Sciencemadness Discussion Board - Colorful Aqueous Molybdenum and Tungsten Chemistry

Colorful Aqueous Molybdenum and Tungsten Chemistry

bfesser - 17-3-2009 at 13:39

Aqueous solutions of ammonium molybdate, (NH4

6Mo7O24 · 4H2O, sodium tungstate, Na2WO4 · 2H2O, sodium phosphate tribasic, Na3PO4 · 12H2O, sodium hydroxide, and L-ascorbic acid were prepared. The colorless clear to white crystalline solids produced clear colorless solutions, with the exception of ammonium molybdate which left some white particulate undissolved—the (NH4)6Mo7O24 · 4H2O used was from a very old opened bottle.

I. Mixing of the molybdate and phosphate solutions produced no noticeable signs of reaction. Heated to boiling briefly--still nothing. Cooled, added a little aqueous NaOH--nothing. Heated to boiling again briefly--nothing.

II. Upon addition of one drop of L-ascorbic acid solution to a tube containing tungstate solution, a clear yellowness was observed near the top of the solution, which disappeared upon swirling. Addition of another drop of L-ascorbic acid solution produced a faint yellow solution which did not lose color upon swirling. Addition of further L-ascorbic acid caused the color to change to a slightly-orangish brown. Upon standing, the color darkened. Heated briefly to slight boil; color darkened & changed to a darker greenish hue, then to an indescribable cammo-green/brown/yellow hue. Addition of a single drop of NaOH(aq.) solution formed a beautiful bright blue color which sank to the bottom of the tube. Upon swirling, the solution appeared green. Addition of another drop of NaOH changed the color to a blue-green (teal? aqua?). Addition of more NaOH caused the color to change entirely to an amazingly beautiful and intense blue hue. Brief heating to boiling caused the solution to turn bluish-grey, to grey, to greenish-grey to yellowish grey. After cooling, the color was brownish/yellowish grey.

III. Upon addition of one drop of L-ascorbic acid to a tube containing molybdate solution, the color of the solution very quickly turned yellow with a very slight hint of green. Upon standing, the solution changed to a light lime green color and then darkened slightly. Brief boiling caused the color to change a very dark (blue-tinged?) green. The PTFE boiling chip refused to sink back to the bottom of the tube until some distilled water was added. Continued heating produced no further color change. Tube was cooled. Addition of one drop NaOH created a clear bright yellow colored area of solution on the bottom of the tube which, when swirled, caused the solution to become yellow-brown overall. One more drop of NaOH created an orange-yellow color, additional NaOH yielding yellow. This tube was not heated again after addition of the hydroxide.

Very fascinating color changes. I'll try to repeat these and get pictures/video next time--unless Woelen wants to give it a shot.

I honestly don't even know why I did tube I, but tubes II & III were inspired by the reduction of Cu2+ to Cu0.

[Edited on 7.8.13 by bfesser]

Magpie - 17-3-2009 at 16:27

The yellow product resulting from ammonium molybdate's reaction with phosphate is a qualitative test for a phosphate, as I reported here:

http://www.sciencemadness.org/talk/viewthread.php?tid=11908&...

My procedure for this test is from "Laboratory Studies in Chemistry," (1923) by Robert H. Bradbury:

"Test for a phosphate. Dissolve in a small test tube 0.5 mL of sodium hydrogen phosphate and 1 mL of ammonium nitrate in 5 mL of water, and add 5 drops of nitric acid. Add 2 mL of ammonium molydate solution and warm gently, not to boiling. The composition of the product depends on temperature, so that no formula may be assigned to it."

As you found out, it seems that acidic conditions are necessary in order to form the yellow precipitate.

woelen - 19-3-2009 at 00:36

Bfesser, what you have done is quite interesting. I also have done quite some experiments with tungsten and molybdenum, and I also have observed many colors. For me, the time has come to systematize the observations and make a write-up about them. I now have all kinds of experiments, scattered around. I sure am willing to make some pictures.

One remarkable experiment I already have done and put in a webpage is the dissolving of molybdenum metal in nitric acid. This gives a remarkable red compound, which only is slightly soluble and which can be obtained as a fine powder under a layer of nitric acid.

http://woelen.homescience.net/science/chem/riddles/Mo+HNO3/i...

This experiment is totally different from what you have done (you start with the highest oxidation state, I start from the metal), but it adds to the complexity and colorfulness of the chemistry of molybdenum.

JohnWW - 19-3-2009 at 01:50

Quote:

Originally posted by Magpie
The yellow product resulting from ammonium molybdate's reaction with phosphate is a qualitative test for a phosphate, as I reported here: http://www.sciencemadness.org/talk/viewthread.php?tid=11908&...
My procedure for this test is from "Laboratory Studies in Chemistry," (1923) by Robert H. Bradbury:
"Test for a phosphate. Dissolve in a small test tube 0.5 mL of sodium hydrogen phosphate and 1 mL of ammonium nitrate in 5 mL of water, and add 5 drops of nitric acid. Add 2 mL of ammonium molydate solution and warm gently, not to boiling. The composition of the product depends on temperature, so that no formula may be assigned to it."

The yellow color is also used as a quantitative test for phosphate, colorimetrically, with a spectrophotometer set to measure absorbance of the solution at the wavelength (near the violet end of the spectrum) where absorbance is strongest. However, it is likely that some other oxy-anions such as arsenate would interfere if present, by also forming complex molybdates.

woelen - 22-3-2009 at 08:08

I have done the molybdate/ascorbic acid experiment and as a counter-experiment I have done the same with hypophosphite as reductor. With ascorbic acid I also observed a strong yellow color, which turns blue in acidic media and which turns brown/red in alkaline media. The colors are very nice. I made pictures of these, but I still have to put them in a webpage and do some scaling to more decent sizes.

The experiment with hypophosphite is different. In alkaline solution, nothing seems to hapen, the liquid remains colorless. In acidic solution, the liquid turns dark blue, starting from pale blue, which gradually darkens. I also made pictures of this.

I have the impression that ascorbic acid forms some yellow complex with molubdenum, but I'm not sure about the oxidation state in which the complex is formed. Without complex formation, you simply get the beautiful blue color.

A link to a page with pictures and more precise description of what I have done will follow...

[Edited on 23-3-09 by woelen]

benzylchloride1 - 22-3-2009 at 19:16

Woelen, where can tungsten metal be obtained cheaply without resorting to a scientific supply? I am interested in the aqueous chemistry and organometallic chemistry of tungsten.

kclo4 - 22-3-2009 at 19:55

I think rods of 2% thoriated tungsten is sold for welding. There is a thread for producing Sodium tungstate somewhere around here if i remember correctly.

woelen - 23-3-2009 at 10:24

@Benzylchloride1: An interesting source of cheap, yet very pure, tungsten trioxide is the following:

http://cgi.ebay.nl/YELLOW-TUNGSTEN-TRIOXIDE-High-grade-mater...

Dissolve some of this in NaOH-solution (requires some heating) and you get a solution of Na2WO4. The quality of this material is very good. I was surprised to obtain such a pure compound.

Another interesting seller is the following:

http://stores.ebay.nl/THE-MISTS-OF-AVALON_tungsten_W0QQcolZ4...

Here we are talking about metal rods, metal grains and metal powder. The same seller also has molybdenum and that is of good quality as well.

Both sellers ship internationally, worldwide.

He also sells MoO3, but this unfortunately is of bad quality. I ordered this as well, but this material is brown/green and has a lot of iron in it (it dissolves in NaOH-solution, but a brown flocculent precipitate remains).

Jor - 23-3-2009 at 13:36

The second supplier is very interesting. I think I am going to place some orders there...
However, before I buy wolfram metal, how do I dissolve this stuff?
One option is HF/HNO3, but for me that is a no-go. Aqua regia or nitric? Nitric probably wont work or very slowly, because otherwise, why would several internetsites mention HF/HNO3...

I was also thinking of hot alkaline hypochlorite. My hypothesis is that this could be good candidate. Any advice?

And do these metals, molybdenum and wolfram, dissolve only slowly or quickly? I can buy powdered form, but it's not cheap.

Quite remarkable that the seller sells 390g of cadmium to any random individual for 10 UK pounds. Not something Greenpeace would be happy with. Who would want to buy such a large amount of cadmium anyway? That metal is about as toxic as mercury, if not more.

benzylchloride1 - 23-3-2009 at 20:14

Thank you very much, Woelen, I am going to purchase some tungsten trioxide from this seller in a few weeks. How much iron precipitate is formed when the molybdenum trioxide is dissolved in NaOH ? The molybdenum trioxide could be purified by dissolving it in concentrated ammonia water, filtering through a fine fritted funnel with vacuum and then concentrating the solution to cause the ammonium molybdate to crystallize out. If the trioxide is needed, the ammonium molybdate could be heated to decomposition. I am interested in obtaining some molybdenum trioxide. I have a small amount of molybdenum metal, but would prefer to use this only for making the anhydrous chlorides or bromides, that will have to wait until I find a tube furnace at a reasonable price. Tungsten can be dissolved in a solution of concetrated hydrogen peroxide and sodium hydroxide. There are several threads that describe this topic.

[Edited on 24-3-2009 by benzylchloride1]

woelen - 24-3-2009 at 02:11

Jor, I have tungsten powder and this dissolves in H2O2 without anything added. Just put it in the liquid and leave overnight. Next morning the material has dissolved. In this way you can make dilute solutions of peroxotungstic acid in water. if you add NaOH to the mix, then sodium peroxotungstate goes in solution. In that way you can dissolve a little more tungsten.

The process is slow, but it works. When you have dissolved the metal, you have to boil down the solution, until a solid remains. You need stronger heating to get rid of all peroxo-ligands. Tungsten peroxo complexes are rather stable.

if you have bulk tungsten, then things are somewhat harder. The rate of dissolving is a few micrometers per hour, so dissolving bulk tungsten will be VERY slow. Electrolytic dissolving is an option in that case, take a tungsten rod, immerse it in a H2O2/NaOH mix and apply a DC voltage. The anode will dissolve in that case.

turd - 24-3-2009 at 05:29

Quote: Originally posted by woelen

One remarkable experiment I already have done and put in a webpage is the dissolving of molybdenum metal in nitric acid. This gives a remarkable red compound, which only is slightly soluble and which can be obtained as a fine powder under a layer of nitric acid.

http://woelen.homescience.net/science/chem/riddles/Mo+HNO3/i...

FWIW, I couldn't repeat your experiment. Adding 65% HNO3 (p.a.) to some Mo powder (p.a.) gave no reaction. Heating for different amounts of times I obtained snow white, blueish white or greenish white precipitates, which were all a mix of Mo, MoO3 and MoO3*0.5H2O. Sometimes the aqueous phase was yellow, sometimes colourless, but I never got your brown suspension. So I guess my powder is either not fine enough or passivated.

Doesn't make me wonder though, with Mo (as with V) you can get everything from 0 to 3 dimensional oxo-clusters with oxidation numbers from 0 to 6. Often different ones in the same compound. It's a mess.

Photo series of colorful molybdenum

woelen - 25-3-2009 at 10:49

I have resized/scaled the images and have made three sequences of experiments:
- molybdate in alkaline solution with ascorbic acid as reductor
- molybdate in acidic solution with ascorbic acid as reductor
- molybdate in acidic solution with hypohosphorous acid as reductor
(no hypophosphite in alkaline series, this remains colorless).

The pictures follow in separate messages

Alkaline solution of ascorbate

woelen - 25-3-2009 at 10:50

Here follows the situation with alkaline ascorbic acid solution (ascorbate). The solution slowly turns from yellow to red and then very dark brown:

[Edited on 25-3-09 by woelen]

Acidic solution of ascorbic acid

woelen - 25-3-2009 at 10:52

In acidic solution with ascorbic acid (in 10% H2SO4) the result is totally different. Now there is a shift from yellow through green to dark blue:

[Edited on 25-3-09 by woelen]

Hypophosphorous acid solution

woelen - 25-3-2009 at 11:00

With hypophosphorous acid a pure blue solution is formed. The difference most likely is not in the reduction of molybdenum(+6) to molybdenum(+5), but in the initial complex formation with ascorbic acid. Apparently, molybdenum(+6) forms a yellow complex with ascorbate and ascorbic acid. This explains the initial yellow color and the later green color in the ascorbic acid sequence of pictures. With hypophosphorous acid no such yellow complex is formed and the liquid simply turns from colorless to dark blue through shades of blue.

[Edited on 25-3-09 by woelen]

UnintentionalChaos - 25-3-2009 at 12:07

Quote: Originally posted by Jor

Quite remarkable that the seller sells 390g of cadmium to any random individual for 10 UK pounds. Not something Greenpeace would be happy with. Who would want to buy such a large amount of cadmium anyway? That metal is about as toxic as mercury, if not more.

For those in the US, you can buy good quality lead, tin, antimony, cadmium, zinc, and zinc here: http://www.rotometals.com/ I placed quite a large order...cadmium isn't regulated or anything, just not something 99.9% of people would want to have kicking around...I have a pound for example for element collection purposes and maybe for some cadmium pigments.

As for the ebay shop "The Mists of Avalon" with all the metals in the UK, I can attest that they are a good source as well.

Beautiful stuff woelen!

pantone159 - 25-3-2009 at 16:11

Very nice photos, woelen, I always really like your stuff.
Makes me sad that I can't get hypophosphorous acid in the USA. :mad:

woelen - 18-8-2009 at 12:11

I now made a webpage about this experiment set:

http://woelen.homescience.net/science/chem/exps/colorfulmoly...

The pictures you may have seen before, but I also now have a good description of the experiments, so that others can repeat them if they wish. I very much like the colorful chemistry of molybdenum, the more so, because it is not the first thing which comes to mind when one is speaking about colorful chemistry.

Btw, no sodium hypophosphite or hypophosphorous acid is needed for the experiments, many other reductors are suitable (e.g. zinc powder, sodium sulfite, sodium dithionite).

bfesser - 18-8-2009 at 18:47

Beautiful photos and excellent writeup, as always, woelen! Keep up the good work.