Sciencemadness Discussion Board

Dissolving Molybdenum in hypochlorite bleach

Boffis - 28-8-2023 at 12:35

I have a piece of molybdenum bar about 0.5 inches square and 3 inches in length and since I had no specific use for it I decided to try and convert it into something more useful. I had read somewhere that molybdenum dissolves in sodium hypochlorite so I put the bar into a 1 litre plastic bottle with a litre of leftover 10-11 per cent hypochlorite bleach. I didn't really expect much to happen since I suspect the source of the information probably used powdered metal but to my pleasant surprise the metal began to dissolve fairly rapidly so I left the bottle for a couple of days when I noted that the bottle was bloating. When I opened the lid I could immediately smell chlorine and the liquor had become faintly acidic.

This suggests that the reaction proceeds something like:

Mo + 3NaOCl + H2O -> 3NaCl + H2MoO4

The chlorine then results from the acidifying effect of the free molybdic acid acting on either the hypochlorite directly or the mixture of chloride and chlorate that result from its decomposition and which is not stable under acid conditions. This is surprising since bleach contains a significant amount of free hydroxide anyway.

To prevent this happening I added some extra sodium hydroxide solution according to the equation:

2Mo + 7NaOCl + 2NaOH -> 7NaCl + 2NaMoO4 + H2O

This appeared to work but after a few more days the bottle was bloated again. This time the vented gas was colourless and odourless. The oxidizing power of the hypochlorite was now spent so the slightly cloudy, straw coloured solution was poured off and replaced with fresh bleach. The bar was now black, deeply corroded and covered with a soft black power that was easily removed with a toothbrush revealing a deeply etched crystalline surface.

I have left the bar in fresh bleach and will monitor for chlorine, basifying as necessary.

The liquor was filtered to remove the brownish flock of what is almost certainly ferric oxide to give a practically clear solution. Rather curiously, on standing the solution has started to slowly evolve more odourless gas. The solution is still strongly alkaline, pH >12. Any idea what the gas is?

Hydrogen or oxygen would seem to be the most obvious candidates but it is generated too slowly and in too small a quantity to collect enough to test. And by what mechanism is it being generated since the liquor has now been separated from the metal bar?

I haven't tested the resulting solution for molybdate ions yet but will do in due course. My plan to recover the molybdate is to evaporate down the solution until salt starts to separate, filter and then acidify with hydrochloric acid. Molybdic acid crystallises very slowly by all accounts (week to many months being required) in spite of being sparingly soluble. Does anyone have any better ideas? Maybe as ppt as calcium molybdate?

woelen - 28-8-2023 at 12:55

The colorless gas most likely is O2. Many transition metals have a catalytic effect on decomposition of hypochlorite:
2 ClO(-) --> 2 Cl(-) + O2

Cobalt has a really strong effect, if you add a cobalt salt to 10% NaClO, then the liquid starts bubbling. With many other metals, there is no immediately visible effect, but when the container is closed tightly, then in a few hours there is noticeable pressure buildup.

fusso - 29-8-2023 at 07:19

Assuming Mo can only reach +6, your 2nd eqn is wrong. Sodium molybdate is Na2MoO4.

Boffis - 29-8-2023 at 13:07

Hi woelen, I originally thought it must be oxygen too but the solution has lost all its oxidizing potential in basic conditions so appears "spent" also the the solution has been vacuum filtered to remove the brown ppt so most of the dissolve gas would have been exhausted and the metal bar has been removed so the generating species must be in solution though presumably not a hypochlorite any more. Only a little more gas is evolved when the solution is heated.

@fusso, well spotted! I hadn't spotted the increase ratio of hypochlorite to Mo the raction should be:

Mo + 3NaOCl + 2NaOH -> 3NaCl + Na2MoO4 + H2O



[Edited on 29-8-2023 by Boffis]

Bedlasky - 29-8-2023 at 13:46

Interesting result! I once tried reaction between molybdenum powder and 4% NaOCl in the test tube, but nothing happened. This really surprised me.

Separating Na2MoO4 from NaCl by fractional crystallization isn't good idea, sodium molybdate have quite high solubility, NaCl will crystallize first. Precipitating CaMoO4 seems like a better solution.
Quote: Originally posted by fusso  
Assuming Mo can only reach +6, your 2nd eqn is wrong. Sodium molybdate is Na2MoO4.


Molybdenum can reach several lower oxidations states in aqueous solution, but they don't like oxidizing and basic conditions.

DraconicAcid - 29-8-2023 at 14:34

Quote: Originally posted by Bedlasky  
Interesting result! I once tried reaction between molybdenum powder and 4% NaOCl in the test tube, but nothing happened. This really surprised me.

Separating Na2MoO4 from NaCl by fractional crystallization isn't good idea, sodium molybdate have quite high solubility, NaCl will crystallize first. Precipitating CaMoO4 seems like a better solution.

Quote: Originally posted by fusso  
Assuming Mo can only reach +6, your 2nd eqn is wrong. Sodium molybdate is Na2MoO4.


Molybdenum can reach several lower oxidations states in aqueous solution, but they don't like oxidizing and basic conditions.


Yes, but it can't be +7, which is implies by NaMoO4.

Boffis - 30-8-2023 at 12:48

OK guys I think we have done-to-death the incorrect formula; it was just a typo in the equation generating software I use. I'm surprised it could balance it
Anyway, I added a 5M solution of calcium chloride solution to the carefully neutralised (with acetic acid) molybdenum solution until no further ppt formed and got a dense white ppt. I filtered it off, washed the cake with copious water and dried it in the oven at 70-80 C to constant weight and obtained 58.53g of white powder or about 50% based on hypochlorite (assuming bleach is 11% w/w and D=1.59). I'll run some tests on it tomorrow.

Bedlasky - 30-8-2023 at 16:03

It could possibly containt water of crystallization. I didn't found anything about it, but precipitated salts are often hydrates (however not everytime).

Dripping SnCl2 solution in to the solution of molybdate in 30% HCl results in formation of green complex [MoOCl5]2-. That's very good test for the presence of molybdate.

Boffis - 31-8-2023 at 09:58

Hi Bedlasky, I doubt that it is hydrated, at one time calcium molybdate precipitation was an analytical method for measuring both calcium and molybdenum its chemistry is well covered in older analytical text and the natural calcium molybdate is always anhydrous (powellite).

I ran some tests today on the ppt. It is rich in molybdate (Stannous chloride-thiocyanate test) and contains some carbonate. The carbonate is easily leached out with dilute acetic acid which does not dissolve the calcium molybdate. So its looking good; so far!

Bedlasky - 1-9-2023 at 10:30

Oh, I didn't know that it was used as analytical method! Do you have some of these papers?

Boffis - 1-9-2023 at 12:39

Quote: Originally posted by Bedlasky  
Oh, I didn't know that it was used as analytical method! Do you have some of these papers?


They are mostly in old books on analytical chemistry but I will dig out some of them and then either scan the relevant bits or give you references if they are available on line.

I leached the white ppt with acetic acid until it stopped effervescing, filtered of the solid and then washed it by decanting and then filtered it again and dried it in the oven at about 100C to obtain 41.9g of pure white calcium molybdate. I have a second batch in the wings lets see what I get from it!

The question now is how to turn this into molybdenum trioxide, sodium molybdate, phosphomolybdic acid etc. theat are useful?

Bedlasky - 1-9-2023 at 14:54

Reaction with sulfuric acid give you solution of molybdic acid. Solubility of molybdic acid is according to wikipedia 151 g/l, so make sure you have enough water to dissolve it all.

Another option is heating CaMoO4 in the solution of Na2CO3, this should give you sodium molybdate solution.

[Edited on 2-9-2023 by Bedlasky]