Sciencemadness Discussion Board - Reducing sodium molybdate by nascent hydrogen

Reducing sodium molybdate by nascent hydrogen

Bedlasky - 1-6-2019 at 04:35

Hi.

I did the experiment with sodium molybdate with nascent hydrogen today. I prepared solution of sodium molybdate and added few mils in to the test tube. Than I added cca the same portion of concentrated HCl. After that I added a very small piece of aluminum foil and cover test tube with paper tissue. After half a minute aluminum foil was start vigorously reacting with HCl and hydrogen was start to releasing. On the surface of the foil was start appearing some yellow compound which was dissolving in water. Solution became yellow and after minute it turned in orange. Before reaction I suspected that sodium molybdate will be reduced to MoO2 or metallic Mo. But I surprised this colour changing. This isn't metallic Mo and MoO2 is purple and insoluble in water. This orange compound didn't visibly react with NaOH and Na2S2O5 but did react with KMnO4 to form colourless solution (I suspected that is molybdate(VI)).

I've done some research in book and on the internet. I've found in book that exists coloured compounds of Mo and W called molybdenum (or tungsten) bronzes which formed by reduction by hydrogen, molybdenum (or tungsten) or by electrochemical reduction. About this compound is also article on wiki.

https://en.wikipedia.org/wiki/Molybdenum_bronze

About molybdenum compounds is something there:

https://books.google.cz/books?id=Bkr-BAAAQBAJ&pg=PA723&lpg=PA723&dq=molybdenum(IV)+oxide+preparation&source=bl&ots=LyR1rbWyIf& sig=ACfU3U1X3qnRh9Z4xJqRrilY8ad_ksrCLA&hl=cs&sa=X&ved=2ahUKEwjJ9L_2qsbiAhUJEVAKHZvJBjo4FBDoATABegQICBAB#v=onepage&q=molybdenum(IV)%20o xide%20preparation&f=false

There is mention about Mo(III) solution which is red. Mo(OH)3 is prepare by action of some metals on molybdate(VI) solution. This compound is soluble in HCl. There is also something about molybdates(IV) - especcialy about baryum and strontium molybdates(IV) which is also in shades of red. They are make by reduction of Sr or Ba molybdate(VI) by hydrogen. Maybe I'll try my reaction with addition of some strontium nitrate.

So what do you mean?

[Edited on 1-6-2019 by Bedlasky]

Bedlasky - 2-6-2019 at 11:33

This compound is more interesting than I thought. I did few tests today and I'll do more test in future. I also will do manganometry analysis of it - I'll calculate average oxidation number of Mo in this compound.

Here are my observations:

Na2MoO4 + Al (or Zn) + conc. HCl --> orange Mo (+ conc. HCl) --> light green Mo (+H2O) --> orange Mo

Light green Mo + KMnO4 --> MoO4(2-) (probably) + MnO2 (+Na2S2O5) --> MoO4(2-) + Mn2+

Orange Mo + KMnO4 --> MoO4(2-)

Orange Mo + Na2MoO4 --> molybdenum blue

Orange Mo + more H2O (or NaOH) --> colourless Mo

Na2MoO4 + more Zn than in first reaction + conc. HCl --> orange Mo (+ HCl) --> very pale red Mo (+Na2MoO4) --> light green Mo

Very pale red Mo + KMnO4 --> MoO4(2-) + MnO2 (+Na2S2O5) --> MoO4(2-) + Mn2+

Na2MoO4 + Zn + H2SO4 --> dark green Mo --> orange Mo (but slightly different shade than first orange Mo) (+H2SO4) --> orange-green Mo

Seems like this molybdenum species forms complexes with sulphates and chlorides and have reducing properties

Edit: I found, what is the light green compound is: https://www.imoa.info/molybdenum-uses/molybdenum-chemistry-uses/molybdenum-oxidation-states.php

[Edited on 2-6-2019 by Bedlasky]

Discussion

Bedlasky - 21-6-2019 at 11:29

So I've done some research.

I completed reaction mechanism which is shown on picture. I gave them working names (except first two ones):

Molybdate-->Molybdenum blue-->Yellow molybdenum-->Orange 1 molybdenum (below it is green molybdenum)-->Orange 2 molybdenum (below it is red molybdenum)-->Dark green molybdenum

All these molybdenum compounds are oxidized by molybdate in to higher molybdenum species (except molybdenum blue and molybdate).

All these molybdenum compounds are reduced by nascent hydrogen in to lower molybdenum compounds (except dark green molybdenum).

Orange 1 and Orange 2 form in concentrated HCl chloride complexes.

Orange 1 and Orange 2 form in basic solution some hydroxide, oxide-hydroxides or hydrated oxides soluble in HCl (maybe dark green and yellow too create hydroxides, I didn't test it).

Solution of green molybdenum is stable, but red molybdenum is slowly oxidize (few days). Dark green is oxidize far more quickly (after few minutes in hot water bath is apparent colour changing).

Green and red molybdenum are oxidized after evaporation of HCl in to molybdenum blue.

Reduction of molybdate is very sensitive to amount of used zinc. It's very easy to add too much zinc powder and you get different compound. To preparation of higher molybdenum species is better to use aluminum foil.

Yellow molybdenum I prepared only once acidently during analysis.

Green molybdenum react with dark green molybdenum to form red molybdenum.

Analysis

In tuesday I've done some analysis. Sadly I didn't have much time, so I analysed only yellow, orange 1 and dark green molybdenum. I chose manganometry, but it was hard determine the equivalence point because of oxidation of chlorides. I weighed sodium molybdate on analytical scales and zinc powder to two decimal places. I used little amount of MnCl2 to inhibition chloride oxidation.

Here are results:

Yellow molybdenum is mixed-valence species with average oxidation number 5,8. Maybe molybdenum bronze or something like that?

Orange 1 molybdenum have average oxidation number 5,14 - so probably it is Mo(V) species.

Dark green molybdenum have average oxidation number 3,82.

Orange 2 molybdenum is somewhere between 5,14 and 3,82.

Little discusion:

I read some materials:

Molybdenum in lower oxidation states

Molybdenum chloride complexes

Molybdenum in different oxidation states (VI-III)

Reduction of Mo(VI) and description of molybdenum in different oxidation states

Orange 1 molybdenum is probably [Mo2O4]2+ or maybe some chloride complexes ([MoOCl]2+; [MoOCl2]+; or [MoOCl3]).

Green molybdenum is probably [MoOCl5]2-.

This agrees with literature and with my analysis.

Dark green molybdenum resembles its colour and some properties with [Mo(H2O)Cl5]2- and [MoCl6]3- (according to literature). But this is not corresponding with my analysis. But there is some explanation - 1. Mo(III) is quickly oxidized by air 2. Maybe was reduction incomplete (mixed solution of Mo(III) + Mo(IV)).

According to some sources [MoCl6]3- which exists in HCl > 4,5M is red - but I never prepare red solution from dark green molybdenum adding concentrated HCl.

If are these presumptions right, orange 2 molybdenum is [MoO(OH)]+ and red molybdenum is [MoOCl]+.

What is yellow molybdenum I don't know. Maybe molybdenum bronze - these compounds have a wide range of colours.

[Edited on 21-6-2019 by Bedlasky]

Bedlasky - 14-2-2020 at 16:54

I posted article about this reaction on my website in czech and english.

wg48temp9 - 16-2-2020 at 08:39

Quote: Originally posted by Bedlasky

I posted article about this reaction on my website in czech and english.

Interesting post and work Bedlasky.

I tried to synthesis some of your colourful molybdenum compounds. I started with molybdenum blue and oxidised to molybdenum acid with H2O2. As my HCl was not available I used dilute sulphuric acid, sodium chloride. and zinc. It first seemed to produce some wisps of molybdenum blue round the zinc but later it turned a muddy dark greenish colour. I added more zinc but it remained cloudy and dark greenish in colour. I will try again with the correct reagents. I have some more small pieces molybdenum (mounting wire of the filament of a MO magnetron) slowly dissolving in 12%H2O2 to make ammonium molybdate.

So now I have a waste solution containing <100mg of molybdenum. I think its not significantly poisonous but I am reluctant to flush it down the sink. Perhaps I could precipitate the molybdenum as the sulphide with hypo.

PS: Sodium thiomolybdate Na2MoS4 is an interesting colourful dark red. Its prepared by the action of hydrogen sulphide upon a solution of sodium molybdate .

Bedlasky - 16-2-2020 at 09:29

Reduction by zinc dust is very sensitive and if you add too much zinc you reduce all molybdenum to III oxidation state (which is very dark green). So this is probably what happened to you.

For making Mo(V) and Mo(VI/V) compounds is better to use aluminium foil because it have relatively small surface comparing to zinc dust, so aluminium foil generate less nascent hydrogen. If you want to use zinc dust, you must use only very very tiny amount of it in more concentrated molybdate solution.

For making Mo(IV) and Mo(III) is better zinc dust. If you want to use aluminium foil, you must add very large amount of it - so reduction with aluminium foil is really annoying (especially on III oxidation state). If you want to make Mo(IV) solution you must be carefull with addition of zinc. Add it only in small portions. You can also test if the reduction is complete - just add few drops of your solution in concentrated hydrochloric acid. If solution is red, then reduction on Mo(IV) is complete, if solution is greenish brown, then you must add more zinc. I use similar test on Mo(V).

There are also other reducing agents. For example:

Molybdenum blue can be formed by reaction of sodium molybdate with Fe2+, Na2S2O3, ascorbic acid, glucose, fructose, sucrose or Na2S2O4 in acidic solution.

Mo(V) can be formed by reaction of sodium molybdate with excess of SnCl2 or TiCl3 in acidic solution. I suppose that V2+ and Cr2+ salts also works.

Quote: Originally posted by wg48temp9

So now I have a waste solution containing <100mg of molybdenum. I think its not significantly poisonous but I am reluctant to flush it down the sink. Perhaps I could precipitate the molybdenum as the sulphide with hypo.

If you have solution which contains only few hundred milligrams of molybdate, than you can flush it down the sink. Molybdenum isn't that toxic for environment as another heavy metals like Pb, Tl, Hg, Co, Ni, Cu etc. Molybdates can be precipitated by concentrated solution of CaCl2.

woelen - 16-2-2020 at 11:32

Very interesting, especially your scheme of all oxidation states.

I myself have done experiments with Mo as well, and I also observed the yellows, reds and blues. I even did an electrolysis experiment of molubdate, dissolved in dilute HCl. At the cathode, a red compound is formed (I have this experiment on my website):
https://woelen.homescience.net/science/chem/exps/electrolysi... (first experiment)

I also did experiments with ascorbic acid and hypophosphite as reductor:
https://woelen.homescience.net/science/chem/exps/colorfulmol...

Finally, I dissolved molybdenum metal in HNO3. This experiment has unexpected results:
https://woelen.homescience.net/science/chem/riddles/Mo+HNO3/...

Molybdenum has a remarkable, and also still poorly understood chemistry.

Bedlasky - 16-2-2020 at 13:06

Your solution from electrolysis looks exactly like Mo(V) or Mo(IV) solution. I read that molybdates can be electrolyticaly reduced even at III oxidation state.

The precipitate from dissolving molybdenum in HNO3 maybe dissolves in concentrated HCl. Lower oxidation states of molybdenum formed stable chloride complexes which are definitely soluble in acidic solutions. HNO3 can oxidize this lower oxidation states of molybdenum in to molybdate (I once tried add nitrate in to Mo(V) solution), but you probably used excess of molybdenum.

I will definitely post few articles about molybdenum chemistry in future. I have plenty of photos from various experiments with sodium molybdate. I also want to try titanometric determination of molybdate and preparation of (NH4)2[MoOCl5].

woelen - 16-2-2020 at 23:43

Quote: Originally posted by Bedlasky

The precipitate from dissolving molybdenum in HNO3 maybe dissolves in concentrated HCl. Lower oxidation states of molybdenum formed stable chloride complexes which are definitely soluble in acidic solutions. HNO3 can oxidize this lower oxidation states of molybdenum in to molybdate (I once tried add nitrate in to Mo(V) solution), but you probably used excess of molybdenum.

I'll make some HNO3 again, and try this experiment again and then add some concentrated HCl after making the precipitate. In another experiment, I'll mix the HNO3 and HCl beforehand (making aqua regia) and add molybdenum metal to that mix.

I did not use excess Mo in that experiment, but excess HNO3 (actually, the excess amount was quite large, the liquid in the pictures is concentrated HNO3, while there is just a spatula of Mo-powder).

wg48temp9 - 17-2-2020 at 00:16

Quote: Originally posted by Bedlasky

Reduction by zinc dust is very sensitive and if you add too much zinc you reduce all molybdenum to III oxidation state (which is very dark green). So this is probably what happened to you.

For making Mo(V) and Mo(VI/V) compounds is better to use aluminium foil because it have relatively small surface comparing to zinc dust, so aluminium foil generate less nascent hydrogen. If you want to use zinc dust, you must use only very very tiny amount of it in more concentrated molybdate solution.

Actually initially I used thick aluminium strip cut from a clean tart base but it was too slow so I switched to a strip of zinc foil. The zinc was more reactive. Unfortunately I was forced to leave the experiment unattended.

What I thought would happen is the molybdenum would be slowly reduced and therefore go though the colours you displayed. That may have happened while it was unattended.

I wanted to repeat the video shown in http://www.sciencemadness.org/talk/viewthread.php?tid=82505#...
but with all the colour changes.

PS: I see that my video has been down loaded and presumably viewed 368 times. It took some effort to capture that effect so to see that it has been viewed 368 times makes my effort worthwhile.

Bedlasky - 17-2-2020 at 14:23

Quote: Originally posted by woelen

I did not use excess Mo in that experiment, but excess HNO3 (actually, the excess amount was quite large, the liquid in the pictures is concentrated HNO3, while there is just a spatula of Mo-powder).

This is mystery for me. If you use excess HNO3 it should oxidize Mo in to VI oxidation state (in theory). I once tried if I precipitate strontium molybdate(IV) with strontium nitrate and solution turned in to colourless. I today tried few tests.

1. I mixed sodium molybdate, hydrochloric acid and aluminium foil to form reduced molybdenum compounds. Then I added some potassium nitrate and solution turned in to the yellow with formation a little bit of NO2 gas. It didn't visibly react with KMnO4 (only some MnO2 was formed due to reaction with HCl). Yellow colour is probably caused by NOCl/Cl2/NO2. Solution smelled of chlorine.

2. I mixed sodium molybdate, sulfuric acid and zinc dust to form reduced molybdenum compounds. Then I added some potassium nitrate and solution turned in to the orange with formation a little bit of NO2 gas. Maybe is orange colour cause by NO2? But colour is too much intense in my opinion. Solution smelled of chlorine, which is quite surprising for me. How smell NO2? Have NO2 similar smell?

I've got one other idea for test your precipitate from HNO3. Try it dissolve in hot acidified phosphate solution for the case that it is compound of molybdenum in VI oxidation state. Phosphates form very stable yellow complex with molybdates. Phosphate-molybdate complex is better test then peroxide-molybdate complex because phosphate haven't any oxidation properties.

Here is also some literature:

https://books.google.cz/books?id=aFSnXpONJEoC&pg=PA74&lpg=PA74&dq=%5BMoOCl5%5D2-&source=bl&ots=Vme0BHAW8v&sig=ACfU3U3UQYi94Ss8 57CiqpsGRMxm0NCnsQ&hl=cs&sa=X&ved=2ahUKEwiNms783sviAhVGwMQBHXzRCKAQ6AEwCHoECAcQAQ#v=onepage&q=%5BMoOCl5%5D2-&f=false

Quote: Originally posted by wg48temp9

What I thought would happen is the molybdenum would be slowly reduced and therefore go though the colours you displayed. That may have happened while it was unattended.

Aluminium foil must be dissolved at least in 1+1 HCl. If concentration of HCl is lower, then dissolving is very slow. You want rapid dissolving. Be careful because reaction between Al and HCl is quite exothermic.

[Edited on 17-2-2020 by Bedlasky]

wg48temp9 - 18-2-2020 at 05:51

Molybdates of zinc and iron are insoluble and aluminium molybdate is very sightly soluble. So zinc or ferrous salts might be used to produce insoluble waste from soluble molybdate waste. Probable zinc is best to avoid ferrous ferric problems and it will be necessary to make certain the molybdenum is in the Vl oxidation state.

It also probably means that zinc, iron and aluminium tends to passivate in molybdate solutions.

woelen - 18-2-2020 at 13:56

I tried the experiment with HNO3 again.

I took 2 ml of room temperature HNO3 (concentration not known exactly, pale yellow home-made stuff, close to azeotropic concentration, but might be a little less or more).
To this HNO3 I added a small amount of powdered Mo-metal (reagent grade, 99.9%) and swirled. Initially nothing happens, but after half a minute or so, a reaction sets in, which becomes more and more violent. A lot of NO2 is produced and a brown solid is dispersed in the acid. Quickly the reaction subsides again and a dark brown turbid liquid is obtained. The dark brown material is a solid, finely dispersed in the acid.
I heated the liquid until it started boiling. This causes production of more NO2 and the color of the solid slowly changes from dark brown to lighter brown/red until it becomes beige/brown. It does not become completely white.

In another test tube, I took 1 ml of this home-made HNO3 and added 2 ml of conc. HCl (37%). To this mix, I added a small amount of Mo-metal.
Initially, nothing happens. The metal is finely dispersed in the liquid and a dark grey liquid is obtained. After a minute or so, the color shifts from grey to reddish brown and a reaction sets in. The material then dissolves, in a few tens of seconds and a lot of orange/yellow gas is produced (must be ONCl, I recognized its smell). The liquid turns deep red and becomes clear. No solid matter is dispersed in the liquid.
I boiled this liquid as well after the reaction. While boiling, the liquid becomes lighter and lighter. Finally it becomes yellow, the typical color of aqua regia which has aged. On dilution the liquid becomes colorless (aqua regia has a yellow color, due to dissolved ONCl, which hydrolyses to colorless compounds on dilution with water). So, with aqua regia you get a red compound, which is oxidized to a colorless compound of molybdenum on further heating.

Finally, I added some conc. HCl to the test tube with HNO3 and the beige solid in it. The HCl causes this solid to dissolve. It does not dissolve immediately, but on gentle heating it dissolves fairly easily and the solution is yellow (color of aqua regia, but with too much HNO3 in it).

So, on further heating the molybdenum indeed is oxidized to its +6 oxidation state, giving white/colorless compounds, but in the initial reaction, even when the temperature is quite high, there is a brown/red compound. In HNO3 it is an insoluble compound, in HNO3/HCl it dissolves.

I find it quite remarkable, that even in large excess concentrated HNO3 the molybdenum is not immediately oxidized to the +6 oxidation state. In the concentrated acid, the red/brown compound can exist without being oxidized further. Only on strong heating (boiling azeotropic acid) it is further oxidized.

Bedlasky - 18-2-2020 at 15:31

Very interesting results Woelen!

It's interesting that oxidation is gradual. I am interested what this brown stuff is. Maybe it's some molybdenum oxide? You could find out easily oxidation state of Mo in it. Just filtred off this brown precipitate, wash it with little bit of water and dissolve it in concentrated HCl. Mo(V) forms green complex, Mo(IV) red complex, Mo(III) brown complex. I personally guess that brown precipitate is probably hydrous Mo2O5 or another Mo(V) compound, because Mo(IV) and Mo(III) have strong reducing properties (they are oxidized even by oxygen), but maybe I am wrong. This test can be also used for analysis beige/brown precipitate.

It's interesting how complex formation (and also formation of stronger oxidant as NOCl/Cl2 mixture) can affect reaction.

May I ask where did you get molybdenum powder?

Thanks for sharing your results!

[Edited on 18-2-2020 by Bedlasky]

Bezaleel - 21-2-2020 at 03:06

Maybe a complex with NOCl is formed? As I checked for presence of a lone pair on NOCl, I found that Wikipedia says:

Quote:

Nitrosyl chloride is used to prepare metal nitrosyl complexes. With molybdenum hexacarbonyl, NOCl gives the dinitrosyldichloride complex:[6]
Mo(CO)6 + 2 NOCl → MoCl2(NO)2 + 6 CO

[6] Johnson, B. F. G.; Al-Obadi, K. H. (1970). "Dihalogenodinitrosylmolybdenum and Dihalogenodinitrosyltungsten". Inorg. Synth. 12: 264–266. doi:10.1002/9780470132432.ch47.

Who knows what it is capable of forming in an aqueous environment?

Bedlasky - 21-2-2020 at 10:20

I tried reaction between acidified ammonium heptamolybdate and sodium nitrite and there wasn't visible reaction. So yellow colour is caused by NOCl.

wg48temp9 - 21-2-2020 at 10:48

Quote: Originally posted by Bedlasky

Very interesting results -------

May I ask where did you get molybdenum powder?

Thanks for sharing your results!

[Edited on 18-2-2020 by Bedlasky]

Its available on ebay uk.

woelen - 21-2-2020 at 12:28

I obtained my Mo-powder several years ago, indeed from eBay, but I do not know the seller anymore. It was either German or British.

Fery - 22-2-2020 at 00:06

http://svetprvku.cz/12-prvky
Molybden - prášek
Čistota 99,9%
6 CZK (0,25 EUR) per 1 g
49 g in stock

Bedlasky - 20-3-2020 at 19:29

I did few experiments with Mo powder today. And I have few interesting observations.

Reactions with acids and peroxides:

31% HCl - Mo very very slowly dissolved in hot water bath to form green [MoOCl5]2-

38% H2SO4 - Without reaction

65% HNO3 - Mo very quickly dissolved to form white powder and yellow solution. After dilution solution became colourless. White powder is probably H2MoO4.

35% H2O2 - Mo very quickly dissolved even at room temperature to form yellow [MoO3(O2)]2- complex.

Reaction with acids and ammonia mixtures:

HCl + H2O2 - Mo very quickly dissolved to form yellow peroxide complex

HCl + HNO3 - Mo quickly dissolved in hot water bath to form yellow solution. Upon dilution yellow colour disappeared - so solution contained Mo(VI).

HCl + (NH4)6Mo7O24 - Mo quickly dissolved in hot water bath to form green [MoOCl5]2- complex

H2SO4 + H2O2 - Mo very quickly dissolved to form yellow peroxide complex

H2SO4 + (NH4)6Mo7O24 - Mo dissolved in hot water bath to form brown solution of [Mo2O4]2+. It seemed to me that reaction with HCl/heptamolybdate mixture was faster.

NH3 + H2O2 - Mo dissolved firstly to form reddish brown solution of [Mo(O2)4]2-. After heating solution became colourless due to decomposition of peroxide complex in to molybdate.

[Edited on 22-3-2020 by Bedlasky]

Preparation of Phosphomolybdic Acid

wg48temp9 - 21-3-2020 at 03:04

Below is a prep of Phosphomolybdic Acid. Its more soluble than Molybdic Acid so it may be useful starting compound for other Mo compounds

Bedlasky - 21-3-2020 at 17:58

wg48temp9: Thanks for nice paper

.

I experimented today little bit with dissolving Mo metal again. Yesterday I swapped test tube with HNO3 and aqua regia, so observations was different today (I'll edit yesterday post and correct it).

Heating Mo with 65% HNO3 - white powder of H2MoO4

Reaction of Mo with aqua regia - brown solution, after dilution it's nearly colourless.

Mo reacts very different with dilute nitric acid (I used 1+1 HNO3). It reacts vigorously even without heating to form brown powder. This brown powder is soluble in HCl to form orange solution. What is this compound I really don't know. I thought that maybe hydrated MoO2 or some Mo(VI)/Mo(V) oxide, but I tried to oxidize Mo(IV) and molybdenum blue with HNO3 and both compounds was oxidized in to Mo(VI). So I am really confused what is it.

Mo is very quickly dissolved by mixture of 65% HNO3 and NaH2PO4 to form yellow solution.

Mo doesn't react with conc. sulfuric acid.

Mo very vigorously reacts with HNO3/KBr mixture

Mo is very slowly attackted also by dilute HCl (I used 1+1 HCl) to form green solution.

Mo is very quickly dissolved by mixutre of H2O2 with 4% NaOH, firstly to form red solution, which become after heating colourless.

Mo doesn't react with bleach

Addition of molybdate in to the dilute HNO3 doesn't affect reaction with Mo metal.

Quick summary:

Mo metal can be dissolved in:

1. conc. HNO3, dilute HNO3, conc. HCl, dilute HCl

2. aqua regia, HBr/HNO3 mix., H3PO4/HNO3 mix., HCl/molybdate mix.

3. almost any acidic or basic mixture of H2O2, H2O2 itself

wg48temp9 - 22-3-2020 at 09:16

Below is a brief paper on the some salts of Phosphomolybdic Acid. Several salts of monovalent elements including potassium and ammonia are sparingly soluble while the salts of sodium and di or trivalent elements are all soluble.

Bedlasky - 22-3-2020 at 16:26

wg48temp9: Thank you very much for your post! You help me to explain one mystery. A few days ago I want to take a picture of sodium phosphomolybdate solution. I dissolved some Na2MoO4 in water, add tiny amount of K2HPO4 and add few drops of conc. HCl. I obtained yellow solution. But when I added more phosphate, I obtained yellow precipitate, which was surprise for me. Now I know why. I used POTASSIUM hydrogen phosphate. When I repeated this experiment today, I used NaH2PO4 instead of K2HPO4. I added just a tiny amount of phosphate and obtained yellow solution. When I added more phosphate, solution became colourless - which corresponds with my previous observations.

So sodium phosphomolybdate is interesting reagent for K+, NH4+ and Rb+ test.

Btw - when you add K+, NH4+ or Rb+ salt in to the solution of sodium silicomolybdate, you obtain also yellow precipitate. Reaction with Rb+ is immediate, while reaction with NH4+ and K+ requieres heating.

Bedlasky - 28-3-2020 at 17:17

I have few new observations about dissolving Mo metal in acids:

85% H3PO4 - no reaction

99% HNO3 - In cold acid there is no reaction, in hot acid there is formation of small amount of nitrogen oxides. If you add small amount of water in to this mixture, vigorous reaction take place and lots of MoO3 precipitate is formed.

1+1 HNO3/NaH2PO4 mixture - Formation of brown solution.

wg48temp9 - 13-6-2020 at 13:16

Quote: Originally posted by Bedlasky

I posted article about this reaction on my website in czech and english.

I just checked out your link, its a good compliment to this thread and interesting display of the various colours of molybdeum solutions.

Bedlasky - 14-6-2020 at 07:05

Thanks.

I am preparing article about Mo complexes (but I don't have much time right know). This is also colourful and spectacular chemistry. Molybdenum is really chameleon among elements.

Bezaleel - 19-6-2020 at 15:29

Quote: Originally posted by Bedlasky

wg48temp9: Thank you very much for your post! You help me to explain one mystery. A few days ago I want to take a picture of sodium phosphomolybdate solution. I dissolved some Na2MoO4 in water, add tiny amount of K2HPO4 and add few drops of conc. HCl. I obtained yellow solution. But when I added more phosphate, I obtained yellow precipitate, which was surprise for me. Now I know why. I used POTASSIUM hydrogen phosphate. When I repeated this experiment today, I used NaH2PO4 instead of K2HPO4. I added just a tiny amount of phosphate and obtained yellow solution. When I added more phosphate, solution became colourless - which corresponds with my previous observations.

So sodium phosphomolybdate is interesting reagent for K+, NH4+ and Rb+ test.

Btw - when you add K+, NH4+ or Rb+ salt in to the solution of sodium silicomolybdate, you obtain also yellow precipitate. Reaction with Rb+ is immediate, while reaction with NH4+ and K+ requieres heating.

Yesterday I prepared ammonium phosphomolybdate. This was a test to check the colour of this compound.
A crystal of 1x0.5cm of ammoniumparamolybdate (NH4)6Mo7O24.4H2O and ~10ml of water were gently heated until a completely clear and colourless solution was formed. The solution was split in two parts of similar volume.

1. To one part, 1ml 85% (ortho)phosphoric acid (H3PO4) was added in a test tube. The mixture was shaken and heated, but no change in colour took place. 2 ml of 10% HNO3 solution were added without change of colour. When it was heated shortly to ~80C, the solution stayed clear but turned yellow; when cooled, it became colourless again after a few minutes. This turning yellow on brief heating and discolouring on cooling was repeated a few times - the reaction seems reversible.
When heated for a few minutes to boiling, a very fine yellow precipitate was formed and the solution turned yellow. After a few hours, a fine precipitate had settled. The supernatant liquid was then colourless, with a some microscopically fine yellow powder on its surface.

2. Likewise, the other part was put into a small flask and converted to ammonium phosphomolybdate by adding 85% H3PO4 and 10% HNO3 solution and boiling briefly. To this an equal volume of 10% HCl solution was added. This was boiled for ~5 minutes and water was added as needed. No changes were observed.

Results 1 and 2 after standing for 1 day are shown here:

I did another experiment. A strong solution of MoO3 in 36%HCl is green in colour. From this solution, 1ml was taken, and 1.5ml of 85% H3PO4 were added. The mixture was shaken. No reaction or change in colour was observed. When water was added (~5ml), the colour immediately turned deep blue. When heated, no changes were seen. The colour disappeared when a soda solution (Na2CO3) was added. No further changes took place when boiling the result. Adding 2ml of 10% HNO3 made no visible changes, neither after boiling
Extra H3PO4 and more HNO3 were added, but no changes were observed, neither after boiling or standing for a day.

Question: Is the presence of chloride known to prevent the formation of phosphomolybdate?

[Edited on 19-6-2020 by Bezaleel]

Bedlasky - 19-6-2020 at 22:49

Interesting observations!

I made phosphomolybdate solutions with H2SO4, HNO3 and HCl without any problems, so chloride ions doesn't interfer.

With sodium molybdate and sodium dihydrogenphosphate I made two different phosphomolybdate solutions - one yellow and one colourless. If you add small amount of sodium dihydrogenphosphate (it must be sodium salt, with potassium or ammonium salt you obtain yellow precipitate) in to acidified sodium molybdate solution, you obtain yellow solution. If you add more phosphate, solution become colourless. If you add more molybdate, solution become yellow again. This probably affected your experiment. But it's interesting that ammonium salt didn't precipitated out. I don't know why, but it is probably caused by conditions.

It's also interesting that this equilibrium also depending on temperature. I'll definitely try it.

MoO3 solution in conc. HCl is green? That's something new to me

. Deep blue solution is mistery. Normally I guess molybdenum blue, but there wasn't any reducing agent and soda solution doesn't affect molybdenum blue.

Thanks for sharing your observations.

[Edited on 20-6-2020 by Bedlasky]

plastics - 19-6-2020 at 23:29

Interesting stuff!

I am interested in an analytical test to identify/check rubidium and caesium salts.

I bought some ‘cheap’ rubidium chloride years ago to make rubidium metal - never quite got round to it - but was going for high temp reduction with lithium.

I have been looking for a suitable test and found this paper that uses phospho and silico molybdic acids

I have have ordered some phospho molybdic acid and looking for the silicon version

Attachment: 7209121.pdf (657kB)
This file has been downloaded 462 times

Bedlasky - 20-6-2020 at 05:25

Look at woelen's webpage. There are some interesting tests for caesium.

https://woelen.homescience.net/science/chem/exps/CsCuCl3/ind...

Sodium silicomolybdate is in my opinion better reagent for distinguish K and Rb, because K salts precipitates out only after heating. Sodium silicomolybdate solution can be easily prepared in this way: In to the solution of sodium molybdate add small amount of sodium silicate and then add 1ml of conc. HCl at once (ideally with stirring). If too much silicate is present then hydrous SiO2 precipitates out, so add only small amount!

Another way is flame test. Colour of K, Rb and Cs flames are simmilar, but Rb have more bluish flame than K (I never saw Cs flame, but I suppose that it will be even more bluish).

Bezaleel - 23-6-2020 at 15:02

Quote: Originally posted by Bedlasky

Interesting observations!

I made phosphomolybdate solutions with H2SO4, HNO3 and HCl without any problems, so chloride ions doesn't interfer.

With sodium molybdate and sodium dihydrogenphosphate I made two different phosphomolybdate solutions - one yellow and one colourless. If you add small amount of sodium dihydrogenphosphate (it must be sodium salt, with potassium or ammonium salt you obtain yellow precipitate) in to acidified sodium molybdate solution, you obtain yellow solution. If you add more phosphate, solution become colourless. If you add more molybdate, solution become yellow again. This probably affected your experiment. But it's interesting that ammonium salt didn't precipitated out. I don't know why, but it is probably caused by conditions.

This well explains what I ran into: too much phosphate forms a Mo complex which is richer in phosphate. Such complexes are colourless and their salts are normally well soluble.
Here are page 660 and 661 from Mellor, vol XI, about the various phosphomolybdates, which supports your observations.
Attachment: Mellor vol XI - phosphomolybdates p660-661.pdf (1.9MB)
This file has been downloaded 424 times
This table is taken from it and gives a synopsis of the ratios between Mo and P at which complexes have been found:

Mellor - phosphomolybdic acids p661.jpg - 150kB

Quote: Originally posted by Bedlasky

It's also interesting that this equilibrium also depending on temperature. I'll definitely try it.

MoO3 solution in conc. HCl is green? That's something new to me

If concentrated HCl solution is used, and copious amounts of MoO3 are added, it is indeed green:

It's really striking how much MoO3 can be dissolved in concentrated HCl solution.

Quote: Originally posted by Bedlasky

Deep blue solution is mistery. Normally I guess molybdenum blue, but there wasn't any reducing agent and soda solution doesn't affect molybdenum blue.

Thanks for sharing your observations.

[Edited on 20-6-2020 by Bedlasky]

This has to do with the HCl concentration, I think. I only observed the green complex in concentrated HCl with larger amounts of MoO3 added. I think that when the green complex is broken down, some molybdenum blue is formed.

Bedlasky - 25-6-2020 at 16:20

Quote: Originally posted by Bezaleel

I don't think so. MoO3 is only weak oxidant, it can't oxidize HCl. But it forms few chloride complexes - maybe is one of them blue?

Book 1

Book 2

These two books mentioned [MoO2Cl2(H2O)2], [MoO2Cl3(H2O)]- and [MoO2Cl4]2- complexes. Book 2 also mentioned that [MoO2Cl2(H2O)2] is formed in 6M HCl and [MoO2Cl4]2- is formed in 12M HCl. But they didn't mentioned colour. (Book 1 - page 734, Book 2 - there aren't number of pages, just find title Mo(VI)).

[Edited on 26-6-2020 by Bedlasky]

Bedlasky - 27-6-2020 at 11:22

Bezaleel: I try today make this Mo(VI) chloride complex. I poured small amount of concentrated HCl in to the test tube and dissolved in it some ammonium heptamolybdate. I obtained yellow solution. On direct sunlight looked yellow, but when I put it in to the shadow it looked blue or bluish-green depending on angle of view.

When I dissolved ammonium heptamolybdate in hot acid, some solid precipitates out after some time (probably MoO3). It was almost insoluble in water. Dissolved in NaOH and even better in H2O2.

I'll probably try make some MoO3 from ammonium heptamolybdate and dissolve it in HCl if there will be some difference.

Bedlasky - 28-6-2020 at 11:18

So there are another observations:

With MoO3 there isn't any change in colour. If you dissolve molybdate or MoO3 in commercial 31% HCl, there is formation of light yellow solution, in pure 36% stuff it's bright yellow. Under UV light it's green with yellow tinge. If you dissolve too much molybdate in HCl and place solution in hot water bath, after some time there is formation of MoO3 precipitate which dissolves after adding more HCl. After dilution with water solution lost his colour. Few times there was partial reduction in to molybdenum blue (which can be oxidized by two drops of KMnO4 solution), but I don't know why. Maybe some contamination?

Wiki says, that MoO2Cl2 is yellow compound.

https://en.wikipedia.org/wiki/Molybdenum_dichloride_dioxide

But there is also mentioned green MoOCl4.

https://en.wikipedia.org/wiki/Molybdenum_oxytetrachloride

I don't know why your observation is different from mine. There are two possible reasons:

1. Your MoO3 is contaminated with some reducing agent and there is molybdenum blue formation. Yellow + blue = green.

2. Or you prepare different chloride complex.

wg48temp9 - 22-8-2020 at 01:56

Here is an observation of mine:

I had a dilute solution of what I assumed was molybdic acid in a test tube, Mo dissolved in 12% H2O2. I had removed the screw cap about a month ago. As the water evaporated the slight yellow colour became more intense with a tinge of orange in sunlight. Recently I put the solution into a small beaker and placed it on a 80C hot plate. After the solution had been reduced to about half its volume it was a brown and cloudy reminiscent of an oxidised ferrous sulphate solution without any green colour. Eventual I was left with dark brown deposit in the beaker which was actually a mush of dark brown granules with dry almost black with a metallic lustre crust.

The only compound I think it could be is MoO2 but it has never been heater past about 80C certainly not 400C because at the same hot plate I was evaporating a sugar solution which was now only transparent brown syrup with a slight caramel smell.

What is the dark brown mush?

According a wiki page MoO2 will combine with iodine to form volatile species MoO2I2. I could not find any info on that compound.

PS I will post pics of the brown mush later

[Edited on 8/22/2020 by wg48temp9]

Bedlasky - 22-8-2020 at 03:38

Try dissolve small amount of it in water. Maybe it is solid peroxide complex. I doubt that it is MoO2, because H2O2 oxidize tetravalent molybdenum to hexavalent.

wg48temp9 - 22-8-2020 at 06:05

Quote: Originally posted by Bedlasky

Try dissolve small amount of it in water. Maybe it is solid peroxide complex. I doubt that it is MoO2, because H2O2 oxidize tetravalent molybdenum to hexavalent.

The original solution had H2O2 in excess which I assumed has decayed because when it was now heated no oxygen or any gas bubbled out. You could be right perhaps the excess H2O2 reacted slowly to form a yellow/orange coloured peroxide complex but if it did its stable to about 80C. The mush stains paper orange yellow. The crust is electrical conductive (>100ohms between sharp points 10mm apart) as is the dioxide, but the crust may not be dry.

The material redissolves in water to form a orange/brown solution.
The mush when slowly heated in air to about 400C: it first bubbles and expands then drys/decomposes to a friable foam brown on the inside and a shinny black outer surface.

I would expect a H2O2 complex to leave the white trioxide.

To be certain I am not fooling myself I will recreate the original solution and see if it behaves the same.

MoO2I2 is molybdenum oxyiodide. Apparently I2 reacts with the dioxide reversibly so it may be a way of depositing the conductive dioxide on glass.

Bedlasky - 22-8-2020 at 13:09

Quote: Originally posted by wg48temp9

The original solution had H2O2 in excess which I assumed has decayed because when it was now heated no oxygen or any gas bubbled out.

Peroxide complexes, which are formed in acidic solution, are quite stable. Heating or UV light doesn't cause decomposition. On the other hand complexes from alkaline solution are thermally unstable and UV sensitive and quickly decomposes in to molybdate and oxygen.

vano - 5-7-2022 at 23:14

Has anyone made solid molybdenum blue? Or do you have a methodic of its prepatarion?

wg48temp9 - 6-7-2022 at 00:24

Quote: Originally posted by vano

Has anyone made solid molybdenum blue? Or do you have a methodic of its prepatarion?

It can be made by dissolving molybdenum metal in 10% hydrogen peroxide at room temperature. Initially a yellowish solution of is formed formed with some bubbling which I assumed was the decomposition of the H2O2. If the Mo is in excess when the H2O2 is used up by oxidation of the Mo or decomposition, the Mo reduces the yellow solution to a solution of Mo blue. That solution can be evaporate to leaving dark blue solid Mo blue.

There is at least one thread on the subject.

Lion850 - 6-7-2022 at 00:37

Quote: Originally posted by vano

Has anyone made solid molybdenum blue? Or do you have a methodic of its prepatarion?

Hi Vano when I made a basic moly blue the dry product was so dark blue it almost looks black. But when wetted and smeared on a surface it's blue. The only info I have right now is that it was made from "MoO3, HCl, and Zinc metal". When I get back to the shed tomorrow I can try to find the log book entry. Attached some photos, sorry for the poor quality.

vano - 6-7-2022 at 00:44

thanks! I saw your post about it, but the link doesn't work if you remember procedures ill glad if you tell me.

Lion850 - 6-7-2022 at 00:55

Hi Vano I found my notes:
- 11g MoO3 in 50ml water in a beaker
- Add 5.1g zinc granules
- Add 10ml concentrated HCl
- Stir. The white suspended MoO3 should turn blue.
(at this point I stopped for the night and continued the next day)
- Filter. Wash once in funnel with water. (Not recorded in the notes, but I seem to recall that at this point I simply used a tweezers to remove some unreacted zinc metal)
- Dry in desiccator for 2 days (which for me is always over NaOH and under vacuum).
- 12g was recovered.

I also found the sample, and now after 2 years the upper third has changed back to white. Seems it slowly reacted with the bit of oxygen in the vial over time? Or maybe it was not fully dry?

If you make some let us know how it goes.

vano - 6-7-2022 at 01:34

Thank you so much!

vano - 7-7-2022 at 03:29

Mine also is completely black. it has a very weak bluish color. I started with 33g of MoO3.

290139379_5062703130495115_4096509170757317000_n.jpg - 414kB

[Edited on 7-7-2022 by vano]