Sciencemadness Discussion Board

Reaction of molybdenum with nitric acid.

I am a fish - 16-7-2004 at 08:53

I have some metal that I believe to be molybdenum. However, I'm not absolutely certain about this, and so I want to test it. However, my efforts are being hampered by my lack of knowledge of molybdenum chemistry.

I added some of the metal to 69% nitric acid, and it dissolved slowly, with the release of nitrogen dioxide, yielding a cloudy, dark red liquid. I diluted some of this and added sodium hydroxide, which gave an orange solution (or very fine suspension) from which a fine, flaky looking, orange precipitate started forming.

Does this seem like molybdenum? I know that it reacts with nitric acid, but despite much searching, I can't find anything about the precise compound formed. As molybdenum exhibits a wide range of oxidation states, guessing is rather difficult. Drawing analogies with similar elements doesn't help either: I can find no more about tungsten (the element below molybdenum) than I can find about molybdenum. Chromium (the element above molybdenum) is significantly different, as Mo(VI) compounds are only weakly oxidising, whilst Cr(VI) compounds are strongly oxidising (for example CrO3 is a powerful and unstable oxidising agent, whilst MoO3 is fairly inert).

vulture - 16-7-2004 at 09:27

You could try the phosphate/molybdenum test, IIRC ammoniummolybdate will form a yellow precipitate with phosphates.

bennator - 16-7-2004 at 09:55

I just dissolved some Ammonium Molybdate in H2O, the solution it formed was clear. Strangely enough, we apparently don't have any nitrates in this lab, so I can't test to see if it forms an orange solution with nitric acid.

Marvin - 16-7-2004 at 10:43

70% nitric produces a red solution and ppt, this should be due to MoO3. 50% acid attacks it faster but goes through an intermediate nitrate stage before producing the trioxide.

Suggestion here is to take a trace of the material (meaning the ppt) or a salt of the metal, mix with conc sulphuric acid and heat neerly to dryness. You should see a strong blue colour. Antimony or Tin interfere with the test.

(Mellor)

[Edited on 16-7-2004 by Marvin]

BromicAcid - 17-7-2004 at 17:26

To get a better idea of what element you have you could just take the density of it. Just differentiating between chromium (D = 7.1) and molybdenum (D = 10.2) would be easy by this physical test, assuming you have a nice graduated cylinder and a decent scale.

unionised - 18-7-2004 at 03:19

I think you may be making another assumption there. How would you determine the density of a bunch of light-bulb fillament supports with a measurig cylinder and a scale?
I'm pretty sure that nitric acid passivates chromium so it wouldn't have dissolved in the first place. I'm also pretty sure that nitric acid won't oxidise chromium to Cr(VI) so it's probably not Cr.

The_Davster - 18-7-2004 at 12:01

I am not really sure what you mean unionised, but the density of filaments is easily determined. Place the filaments in a graduated cylinder with a known volume of water in it, record the increase in water level, and you have the volume of your filaments in mL or cm3. Then mass the filaments and use d=m/v to get the density of the filaments.

Mo qualitative tests

Magpie - 18-7-2004 at 17:34

From C. W. Ammen's Recovery and Refining of Precious Metals:

1. When sublimated on a plaster tablet MoI4 turns "a beautiful deep ultramarine blue" when exposed to ammonia fumes.

2. Sublimate of MoO3 is pale yellow when hot and white when cold. When heated to a red heat will fuse; and on continued heating will volatilize.

3. Flame test with Mo oxide or sulfide gives a pale yellow-green.

Hope this helps.

I am a fish - 19-7-2004 at 11:14

Quote:
Originally posted by vulture
You could try the phosphate/molybdenum test, IIRC ammoniummolybdate will form a yellow precipitate with phosphates.


Are you sure about that? I added a solution of potassium phosphate to a solution of sodium molybdate (my only molybdenum compound) and no precipitate or colour change was seen. I repeated the experiment with diammonium hydrogen phosphate (my only other orthophosphate salt), and again, nothing was seen.
Quote:
Originally posted by Marvin
...take a trace of the material (meaning the ppt) or a salt of the metal, mix with conc sulphuric acid and heat neerly to dryness. You should see a strong blue colour. Antimony or Tin interfere with the test.


I dissolved the precipitate in concentrated sulphuric acid, and it gave a very pale yellow solution. I started boiling it, but gave up when the sulphur trioxide fumes became too much. (I don't have a fume hood, and I think the neighbours are probably suspicious enough already, without me wearing a gas mask in the garage.)
Quote:
Originally posted by BromicAcid
To get a better idea of what element you have you could just take the density of it. Just differentiating between chromium (D = 7.1) and molybdenum (D = 10.2) would be easy by this physical test, assuming you have a nice graduated cylinder and a decent scale.


I've done density measurements with other samples. However, the sample in question is a circular sheet, which is thin, but inflexible. The liquid in any container large enough to hold it, would only rise a very short distance.
Quote:
Originally posted by Magpie
2. Sublimate of MoO3 is pale yellow when hot and white when cold. When heated to a red heat will fuse; and on continued heating will volatilize.

3. Flame test with Mo oxide or sulfide gives a pale yellow-green.


I placed some of the precipitate on the end of a glass tube, and heated it in a propane/butane flame. It turned the flame a lemon yellow colour and left a light yellow residue on the end of the tube. I left it to cool, and the residue turned white. When heated again, it returned to it's yellow colour. This seems to be molybdenum(VI) oxide.



Thanks for everyone's help.