Preparation of Ammonium Molybdate from Molybdenum Disulfide

August 13, 2013
by Magpie


A. Introduction
The applicable reactions appear to be:

2MoS2 + 7O2 + high heat ---> 2MoO3 + 4SO2

MoO3 + 2NH4OH ---> (NH4)2MoO4 + H2O

Crystals removed from pieces of molybdenite ore provided the source of MoS2. From a charge of 0.51g of MoS2 crystals 0.25g of (NH4)2MoO4 was recovered. This is a yield of 44%.

The scale of this procedure is small, being the same as that used by the author during procedure development in 2009 (ref 1).

This procedure follows that of Walton (ref 2).

B. Reagents
~0.51g of MoS2 crystals scrapings from molybdenite ore
3mL of distilled water
1.2mL of 13.3N NH4OH

C. Equipment
1. Bunsen burner (note 1)
2. small crucible
3. clay triangle
4. iron ring w/clamp
5. ring stand
6. steam bath
7. 100 mL beaker
8. glass funnel
9. filter paper
10. evaporating dish
11. mortar & pestle

D. Procedure

a. Conversion to MoO3
1. Grind the MoS2 crystal scrapings to a powder using a mortar & pestle.
2. Place MoS2 powder in the crucible.
3. Install the iron ring on the ring stand; place the clay triangle on the ring.
4. Place the crucible w/MoS2 scrapings on the clay triangle.
5. Heat the crucible as strongly as possible with the Bunsen burner. Per Walton: “heat at a dull red heat for 30 minutes.” The gray MoS2 should all eventually turn to a dull yellow. The cooled finished product will look almost white.

b. Conversion to (NH4)2MoO4
1. Place the cooled MoO3 powder in a 100mL beaker.
2. Add 3mL of distilled water.
3. Add 1.2mL of 13.3N NH4OH.
4. Mix these ingredients and then heat on a steam bath for ~15 minutes with stirring.
5. When the MoO3 has dissolved filter the solution into an evaporating dish.
6. Allow to air dry. Pure white crystals of ammonium molybdate will form.

E. Results
This author recovered 0.25g of ammonium molybdate for a % yield of 44% based on the weight of the intermediate MoO3 of 0.57g.

F. Discussion
Shown below is a photo of the molybdenite ore. The dark spots are the crystals of relatively pure MoS2. This is the only source of MoS2 tested by this author. Presumably this procedure will also be useful for making ammonium molbydate from other sources of MoS2 such as reagent grade and lubricants.



Also shown in the picture, in the evaporating dish is the ammonium molybdate product. The blue cast is due to some contamination, possibly MoO2. This blue color was also noted in the “Questions” section of Walton’s procedure.

Ammonium molydate can be used in a qualitative test for the presence of phosphate, and this is purpose for which the author prepared it. The presence of phosphate results in the formation of the brilliantly yellow ammonium phosphomolybdate also shown in the photo in the test tube. (Note 2)

G. Notes
1. The author used a Tirill type Bunsen burner on propane. This was marginal as a source of heat. A Mekker burner on MAP/PRO gas would be better.
2. A UNESCO website says the test is good for detection of phosphate in water down to 2.4 mg/L, or 2.5 x 10^-5M. It also provides the following reaction:

Na2HPO4(aq) + 12(NH4)2MoO4(aq) --->(NH4)3PMo12O40(s)
+ 2NaNO3(aq) + 21NH4NO3(aq) + 12H2O(l)

H. References
1. Inorganic Preparations, by Harold Walton, 1948, p. 146 (see forum library).
2. Science Madness forum thread: http://www.sciencemadness.org/talk/viewthread.php?tid=11908#...




[Edited on 14-8-2013 by Magpie]

[Edited on 14-8-2013 by Magpie]

Quote: Originally posted by j_sum1

Thanks for this Magpie. I am considering making some of this rather than purchasing it. A couple of questions: My starting material is likely to be Mo wire rather than MoS2 -- since I have some on hand and it is pretty cheap anyway. So, I am looking for the best method for oxidising it to MoO3 as a first step. Not that I have started researching yet, but is there a straightforward method for doing this? (Will burning in air get me what I want? Or maybe I need to go for a hydroxide first?)

Quote: Originally posted by j_sum1

My starting material is likely to be Mo wire rather than MoS2 -- since I have some on hand and it is pretty cheap anyway. So, I am looking for the best method for oxidising it to MoO3 as a first step. Not that I have started researching yet, but is there a straightforward method for doing this? (Will burning in air get me what I want? Or maybe I need to go for a hydroxide first?)

The best way to get from molybdenum or tungsten metal to the oxides is by simply dissolving the metal in concentrated hydrogen peroxide. If you can get the 30% pool store kind, that is supposed to dissolve it fairly rapidly.
http://www.sciencemadness.org/talk/viewthread.php?tid=38280

Now I see that you were the first one to reply to that thread... perhaps you forgot about it?