sodium Hydroxide.

Justin Blaise - 13-10-2014 at 19:15

You may have already seen this paper, but it looks like you can get pertungstic acid if you crystallize and dry the compound at <100 degrees C, tungstic acid at temperatures around 180 degrees C, and tungsten trioxide at higher temperatures. It also appears that acetic acid is unnecessary, as H2O2 will do the job on its own.

Attachment: tungsten hydrogen peroxide.pdf (281kB)
This file has been downloaded 851 times

Mesa - 13-10-2014 at 23:53

Quote: Originally posted by AsocialSurvival  
They form soluble molybdate and tungstate with bases like Sodium Hydroxide.

Said bases are precursors to some pretty interesting heteropoly acids(silico/phosphotungstic acids.)

Also to note, H2WO4 decomposes to WO3 after a short duration, becoming insoluable/precipitating from solution.

Kitsune - 14-10-2014 at 13:46

Thank you for the paper Justin, I read the first half of it previously (only a preview was available), It is pretty interesting stuff and worth trying out, if only for the yellow of the molybdenum+2 ion in solution.

An update: Today I got a little impatient as I have been waiting the solutions to crystallise out for several weeks now; I set about gently heating the solutions.

The Molybdenum solution: yielded an equally as beautiful as before, lemon yellow compound which when heated to decomposition evolves a vapour with an acetic scent; I guess I managed to form Dimolybdenum (ii) Tetraacetate, of which I later decomposed to Molybdenum (vi) Trioxide.

The Tungsten solution: a white solid which upon stronger heating decomposed to a yellow solid with no acetic off gas, Tungsten (vi) Oxide would be my guess.

I will reproduce these reactions, when I get the time and then try to form alkali metal molybdates and tungstates (Sodium Molybdate and Tungstate at first), then Tungsten Carbide will likely become a targeted source of tungsten for dissolution, from no less than dead ball point pens (ball points).

Justin Blaise - 14-10-2014 at 17:56

No problem. If you have any other references you can't access, I'll see if my university has a license with the journals. I'd love to read more on this stuff.

Also, how much tungsten and molybdenum do you get out of each light bulb?

Kitsune - 15-10-2014 at 05:05

Thanks for the offer, I may need to take you up on that sometime.

The amount and quality of what you can get from an incandescent light bulb is variable; it depends on the size, make, construction, whether it is new or used. (Remember that the large wires on each end of the filament are not Molybdenum, it's only the small silver ones that are in the middle)

That all said, the Molybdenum experiment used one filament support (usually you will get two or three per bulb), this was enough for proof of concept and I also ended up with 250mg of compound for good measure.

The Tungsten reaction used only one rather small length of filament (sometimes they snap during recovery from the bulb) and this yielded just as well as the Molybdenum, though it did not form an Acetate but Tungstic acid.

A better source of the WO4 2- Ion may be Tungsten Carbide, a cheap source would be ballpoint pens (if you can cope with the ink you will need to repetitively wash from them beforehand), the Tungsten filament is an incredibly fine wire, wound into a single or double coil for greater surface area so you never get too as much material as you think you have. At this time, I have yet to test whether the WC/W2C approach is valid for a home chemist.

MrHomeScientist - 15-10-2014 at 06:21

It's been mentioned before on the forum that tungsten can be dissolved into pertungstic acid, tungstic acid, or tungsten trioxide (depending on drying temperature) by dissolusion in 30% H2O2: http://www.sciencemadness.org/talk/viewthread.php?tid=395

Something I definitely want to try at some point. I didn't know that molybdenum was also susceptible to peroxide.

Mesa - 15-10-2014 at 17:02

Tungsten also makes an interesting thermite with KNO3/NaNO3, however NaOH/W mixtures merely melt and bubble.
The KNO3/W mixture leaves a black residue weighing less than the original amount of tungsten(I assume a volatile tungsten salt is one of the products of the thermite reaction.) and dissolves in hydroxides.

j_sum1 - 15-10-2014 at 17:22

Let's see if I have this right Mesa.
You mix together a stoichiometric ratio of W and KNO3 for your thermite. Ignite and watch the fun. And the reaction products are... Does this form potassium?

(Runs off to break a million light bulbs and scavenge for the filaments...)

elementcollector1 - 15-10-2014 at 19:23

Doubtful - at most, it's likely potassium oxide. Was the dissolution particularly vigorous?

Mesa - 15-10-2014 at 20:45

I have no idea what it forms. It looks a lot like a fused salt(i.e. similar texture to fused KNO3 I tried to reduce with carbon a few times) and is a very dark grey/black colour. The reaction itself is very fast and produces a remarkably bright white flame, I assume that indicates it's a lot more exothermic than other thermites(based on very little evidence admittedly) so any potassium formed likely evaporates instantly.

Edit: I'm not sure if this would work with lightbulb filaments. I pillaged around 3kg of tungsten powder from a precision engineering company my father used to work for. The powder is about as fine as sifted flour.

[Edited on 16-10-2014 by Mesa]

j_sum1 - 15-10-2014 at 21:36

I wasn't serious about the potassium or the lightbulbs. But interesting thermites are always, well, interesting.
My next element order is likely to be both tungsten and and molybdenum. Not really sure I want to be flaring it away. But a teeny bit of experimentation might be fun.

Mesa - 15-10-2014 at 22:02

If you want I can do another small KNO3/W ignition and take pictures of the process/aftermath. I don't currently have a camera/mobile but I can get one later on today.

j_sum1 - 15-10-2014 at 22:11

Cool. Please do. Video too if you are able.

blogfast25 - 16-10-2014 at 11:49

Quote: Originally posted by Mesa  
Tungsten also makes an interesting thermite with KNO3/NaNO3, however NaOH/W mixtures merely melt and bubble.


You shouldn't call that 'thermite'. It's simply an oxidation of tungsten with saltpetre. The product, mainly WO<sub>3</sub>, is black due to unreacted W and may be some lower oxides.

Dan Vizine - 23-10-2014 at 19:46

Quote: Originally posted by j_sum1  
But interesting thermites are always, well, interesting.


Here's something you might enjoy j_sum1, all the thermite, intermetallic & metal combustion info you could want.

Attachment: Survey of Combustible Metals.pdf (9.8MB)
This file has been downloaded 564 times

[Edited on 24-10-2014 by Dan Vizine]

j_sum1 - 23-10-2014 at 20:54

Fantastic. I think. I have tried downloading but the pdf seems corrupted. Would you care to upload again?

Dan Vizine - 24-10-2014 at 04:07

Quote: Originally posted by j_sum1  
Fantastic. I think. I have tried downloading but the pdf seems corrupted. Would you care to upload again?


Let me try again....

Attachment: Survey of Combustible Metals.pdf (9.8MB)
This file has been downloaded 524 times

j_sum1 - 24-10-2014 at 04:38

Whoa. Pages of fun.
Lots of interesting reactants to get.

j_sum1 - 21-3-2015 at 23:44

I have just attempted dissolution of Mo wire in 6% H2O2 and dilute acetic acid. (Mo sourced online as a cutting tool for mobile phone repair)
Test tube scale -- approximately 600mm of wire (0.11mm diameter) with around 5mL of peroxide and a slightly smaller amount of vinegar.
Progress is really slow. It has been bubbling away for nearly 24 hours so far and it is hard to see much change. The wire does appear thinner and has lost its lustre. The solution has a yellowish tinge -- interestingly near the top. After shaking the solution appeared clear (presumably from dilution). However, a couple of hours later the yellowish colour appeared at the top again. I guess the reaction products are transported upwards with the bubbles.

I'd like to attempt this again with stronger peroxide and without the acetic acid. I also have some tungsten to try.
The colour is an interesting subtle gold hue and rather nice.

Kitsune1 - 22-3-2015 at 15:24

It really is a slow reaction, my aim was to make an acetate of Molybdenum by first oxidising the metal, then forming an acetate; it does (like the Tungsten) dissolve in H2O2, a higher strength makes it react quicker, though not that much quicker, I'm glad I only used incandescent light bulb filament supports, they took long enough but the colour is quite nice.

OP was my old account, couldn't log in and then never received an email to change my password (checked spam, resubmitted so decided I'd just make this account instead).

j_sum1 - 22-3-2015 at 15:33

Quote: Originally posted by Kitsune1  
OP was my old account, couldn't log in and then never received an email to change my password (checked spam, resubmitted so decided I'd just make this account instead).
I figured that.

I need to do a bit more research on this one. I am hazy about the reaction products including the bubbles that come off the wire.
I mainly did this as a little sideline while I spent a couple of hours tidying the lab and also to test that what I had bought was indeed Mo or predominantly so.

Mesa - 23-3-2015 at 05:35

Reaction products are O2 and H2MoO4.

j_sum1 - 23-3-2015 at 06:07

So...
6H2O2 + Mo --> O2 + H2MoO4 + 5H2O??

Why not
4H2O2 + Mo --> H2MoO4 + 3H2O

Is it not correct to consider the evolution of O2 an undesired side reaction catslysed by the Mo? And just because I have bubbles evolving, that is no indicator that I am actually dissolving my Mo.

Is the acetic acid doing nothing? I ask because I merely copied what was done upthread.

Nearly 48 hours and my wire is still there. This is depressingly slow.

Mesa - 23-3-2015 at 07:11

Molybdenum acetate only forms from molybdenum hexacarbonyl. Evolution of bubbles can only be one of two possibilities given whats in the reaction mixture.

edit:
Yes, the acetic acid is doing nothing. Acetic acid/hydrogen peroxide has been used industrially to oxidize group 6 metals, but the concentrations were essentially 1:1. The idea was that the peracid was a better oxidizer than the H2O2.

[Edited on 23-3-2015 by Mesa]

Kitsune1 - 23-3-2015 at 07:51

Molybdenum Disulphide can be used as a catalyst in the decomposition of H2O (it has potential as a Pt replacement in Hydrogen Fuel Cells), so it is plausible that there is some decomposition of H2O2 however this reaction is so painfully slow and so little gas is actually evolved that it would be very hard to test.

My reason for the addition of Acetic Acid was purely to be in line with my other experiments which was to look at Acetate-metal complexes such as Copper (ii) Acetate or Iron (iii) Oxyacetate. I was interested to see whether Molybdenum (as well as Tungsten) could be dissolved into an organic acid in the same way that copper could through the simple action of a household oxidiser (H2O2) as a potential route to open up further studies into Mo compounds without reliance on purchasing compounds or using reagents such as HF. However what made things more interesting for me is that on the addition of Hydrogen Peroxide, Molybdic/Tungstic acid was created, not the respective acetate, thus rendering the addition of Acetic Acid unnecessary.

I don't know whether Acetic acid's action on Mo(CO)6 is the only route or whether it is the standard route used as I have not looked into Group 6 chemistry too much and the information out there seems very limited.

bolbol - 23-3-2015 at 22:49

Wouldn't Aqua regia (Nitric + Hydrchloric acid) get the job done?
I heard Ruthenium is the only metal that Aqua regia doesn't do anything against

Kitsune1 - 24-3-2015 at 03:21

It does, especially in hot Aqua Regia, it will even react with hot Hydrochloric acid or Hot Sulphuric; Nitric acid can also dissolve it but it quickly passivates; the aim was to bring the metal into solution without the use of "dangerous" reagents, really testing how far I could go with organic acids. Ruthenium is indeed a strange metal, most room temperature or hot acids have no action on it but then it is vulnerable to to ClO- ion.

Molecular Manipulations - 24-3-2015 at 08:44

Quote: Originally posted by Kitsune1  
Molybdenum Disulphide can be used as a catalyst in the decomposition of H2O.

Ref? A catalyst causes a reaction to proceed faster or slower (usually faster) then it otherwise would. It can't cause a thermodynamically unfavorable reaction to proceed any more than it's own equilibrium would allow.
H2O(g) → H2 + 1/2 O2 ΔG°= 228.7 kJ/mol. For the opposite of this reaction (formation of water) the Keq = 1.22x10^83 That's an enormous constant, no visible amount of water can decompose without adding the 228.7 kJ/mol, catalyst or not.

[Edited on 24-3-2015 by Molecular Manipulations]

Kitsune1 - 24-3-2015 at 12:53

Quote: Originally posted by Molecular Manipulations  
Quote: Originally posted by Kitsune1  
Molybdenum Disulphide can be used as a catalyst in the decomposition of H2O.

Ref? A catalyst causes a reaction to proceed faster or slower (usually faster) then it otherwise would. It can't cause a thermodynamically unfavorable reaction to proceed any more than it's own equilibrium would allow.
H2O(g) → H2 + 1/2 O2 ΔG°= 228.7 kJ/mol. For the opposite of this reaction (formation of water) the Keq = 1.22x10^83 That's an enormous constant, no visible amount of water can decompose without adding the 228.7 kJ/mol, catalyst or not.


The reference,
Gao, M.-R. et al. An efficient ​molybdenum disulfide/cobalt diselenide hybrid catalyst for electrochemical ​hydrogen generation. Nat. Commun. 6:5982 doi: 10.1038/ncomms6982 (2015).

Molecular Manipulations - 24-3-2015 at 12:57

And that's what I was expecting, electrochemical ​hydrogen generation. There's the energy source.

[Edited on 24-3-2015 by Molecular Manipulations]